tag:blogger.com,1999:blog-7729093380675162051Tue, 26 Sep 2017 16:40:04 +0000codemoyhuhttp://moyhu.blogspot.com/noreply@blogger.com (Nick Stokes)Blogger688125tag:blogger.com,1999:blog-7729093380675162051.post-76948542845917679Mon, 25 Sep 2017 22:24:00 +00002017-09-26T21:05:28.663+10:00The best grid for Earth temperature calculation.<a href="https://moyhu.blogspot.com.au/2017/09/grids-platonic-solids-and-surface.html">Earlier this month</a>, I wrote about the general ideas of gridding, and how the conventional latitude/longitude grids were much inferior to grids that could be derived from various platonic solids. The uses of gridding in calculating temperature (or other field variables) on a sphere are <br /><ol><li>To gather like information into cells of known area </li><li> To form an area weighted sum or average, representative of the whole sphere </li><li> a necessary requirement is that it is feasible to work out in which cell an arbitrary location belongs </li></ol>So a good grid must have cells small enough that variation within them has little effect on the result ("like"), but large enough that they do significant gathering. It is not much use having a grid where most cells are empty of data. This leads to two criteria for cells that balance these needs:<br /><ul><li>The cells should be of approximately equal area.</li><li>The cells should be compact, so that cells of a given area can maximise "likeness".</li></ul>Lat/lon fails because:<br /><ul><li>cells near poles are much smaller</li><li>the cells become long and thin, with poor compactness</li></ul>I showed platonic solid meshes with triangles and squares that are much less distorted, and with more even area distribution, Clive Best, too, has been looking at <a href="http://clivebest.com/?p=8014">icosahedra</a>. I have also been looking at ways of improving area uniformity. But I haven't been thinking much about compactness. The ideal there is a circle. Triangles deviate most; rectangles are better, if nearly square. But better still are regular hexagons, and that is my topic here. <br /><br />With possibly complex grids, practical usability is important. You don't want to keep having to deal with complicated geometry. With the cubed sphere, I posted <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">here</a> a set of data which enables routine use with just lookup. It includes a set of meshes with resolution increasing by factors of 2. The nodes have been remapped to optimise area uniformity. There is a lookup process so that arbitrary points can be celled. But there is also a list showing in which cell the stations of the inventory that I use are found. So although the stations that report vary each month, there is a simple geometry-free grid average process <br /><ul><li>For each month, sort the stations by cell label</li><li>Work out cell averages, then look up cell areas for weighted sum.</li></ul>I want to do that here for what I think is an optimal grid. <br /><br />The optimal grid is derived from the triangle grid for icosahedra, although it can also be derived from the dual dodecahedron. If the division allows, the triangles can be gathered into hexagons, except near vertices of the icosahedron, where pentagons will emerge. This works provided the triangles faces are initially trisected, and then further divided. There will be 12 pentagons, and the rest hexagons. I'll describe the mapping for uniform sphere surface area in an appendix. <br /><h4>Lookup</h4>I have realised that the cell finding process can be done simply and generally. Most regular or near-regular meshes are also <a href="https://en.wikipedia.org/wiki/Voronoi_diagram">Voronoi nets</a> relative to their centres. Thatis, a cell includes the points closest to its centre, and not those closer to any other centre. So you can find the cell for a point by simply looking for the closest cell center. For a sphere that is even easier; it is the centre for which the scalar product (cos angle) of 3D coordinates is greatest. <br /><br />If you have a lot of points to locate, this can still be time-consuming, if mechanical. But it can be sped up. You can look first for the closest face centre (of the icosahedron). Then you can just check the cells within that face. That reduces the time by a factor of about 20. <br /><h4>The grids</h4>Here is a WebGL depiction of the results. I'm using the <a href="https://moyhu.blogspot.com.au/p/moyhu-webgl-earth-facility.html">WebGL facility, V2.1</a>. The sphere is a trackball. You can choose the degree of resolution with the radio buttons on the right; hex122, for example, means a total of 122 cells. They progress with factors of approx 2. The checkboxes at the top let you hide various objects. There are separate objects for red, yellow and green, but if you hide them all, you see just the mesh. The colors are designed to help see the icosahedral pattern. Pentagons are red, surrounded by a ring of yellow. <br /><br /><div id="PxBody"></div><script src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyJSlib.js" type="text/javascript"> </script><script src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/hex.js" type="text/javascript"> </script><script src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/Map.js" type="text/javascript"> </script><script src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyGLV2.js" type="text/javascript"></script><br /><br />The grid imperfections now are just a bit of distortion near the pentagons. This is partly because I have forced them to expand to have simiar area to the hexagons. For grid use, the penalty is just a small loss of compactness. <br /><h4>Data</h4>The data is in the form of a R save file, for which I use the suffix .sav. There are two. One <a href="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/hexmin.sav">here</a> is a minimal set for use. It includes the cell centre locations, areas, and a listing of the cells within each face, for faster search. That is all you need for routine use. There is a data-frame with this information for each of about 8 levels of resolution, with maximum 7682 cells (hex7682). There is a doc string. <br /><br />The longer data set is <a href="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/hex.sav">here</a>. This has the same levels, but for each there are dataframes for cells, nodes, and the underlying triangular mesh. A dataframe is just R for a matrix that can have columns of various types, suitably labelled. It gives all the nodes of the triangular mesh, with various details. There are pointers from one set to another. there is also a doc string with details. <br /><h4>Appendix - equalising area</h4>As I've occasionally mentioned, I've spent a lot of time on this interesting math problem. The basic mapping from platonic solid to sphere is radial projection. But that distorts areas that were uniform on the solid. Areas near the face centres are projected further (thinking of the solid as within the sphere) and grow. There is also, near the edges, an effect due to the face plane slanting differently to the sphere (like your shadow gets smaller when the sun is high). These distortions get worse when the solid is further from spherical. <br /><br />I counter this with a mapping which moves the mesh on the face towards the face centre. I initially used various polynomials. But now I find it best to group the nodes by symmetry - subsets that have to move in step. Each has one (if on edge) or two degrees of freedom. Then the areas are also constrained by symmetry, and can be grouped. I use a Newton-Raphson method (actually secant) to move the nodes so that the triangles have area closest to the ideal, which is the appropriate fraction of the sphere. There are fewer degrees of freedom than areas, so it is a kind of regression calculation. It is best least squares, not exact. You can check the variation in areas; it gets down to a few percent. <br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/09/the-best-grid-for-earth-temperature.htmlnoreply@blogger.com (Nick Stokes)4tag:blogger.com,1999:blog-7729093380675162051.post-8797601485274348857Mon, 18 Sep 2017 17:16:00 +00002017-09-19T08:21:49.945+10:00GISS August up 0.01°C from July.GISS showed a <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#L1">very small rise</a>, going from 0.84°C in July to 0.85°C in August (GISS report <a href="https://data.giss.nasa.gov/gistemp/news/20170918/">here</a>). TempLS mesh showed a very slight fall, <a href="https://moyhu.blogspot.com.au/2017/09/august-global-surface-temperature-down.html">which I posted</a> at 0.013°C, although with further data is is now almost no change at all. I see that GISS is now using ERSST V5, as TempLS does. <br /><br />The overall pattern was similar to that in TempLS. Warm almost everywhere, with a big band across mid-latitude Eurasia and N Africa. Cool in Eastern US and high Arctic, which may be responsible for the slowdown in ice melting.. <br /><br />As usual here, I will compare the GISS and previous TempLS plots below the jump. <br /><a name='more'></a><br />Here is GISS<br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/GISSaug.jpg" /><br /><br />And here is the TempLS spherical harmonics plot <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/map.png" /><br /><br /><div style="color: #aa0000;">This post is part of a series that has now run for six years. The TempLS mesh data is reported <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">here</a>, and the recent history of monthly readings is <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#L1">here</a>. Unadjusted GHCN is normally used, but if you click the TempLS button there, it will show data with adjusted, and also with different integration methods. There is an interactive graph using 1981-2010 base period <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#Drag">here</a> which you can use to show different periods, or compare with other indices. There is a general guide to TempLS <a href="https://moyhu.blogspot.com.au/p/a-guide-to-global-temperature-program.html">here</a>. <br /><br />The reporting cycle starts with a report of the daily reanalysis index on about the 4th of the month. The next post is this, the TempLS report, usually about the 8th. Then when the GISS result comes out, usually about the 15th, I discuss it and compare with TempLS. The TempLS graph uses a spherical harmonics to the TempLS mesh residuals; the residuals are displayed more directly using a triangular grid in a better resolved WebGL plot <a href="http://www.moyhu.blogspot.com.au/p/blog-page_24.html">here</a>. </div><br /><br /><br /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/09/giss-august-up-001-from-july.htmlnoreply@blogger.com (Nick Stokes)8tag:blogger.com,1999:blog-7729093380675162051.post-387955575915084055Mon, 18 Sep 2017 07:35:00 +00002017-09-18T19:39:25.098+10:00Grids, Platonic solids, and surface temperature (and GCMs)This follows a series, predecessor <a href="https://moyhu.blogspot.com.au/2017/08/temperature-averaging-and-integrtaion.html">here</a>, in which I am looking at ways of dealing with surface variation of Earth's temperature, particularly global averaging. I have written a few posts on the cubed sphere, eg <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">here</a>. I have also shown some examples of using an icosahedron, as <a href="https://moyhu.blogspot.com.au/2017/04/spherical-harmonics-movie.html">here</a>. <a href="http://clivebest.com/blog/?p=8014">Clive Best</a> has been looking at similar matters, including use of icosahedron. For the moment, I'd like to write rather generally about grids and ways of mapping the sphere. <br /><h4>Why grids?</h4>For surface temperature, grids are commonly used to form local averages from data, which can then be combined with area weighting to average globally. I have described the general considerations <a href="https://moyhu.blogspot.com.au/2017/08/temperature-averaging-and-integrtaion.html">here</a>. All that is really required is any subdivision of reasonable (compact) shapes. They should be small enough that the effect of variation within is minimal, but large enough that there is enough data for a reasonable estimate. So they should be of reasonably equal area. <br /><br />The other requirement, important later for some, is that any point on the sphere can be associated with the cell that contains it. For a regular grid like lat/lon, this is easy, and just involves conversion to integers. So if each data point is located, and each cell area is known, that is all that is needed. As a practical matter, once the cell locations are known for an inventory of stations, the task of integrating the subset for a given month is just a look-up, whatever the geometry. <br /><br />I described <a href="https://moyhu.blogspot.com.au/search?q=coverage">way back</a> a fairly simple subdivision scheme that works for this. Equal latitude bands are selected. Then each band is divided as nearly as possible into square elements (on the sphere). The formula for this division can then be used to locate arbitrary points within the cells. I think this is all that is required for surface averaging. <br /><br />However, for anything involving partial differentiation, such as finite element or GCM modelling, more is required. Fluxes between cells need to be measured, so hey have to line up. Nodes have to be related to each cell they abut. This suggests regular grids. In my case, I sometimes want to use a kind of diffusive process to estimate what is happening in empty cells. Again, regular is better. <br /><h4>Platonic solids</h4>Something that looks a bit like a sphere and is easy to fit with a regular grid is a <a href="https://en.wikipedia.org/wiki/Platonic_solid">Platonic solid</a>. There are five of them - I'll show the Wiki diagram:<br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/plato1.png" /><br /><br />Regular means that each side has the same length, and each face is a congruent regular polygon. The reason why there are only five is seen if you analyse what has to happen at vertices (Wiki): <br /><br /><a name='more'></a><br /><br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/plato2.png" /><br /><br />There are 8 shown, but the three with lilac background have defect zero, and are the polygons you can use to grid the plane. The rest correspond to what you would see if you unfolded the polyhedron. The size of defect says how much distortion results, and is to be expected if you map the sphere onto the solid. <br /><br />The simplest mapping is a projection. You can enclose any of these solids with a sphere that goes through all the vertices. The sphere is mapped by following, for any point, the radius until the surface is reached, and inverted by going the other way. The relation between defect and distortion is that on the sphere the angles will be stretched to add to 360°. The effcets of this can be seen on my <a href="https://moyhu.blogspot.com.au/2017/06/world-map-equal-area-projection-more.html">cubed maps.</a><br /><br />So the cube (4,3) has a moderate defect of 90°. The icosahedron (3,5) has a smaller (better) defect of 60°. The dodecahedron (5,3) is even better, but the resulting pentagons are harder to subdivide, which we'll want to do. Tetrahedron and octahedron have no advantage to compensate the high defect. <br /><h4>Subdividing and equal area</h4>Even an icosahedron is likely too coarse a gridding on its own, even though the faces are guaranteed to be of equal area. The point of regular faces is that they can themselves be gridded, with squares for the cube, or triangles for the icosahedron. You can't grid a plane with pentagons, which is the problem for the dodecahedron. But it can be triangulated. So the inquiry now is to which solid gives equal area mapping, with an eye on the requirement that the mapping should also permit arbitrary points to be located in mapped cells. <br /><br />There are actually two ways to proceed. The way I used in the cube data structure I posted <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">here</a>, and the way Clive Best divided the icosahedron as linked above, is to successively bisect, in a way shown below, projecting at each stage onto the sphere. In gneral that is the better way; I'll describe some particular issues below. The second is to divide the polygon faces as planes, and then project. That leads to more unequal areas, but it can be remapped. The second method simplifies a basic requirement, that an arbitrary point on the surface can have its enclosing cell identified (providing the remapping can be inverted). <br /><h4>A special case - the cylinder</h4>Familiar flat earth projections are actually projections onto the cylinder. You can, at a stretch, regard this as a degenerate Platonic solid, in which the faces are two-sided polygons, ie lines (longitude). The defect is 360°. The aim is to map onto a finite sheet of paper, so some mapping is always subsequently applied to the y-axis (y=tan(θ), latitude). The famous one is <a href="https://en.wikipedia.org/wiki/Mercator_projection#Derivation_of_the_Mercator_projection">Mercator's</a>; here the y axis is mapped by y=log(tan(π/4+θ/2)). This makes the mapping conformal - locally each feature is in proportion. But the area ratio varies greatly, so Greenland is huge. An <a href="https://en.wikipedia.org/wiki/Cylindrical_equal-area_projection">equal area map</a> has y=sin(θ); this minimises distortion (of which there is much) near the equator. You can scale to move this minimum zone to convenient latitudes. <br /><br />I've actually written these as modified mappings of θ, but you can regard them as projections and then re-mappings. I've introduced the idea because it can be applied to mappings onto solids too, also to improve area ratio. It's a more natural way of looking at it there, because the projection is already a reasonable mapping. I'll come back to this. <br /><h4>First subdivision step</h4>I'm now thinking about cubes, dodecahedra and icosahedra. The following image shows how the bisection process described is initiated for the three solids one might consider: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/09/plato3.png" /><br /><br />The subdivision then proceeds by dividing triangles as shown for the icosahedron, or squares as shown for the cube. A key thing now is that while the subdivisions are equal area on the plane surface, they generally aren't as mapped onto the sphere. In the case of the icosahedron, in the first subdivision, the centre triangle projects to an area of 0.180 on the unit sphere, while the others are 0.149. Further subdivision creates further inequality, although the first step does about half the eventual damage. <br /><br />However, the cube and dodecahedron have an interesting feature which, I think, more than compensates for the reduced number of sides. In the first subdivision step, the new polygons are assured equal area by symmetry. So at that stage the square has 24 equal components, and the dodec has sixty. I'll list the pros and cons of each: <br /><ul><li> Icosahedron - starts with 20 equilateral cells. When first divided, the cells are still equilateral, but unequal area on the sphere, ratio about 6:5. </li><li> Cube - after first subdivision, 24 "squares" of equal area, although the nodes are not quite coplanar. Squares are better than triangles because they are more compact. That means that if you have a number of points within a cell of given area, they are generally closer to each other, which is what gridding is trying to ensure. It also means that a cell of given area sits closer to the sphere surface, so there is less distortion on projecting. </li><li> Dodecahedron - after first subdivision, there are 60 cells of equal area. They aren't quite equilateral; they are isosceles with angles on the surface of 72,60,60° ( they don't have to add to 180, and in fact the excess is the area). Thereafter subdivision proceeds much as for the icosahedron. </li></ul>On this basis, I think the dodec wins, followed by cube. However, next comes the possibility of re-mapping. <br /><h4>Re-mapping</h4>I have spent an inordinate amount of time on this in recent weeks. You can re-map after the bisection as above, but it might as well be done subdividing the plane surface, and then projecting at the end. The idea is that you divide, then develop a parametric scheme for re-mapping to equalise area. I use a Newton-Raphson (actually really secant) to do this. For the cube array, as I stored data and functions linked <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">here</a>, I used a polynomial form that preserved the required symmetries. For the icosahedron I reasoned thus. You can take just one face, and identify six axes of symmetry, dividing into six equal sections. What you do to the nodes in just one of these determines what happens everywhere, and so solving the permitted perturbations that nearly equalise area (least squares, basically Newton-Raphson), gives the required mapping. <br /><br />There is still the pesky issue of how to put a point on the sphere into its cell. With the polynomial mapping, that can be inverted, usually also requiring Newton-Raphson. But there is another, more general method. <br /><br />The more elementary question is, how do you locate a point in a regular grid. Say, (37.3, 124.4) in a 2x2 grid. It's just a matter of truncating each to the nearest below even integer. Or to the nearest odd integer, if you mark the cell by its mid-point. It's easier to think about it if you rescale to a unit grid. Triangles seem harder, but you can do the same in homogeneous coordinates. Simplest is to express everything in the homogeneous coords of one base cell. Then omitting the fractional parts tells you which cell it is in. <br /><br />If the grid is re-mapped, you can proceed iteratively. Choose a base cell, and locate the point relative to that. Then re-do, with that cell as base. If it is still in, we have it, otherwise repeat going to the revised cell as base. Because the re-mappings are relatively minor changes, this converges quickly. <br /><h4>Next</h4>This was all meant to be preparatory to posting about the icosahedron in the same way I did for cube <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">here</a>, with data structures for six or so stages of bisection. Then I realised that dodecahedra actually looked better, and even cubes. I'll probably still post the icosahedron data, and maybe dodec. <br /><br />I should add that, while one can seek to optimise equal area mapping, in practice any of these solids will be adequate for surface temperature averaging.<br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/09/grids-platonic-solids-and-surface.htmlnoreply@blogger.com (Nick Stokes)14tag:blogger.com,1999:blog-7729093380675162051.post-8287724631798237452Fri, 08 Sep 2017 01:01:00 +00002017-09-08T11:01:15.498+10:00August global surface temperature down 0.013°C<a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">TempLS mesh</a> anomaly (1961-90 base) was down from 0.69°C in July to 0.677°C in August. This very small drop compares with the <a href="https://moyhu.blogspot.com/2017/09/august-ncepncar-global-anomaly-up-0038.html"> small rise</a> of 0.038°C in the NCEP/NCAR index, and a <a href="http://www.drroyspencer.com/2017/09/uah-global-temperature-update-for-august-2017-0-41-deg-c/">bigger rise</a> (0.12) in the UAH LT satellite index. The August value is less than August 2015 or 2016, but higher than 2014. <br /><br />There was a moderate fall in Antarctica, which as usual affects TempLS mesh and GISS more than others. I'd expect NOAA and HADCRUT to show increases for August. Regionally, the Old World was mostly warm; US was cold cental and East, but N Canada was warm. S America mostly warm (still awaiting a few countries). : <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/map.png" /><br /><a name='more'></a><div style="color: #aa0000;">This post is part of a series that has now run for six years. The TempLS mesh data is reported <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">here</a>, and the recent history of monthly readings is <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#L1">here</a>. Unadjusted GHCN is normally used, but if you click the TempLS button there, it will show data with adjusted, and also with different integration methods. There is an interactive graph using 1981-2010 base period <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#Drag">here</a> which you can use to show different periods, or compare with other indices. There is a general guide to TempLS <a href="https://moyhu.blogspot.com.au/p/a-guide-to-global-temperature-program.html">here</a>. <br /><br />The reporting cycle starts with a report of the daily reanalysis index on about the 4th of the month. The next post is this, the TempLS report, usually about the 8th. Then when the GISS result comes out, usually about the 15th, I discuss it and compare with TempLS. The TempLS graph uses a spherical harmonics to the TempLS mesh residuals; the residuals are displayed more directly using a triangular grid in a better resolved WebGL plot <a href="http://www.moyhu.blogspot.com.au/p/blog-page_24.html">here</a>. </div><br /><br /><br />http://moyhu.blogspot.com/2017/09/august-global-surface-temperature-down.htmlnoreply@blogger.com (Nick Stokes)10tag:blogger.com,1999:blog-7729093380675162051.post-2344507013242072154Wed, 06 Sep 2017 19:04:00 +00002017-09-07T05:04:19.568+10:00 August NCEP/NCAR global anomaly up 0.038°CIn the <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">Moyhu NCEP/NCAR index</a>, the monthly reanalysis average rose from 0.299°C in July to 0.337°C in August, 2017. The results were late this month; for a few days NCEP/NCAR was not posting new results. It was a very up and down month; a dip at at the start, then quite a long warm period, and then a steep dip at the end. Now that a few days in September are also available, there is some recovery from that late dip. August 2017 was a bit cooler than Aug 2016, but warmer than 2015. <br /><br />It was cool in Eastern US, but warm in the west and further north. Cool in Atlantic Europe, but warm further east. Mostly cool in Antarctica. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/data/freq/days.png" /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/09/august-ncepncar-global-anomaly-up-0038.htmlnoreply@blogger.com (Nick Stokes)0tag:blogger.com,1999:blog-7729093380675162051.post-688881876758148447Wed, 30 Aug 2017 02:51:00 +00002017-08-30T12:52:54.464+10:00Gulf SST - warm before Harvey, cool after.I maintain <a href="http://www.moyhu.blogspot.com.au/p/blog-page.html">a page </a> showing high resolution (1/4degree) AVHRR SST data from NOAA - in detail: <i>"NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/"</i>. It renders it in WebGL and goes back with daily maps for a decade or so, then les frequently. It shows anomalies relative to 1971-2000. I have been tracking the effect of Hurricane Harvey. It was said to have grown rapidly because of warm Gulf waters; they were warm, but not exceptionally, as this extract from 15 August shows: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/aug15.png" /><br /><br />It remained much the same to 24th August, when Harvey grew rapidly, and gained Hurricane status late in the day (). But by 25th, there is some sign of cooling. 26th (not shown) was about the same. But by 27th, There was marked cooling, and by 28th more so. The cooling seems to show up rather belatedly alomg the path of the hurricane. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/harvey.png" /><br /><br />Here's is the latest day at higher resolution: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/aug28a.png" /><br /><br /><a href="https://moyhu.blogspot.com.au/2013/02/hurricanes-and-sst-movies.html">A few years ago</a>, I developed a set of movies based on recent hurricanes of the time, showing their locations and SST at the time. Some showed a big effect, some not so much. Harvey was interesting in that it covered a fairly confined area of ocean, and moved slowly. <br /><br /><br /><br />http://moyhu.blogspot.com/2017/08/gulf-sst-warm-before-harvey-cool-after.htmlnoreply@blogger.com (Nick Stokes)22tag:blogger.com,1999:blog-7729093380675162051.post-1011102749937955409Thu, 24 Aug 2017 09:41:00 +00002017-08-26T15:45:29.461+10:00Surface temperature sparsity error modesThis post follows last week's on <a href="http://moyhu.blogspot.com/2017/08/temperature-averaging-and-integrtaion.html">temperature integration methods</a>. I described a general method of regression fitting of classes of integrable functions, of which the most used to date is <a href="https://moyhu.blogspot.com.au/2015/09/spherical-harmonics.html">spherical harmonics (SH)</a>. I noted that the regression involved inverting a matrix HH consisting of all the scalar product integrals of the functions in the class. With perfect integration this matrix would be a unit matrix, but as the SH functions become more oscillatory, the integration method loses resolution, and the representation degrades with the condition number of the matrix HH. The condition number is the ratio of largest eigenvalue to smallest, so what is happening is that some eigenvectors become small, and the matrix is near singular. That means that the corresponding eigenvector might have a large multiplier in the representation. <br><br>I also use fitted SH for plotting each month's temperature. I described some of the practicalities <a href="https://moyhu.blogspot.com.au/2010/04/ver-2-regional-spatial-variation.html">here</a> (using different functions). Increasing the number of functions improves resolution, but when HH becomes too ill-conditioned, artefacts intrude, which are multiples of these near null eigenvectors. <br><br>In the previous post, I discussed how the condition of HH depends on the scalar product integral. Since the SH are ideally orthogonal, better integration improves HH. I have been taking advantage of that in recent TempLS to increase the order of SH to 16, which implies 289 functions, using mesh integration. That might be overdoing it - I'm checking. <br><br>In this post, I will display those troublesome eigen modes. They are of interest because they are associated with regions of sparse coverage, and give a quantification of how much they matter. Another thing quantified is how much the integration method affects the condition number for a given order of SH. I'll develop that further in another post. <br><br>I took N=12 (169 functions), and looked at TempLS stations (GHCN+ERSST) which reported in May 2017. Considerations on choice of N are that if too low, the condition number is good, and the minimum modes don't show features emphasising sparsity. If the number is too high, each region like Antarctica can have several split modes, which confuses the issue. <br><br>The integration methods I chose were mostly described <a href="https://moyhu.blogspot.com.au/2015/10/new-integration-methods-for-global.html">here</a><ul><li>OLS - just the ordinary scalar product of the values <li>grid - integration by summing on a 5x5° latitude/longitude grid. This was the earliest TempLS method, and is used by HADCRUT. <li>infill - empty cells are infilled with an average of nearby values. Now the grid is a <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">cubed sphere</a> with 1536 cells <li>mesh - my generally preferred method using an irregular triangular grid (complex hull of stations) with linear interpolation. </ul>OLS sounds bad, but works quite well at moderate resolution, and was used in TempLS until very recently. <br><br>I'll show the plots of the modes as an active lat/lon plot below, and then the OLS versions in WebGL, which gives a much better idea of the shapes. But first I'll show a table of the tapering eigenvalues, numbering from smallest up. They are scaled so that the maximum is 1, so reciprocal of the lowest is the condition number. <br><table style="color:blue"><tr><td width=80> <td width=80> OLS<td width=80> grid<td width=80> infilled<td width=80> mesh <tr><td>Eigen1<td>0.0211<td>0.0147<td>0.0695<td>0.135 <tr><td>Eigen2<td>0.0369<td>0.0275<td>0.138<td>0.229 <tr><td>Eigen3<td>0.0423<td>0.0469<td>0.212<td>0.283 <tr><td>Eigen4<td>0.0572<td>0.0499<td>0.244<td>0.329 <tr><td>Eigen5<td>0.084<td>0.089<td>0.248<td>0.461 <tr><td>Eigen6<td>0.104<td>0.107<td>0.373<td>0.535 <tr><td>Eigen7<td>0.108<td>0.146<td>0.406<td>0.571 <tr><td>Eigen8<td>0.124<td>0.164<td>0.429<td>0.619 </table><div>And here is a graph of the whole sequence, now largest first: <br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/eigens.png"></img><br><br>The hierarchy of condition numbers is interesting. I had expected that it would go in the order of the columns, and so it does until near the end. Then mesh drops below infilled grid, and OLS below grid, for the smallest eigenvalues. I think what determines this is the weighting of the nodes in the sparse areas. For grid, this is not high, because each just gets the area of its cell. For both infilled and mesh, the weight rises with the area, and apparently with infilled, more so. <br><br><a name='more'></a><br>Here is an active graph to show the errant modes. You can cycle through "Style", which means style of integration (grid, mesh etc) and mode, starting from 1 (buttons top right). <br><br><div id="Yxe1" style="position:absolute"></div><div style="height:550px;"></div><br></div><br>It's dominated by Antarctica; the lowest modes focus, with some Arctic activity too, and it isn't for a while that modes bob up in Africa, with some effect in S America. The weakest style (OLS) is almost all polar in the first 9 modes, while mesh starts showing Africa from about mode 4 up, and later Brazil shows up. <br><br>Here is the WebGL plot - I show just the mesh style. It gives a better proportion for the polar behaviour, and shows finer features elsewhere. It is the usual trackball, with radio buttons for the modes. Dots are the stations. <br><br><div id="PxBody" /></div><!--This is a generic world WebGL plotting facility, coded in JS by Nick Stokes, 11 Jan 2014.--><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyJSlib.js"></script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/Map.js"></script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/eigen.js"></script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyGLV2.1.js"></script></html><br><br>Next post will take this further. I'll do a more systematic look at which styles work best in which circumstances. The next main interest is whether I can get better resolution by restricting to a space without the problem nodes. In principle, one could take a very large collection of SH, and collect the eigenfunctions, which are truly orthogonal with respect to the integration style. A subset with moderate eigenvectors would still have a large orthogonal basis. <br><br><script type="text/javascript">function YxInit(){ var G={} MoyJSlib(G) //eval(G.var) var cr=G.cr var i,j,k,px,q,r,s,t,P,x=[0,0],dir="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/"; function C(event){ var i,m,n,t,y; t=event.target;y=[4,7]; m=t.m; n=m[0] x[n]+=m[1]+y[n]; x[n]=x[n]%y[n]; s=dir+"wm/wm"+["O","g","i","m"][x[0]]+(x[1]+1)+".png" px.src=s } P=document.getElementById("Yxe1") P.style="position:absolute;height:400px" px=cr(P,"img") px.src=dir+"wm/wmO1.png" px.style="width:800px;height:500px; border: 2px solid red" q=cr(P,"table") q.style="position:absolute;left:680px;top:0px;border: 2px solid blue" for(i=0;i<2;i++){r=cr(q,"tr"); for(j=0;j<3;j++){ s=cr(q,"td"); if(j==1){t=cr(s,"span");G.iH(t,["Style","Mode"][i])}else{ t=cr(s,"button");t.innerHTML=["<","",">"][j];t.m=[i,j-1]; t.onclick=C } } } } YxInit() </script><br>http://moyhu.blogspot.com/2017/08/surface-temperature-sparsity-error-modes.htmlnoreply@blogger.com (Nick Stokes)9tag:blogger.com,1999:blog-7729093380675162051.post-7937887797018812282Thu, 17 Aug 2017 07:39:00 +00002017-08-17T17:49:10.256+10:00Temperature averaging and integration - the basics <meta charset="UTF-8">I write a lot about spatial integration, which is at the heart of global temperature averaging. I'll write more here about the principles involved. But I'm also writing to place in context methods I use in TempLS, which I think are an advance on what is currently usual. I last wrote a comparison of methods <a href="https://moyhu.blogspot.com.au/2015/10/new-integration-methods-for-global.html">in 2015 here</a>, which I plan to update in a sequel. Some issues here arose in a <a href="https://climateaudit.org/2017/05/18/how-dependent-are-gistemp-trends-on-the-gridding-radius-used/#comment-772475">discussion </a> at Climate Audit. <br><br>It's a long post, so I'll include a table of contents. I want to start from first principles and make some math connections. I'll use paler colors for the bits that are more mathy or that are just to make the logic connect, but which could be skipped. <br><ul><li> <a href="#I1">Basics - averaging temperature and integration</a><li> <a href="#I2">Averaging and integration.</a><li> <a href="#I3">Integration - theory</a><li> <a href="#I4">Numerical integration - sampling and cells</a><li> <a href="#I5">Triangular mesh</a><li> <a href="#I6">Integration by function fitting</a><li> <a href="#I7">Fitting - weighted least squares</a><li> <a href="#I8">Basis functions - grid</a><li> <a href="#I9">Basis functions - mesh</a><li> <a href="#I10">Basis functions - spherical harmonics</a><li> <a href="#I11">Basis functions - radial basis functions</a><li> <a href="#I12">Basis functions - higher order finite element</a><li> <a href="#I13">Next in the series.</a></ul><a name='more'></a><h4 id="I1">Basics - averaging temperature.</h4>Some major scientific organisations track the global temperature monthly, eg <a href="https://data.giss.nasa.gov/gistemp/">GISS</a>, <a href="https://www.ncdc.noaa.gov/climate-information/analyses/monthly-global-climate-reports">NOAA NCEI</a>, <a href="http://www.metoffice.gov.uk/hadobs/hadcrut4/">HADCRUT 4</a>, <a href="http://berkeleyearth.org/land-and-ocean-data/">BEST</a>. What they actually publish is global temperature average anomaly, made up of land station measurements and sea surface temperatures. I write often about anomalies, eg <a href="https://moyhu.blogspot.com.au/2017/01/global-anomaly-spatial-sampling-error.html">here</a>. They are data associated with locations, formed by subtracting an expected value for the time of year. Anomalies are averaged; it isn't an anomaly of an average - a vital distinction. Anomalies must be created before averaging. <br><br>I also do this, with <a href="https://moyhu.blogspot.com.au/p/a-guide-to-global-temperature-program.html">TempLS</a>. Global average anomaly uses two calculations - an average over time, to get normals, and an average of anomalies (using normals) over space (globe). It's important that the normals don't get confounded with global change; this is often handled by restricting the anomaly to a common base period, but TempLS uses an iterative process. <h4 id="I2">Averaging and integration.</h4>People think of averaging as just adding N numbers and dividing by N. I often talk of weighted averaging - Σ wₖxₖ / Σ wₖ (x data, w weights) Dividing by the weights ensures that the average of 1 is 1; a more general way of implementing is just to divide by trhe result of applying what you did to x to 1. <br><br>But the point of averaging is usually not to get just the average of the numbers you fed in, but an estimate of some population mean, using those numbers as a sample. So averaging some stations over Earth is intended to give the mean for Earth as a whole, including the points where you didn't measure. So you'd hope that if you chose a different set of points, you'd still get much the same answer. This needs some care. Just averaging a set of stations is usually not good enough. You need some kind of area weighting to even out representation. For example, most data has many samples in USA, and few in Africa. But you don't want the result to be just USA. Area-weighted averaging is better thought of as integration. <br><br>Let me give an example. Suppose you have a big pile of sand on a hard level surface, and you want to know how much you have - ie volume. Suppose you know the area - perhaps it is in a kind of sandpit. You have a graduated probe so that you can work out the depth at points. If you can work out the average depth, you would multiply that by the area to get the volume. Suppose you have a number of sample depths, perhaps at scattered locations, and you want to calculate the average. <br><br>The average depth you want is volume/area. That is what the calculation should reveal. The process to get it is numerical integration, which I'll describe. <h4 id="I3">Integration - theory</h4> <table style="color:#cc66aa"><tr><td>A rigorous approach to integration by <a href="https://en.wikipedia.org/wiki/Riemann_integral">Riemann</a> in about 1854 is often cited. He showed that if you approximated an integral on a line segment by dividing it into ever smaller segments, not necessarily of equal size, and estimated the function on each by a value within, then the sum of those contributions would tend to a unique limit, which is the integral. Here is a picture from Wiki that I usd to illustrate. <br><br> The idea of subdividing works in higher dimensions as well, basically because integration is additive. And the key thing is that there is that unique limit. Numerically, the task is to get as close to it as possible given a finite number of points. Riemann didn't have to worry about refined estimation, because he could postulatie an infinite process. But in practice, it pays to use a better estimate of the integral in each segment.</td><td><img src="https://upload.wikimedia.org/wikipedia/commons/c/cd/Riemann_integral_irregular.gif" style="width:300px"></img></td></tr></table><br><br> <div style="color:#cc66aa">Riemann dealt with analytic functions, that can be calculated at any prescribed point. With a field variable like temperature, we don't have that We have just a finite number of measurements. But the method is the same. The region should be divided into small areas, and an integral estimated on each using the data in it or nearby. </div><br>The key takeaway from Riemann theory is that there is a unique limit, no matter how the region is divided. That is the integral we seek, or the volume, in the case of the sand pile. <h4 id="I4">Numerical integration - sampling and cells</h4><div style="color:#aa4444">The idea of integation far predates Riemann; it was originally seen as "antiderivative". A basic and early family of formulae is called Newton-Cotes. Formulae like these use derivatives (perhaps implied) to improve the accuracy of integration within sub-intervals. "Quadrature points" can be used to improve accuracy (Gauss quadrature). But there is an alternative view more appropriate to empirical data like temperature anomaly which is of inferring from sample values. This acknowledges that you can't fully control the points where function values are known. </div><br>So one style of numerical integration, in the Riemann spirit, is to divide the region into small areas, and estimate the integral of each as that area times the average of the sample values within. This is the method used by most of the majors. They use a regular grid in latitude/longitude, and average the temperatures measured within. The cells aren't equal area because of the way longitudes gather toward the poles; that has to be allowed for. There is of course a problem when cells don't have any measures at all. They are normally just omitted, with consequences that I will describe. GISS uses a variant of this with cells of larger longitude range near the poles. For TempLS, it is what I call the <a href="https://moyhu.blogspot.com.au/2015/09/better-gridding-for-global-temperature.html">grid method</a>. It was my sole method for years, and I still publish the result, mainly because it aligns well with NOAA and HADCRUT, also gridded. <br><br>Whenever you average a set of data omitting numbers representing part of the data, you effectively assume that the omitted part has the same behaviour as the average of what you included. You can see this because if you replaced the missing data with that average, you'd get the same result. Then you can decide whether that is really what you believe. I have described <a href="https://moyhu.blogspot.com.au/2014/07/infilling-graphics-version.html">here</a> how it often isn't, and what you should do about it (infilling). Eere that means usually that empty cells should be estimated from some set of nearby stations, possibly already expressed as cell averages. <br><br>This method can be re-expressed as a weighted sum where the weights are just cell area divided by number of datapoints in cell, although it gets more complicated if you also use the data to fill empty cells (the stations used get upweighted). <br><br>While this method is usually implemented with a latitude/longitude grid, that isn't the best, because there is a big variation in size. I have explored better grids based on mapping the sphere onto a gridded Platonic polyhedron - <a href="https://moyhu.blogspot.com.au/2015/09/better-gridding-for-global-temperature.html">cube</a> or <a href="https://moyhu.blogspot.com.au/2017/04/icosahedral-earth.html">icosahedron</a>. But the principle is the same. <h4 id="I6">Triangular mesh</h4> My preferred integration has been by irregular triangular mesh. This <a href="https://moyhu.blogspot.com.au/2017/04/global-60-stations-and-coverage.html">recent post</a> shows various meshes and explores how many nodes are really needed, but the full mesh is what the usual TempLS reports are based on. The msh is formed as a convex hull, which is what you would get if you drew a thin membrane over the measuring points in space. For the resulting triangles, there is a node at each corner, and to the values there you can fit a planar surface. This can be integrated, and the result is the average of the three values times the area of the triangle. When you work out the algebra, the weight of each station reading is the area of all the triangles that it is a corner of. <br><br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/usmesh.png" width=700></img><h4 id="I6">Integration by function fitting</h4> There is a more general way of thinking about integration, with more methods resulting. This is where the data is approximated by a function whose integral is known. The usual way of doing this is by taking a set of basis functions with known integral, and linearly combining them with coefficients that are optimised to fit. <br><br> You can think of gridding as a special case. The functions are just those that take constant value 1 on the designated areas. The combination is a piecewise constant function. <h4 id="I7">Fitting - weighted least squares</h4> <div style="color:#cc66aa"></div>Optimising the coefficients means finding ones that minimise s a sum of squares of some kind of the differences between the fit function and observed - called residuals. In math, that is <br> S = Σ wₖrₖrₖ where residual rₖ = yₖ-Σ bₘfₘ(xₖ)<br>Here yₖ and xₖ are the value and location; fₘ are the basis functions and bₘ the coefficients. Since there are usually a lot more locations than coefficients, this can't be made zero, but can be minimised. The weights w should again, as above, generally be area weights appropriate for integration. The reason is that S i a penalty on the size of residuals, and should generally be uniform, rather than varying with the distribution of sampling points. <br><br>By differentiation,<br>∂S/∂bₙ=0= Σₖ Aₖₙwₖ(yₖ-ΣₘAₖₘbₘ) where Aₖₙ=fₙ(xₖ) <br>or Σₘ AAₘₙ bₘ = Σₖ Aₖₙyₖ, where AAₘₙ = Σₖ AₖₘAₖₙ <br>In matrix notation, this is AA* b = A*y, where AA = A<sup>T</sup>A <br>So AA is symmetric positive definite, and has to be inverted to get the coefficients b. In fact, AA is the matrix of scalar products of the basis functions. This least squares fitting can also be seen as regression. It also goes by the name of a <a href="https://en.wikipedia.org/wiki/Moore%E2%80%93Penrose_pseudoinverse">pseudo-inverse</a>. <br><br> This then determines what sort of basis functions f should be sought. AA ia generally large, so for inversion should be as <a href="https://en.wikipedia.org/wiki/Condition_number">well-conditioned</a> as possible. This means that the rows should be nearly orthogonal. That is normally achieved by making AA as nearly as possible diagonal. Since AA is the matrix of scalar products, that means that the basis functions should be as best possibe orthogonal relative to the weights w. If these are appropriate for integration, that means that a good choice of functions f will be analytic functions orthogonal on the sphere. <br><br> I'll now review my various kinds of integration in terms of that orthogonality <br><br> <h4 id="I8">Basis functions - grid.</h4> As said above, the grid basis functions are just functions that are 1 on their cells, and zero elsewhere. "Cell" could be a more complex region, like combinations of cells. They are guaranteed to be orthogonal, since they don't overlap. And within each cell, the simple average crates no further interaction. So AA is actually diagonal, and inversion is trivial. <br><br>That sounds ideal, but the problem is that the basis is discontinuous, whereas there is every reason to expect the temperature anomaly to be continuous. You can see how this is a problem, along with missing areas, in this NOAA map for May 2017: <br><br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/noaamap.gif" width=700></img><br><br>I described <a href="https://moyhu.blogspot.com.au/2015/09/better-gridding-for-global-temperature.html">here</a> a method for overcoming the empty cell problem by systematically assigning neighboring values. You can regard this also as just carving up those areas and adjoining them to cells that do have data, so it isn't really a different method in principle. The paper of <a href="https://moyhu.blogspot.com.au/2013/11/cowton-and-way-trends.html">Cowtan and Way</a> showed another way of overcoming the empty cell problem. <br><br><h4 id="I9">Basis functions - mesh.</h4> In this case, the sum squares minimisation is not needed, since there are exactly as many basis functions as data, and the coefficients are just the data values. This is standard finite element method. The basis functions are continuous - pyramids each with unit peak at a data point, sloping to zero on the opposite edges of the adjacent triangles. This combination of a continuous approximation with a diagonal AA is why I prefer this method, although the requirement to calculate the mesh is a cost. You can see the resulting visualisation at <a href="https://moyhu.blogspot.com.au/p/blog-page_24.html">this WebGL page.</a> It shows a map of the mesh approximation to TempLS residuals, for each month back to 1900. <h4 id="I10">Basis functions - spherical harmonics.</h4> This has been my next favorite method. Spherical harmonics (SH) are described <a href="https://moyhu.blogspot.com.au/2015/09/spherical-harmonics.html">here</a>, and visoalised <a href="https://moyhu.blogspot.com.au/2017/04/spherical-harmonics-movie.html">here</a>. I compared the various methods in a post <a href="https://moyhu.blogspot.com.au/2015/10/new-integration-methods-for-global.html">here</a>. At that stage I was using uniform weights w, and even then the method performed quite well. But it can be improved by using weights for any integration method. Even simple grid weights work very well. After fitting, the next requirement is to integrate the basis functions. In this case it is simple; they are all zero except for the first, which is a constant function. So for the integral, all you need is the first coefficient. <br><br>I also use the fit to display the temperature field with each temperature report. SH are smooth, and so is the fit. Here is a recent example: <br><br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/map.png" width=700></img><br><br>This is the first case where we have a non-trivial AA to invert. I tend to use 100-200 functions, so the matrix can be inverted directly, and it is very quick in R. For a bigger matrix I would use a conjugate gradient method, which would be fine while the matrix is well-conditioned. <br><br>But it is conditioning that is the limitation. The basis functions have undulations, and at higher order, these start to be inadequately resolved by the spacing of data points. That means that, in terms of the approximate integration, they are no longer nearly orthogonal. Eventually AA becomes nearly singular, which means that there are combinations that are not penalised by the minimising of S. These grow and produce artefacts. SH have two integer parameters l and m, which roughly determine how many periods of oscillation you have in latitude and longitude. With uniform w, I can allow l up to 12, which means 169 functions. Recently with w right for grid integration, I can go to 16, with 289 functions, but with some indication of artefacts. I'll post a detailed analysis of this soon. Generally the artefacts are composed of higher order SH, so don't have much effcet on the integral. <h4 id="I11">Basis functions - radial basis functions.</h4> This is a new development. RBFs are functions that are radially symmetric about a centre point, and fade from 1 toward 0 over a distance scaled to the spacing of data. They are smooth - I am using gaussians (normal distribution), although it is more common to use functions that go to zero in finite distance. The idea is that these functions are close to orthogonal if spaced so only their tails overlap. <br><br>It's not necessarily much different to SH; the attraction is that there is flexibility to increase the spread of the functions where data is scarce. <h4 id="I12">Basis functions - higher order finite element.</h4> I used above the simplest element - linear triangles. There is a whole hierarchy that you can use with extra parameters, for example, quadratic functions on triangles with mid-side nodes. In these implementations, the mesh nodes will no longer be data nodes, so there is non-trivial fitting within elements. It is more complicated than, say, RBF. However there is a powerful technique in FEM called hp-fitting. Here the order of polynomial fit functions is increased as the size of the elements decreases. In theory there is a very rapid gain in accuracy, limited by discontinuity at boundaries, which we don't have. However, there is the problem of having to fit the polynomials within each element, so I don't know how that will work out. <h4 id="I13">Next in the series.</h4> I've tried to lay out the theory in this post for reference. I'll do a post on more detail of spherical harmonics, calculating condition numbers and trying to optimise the weights and number of bases. I'll also do an updated post on a comparison of the methods. If RBFs really prove successful, I'll write about that too. <br><br><br><br><br><br> http://moyhu.blogspot.com/2017/08/temperature-averaging-and-integrtaion.htmlnoreply@blogger.com (Nick Stokes)9tag:blogger.com,1999:blog-7729093380675162051.post-125742408866516937Tue, 15 Aug 2017 18:10:00 +00002017-08-16T04:10:00.639+10:00GISS July up 0.15°C from June.GISS was up from 0.68°C in June to 0.83°C in July. It was the warmest July in the record, though the <a href="https://data.giss.nasa.gov/gistemp/news/20170815/">GISS report</a> says it "statistically tied" with 2016 (0.82). The increase was similar to the <a href="http://moyhu.blogspot.com/2017/07/june-global-surface-temperature-down-012.html">0.12°C rise in TempLS</a>. <br /><br />The overall pattern was similar to that in TempLS. Warm almost everywhere, with a big band across mid-latitude Eurasia and N Africa. Cool in parts of the Arctic, which may save some ice. <br /><br />I'll show the plot of recent months on the same 1981-2010 base, mainly because they are currently unusually unanimous. The group HADCRUT/NOAA/TempLS_grid tend to be less sensitive to the Antarctic variations that have dominated recent months, and I'd expect them to be not much changed in July also, which would leave them also in much the same place. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/sixmo.png" width="600" /><br /><br />Recently, August reanalysis has been <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">unusually warm</a>. As usual here, I will compare the GISS and previous TempLS plots below the jump. <br /><a name='more'></a><br />Here is GISS<br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/GISSjul.png" /><br /><br />And here is the TempLS spherical harmonics plot <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/map.png" /><br /><br /><div style="color: #aa0000;">This post is part of a series that has now run for six years. The TempLS mesh data is reported <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">here</a>, and the recent history of monthly readings is <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#L1">here</a>. Unadjusted GHCN is normally used, but if you click the TempLS button there, it will show data with adjusted, and also with different integration methods. There is an interactive graph using 1981-2010 base period <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#Drag">here</a> which you can use to show different periods, or compare with other indices. There is a general guide to TempLS <a href="https://moyhu.blogspot.com.au/p/a-guide-to-global-temperature-program.html">here</a>. <br /><br />The reporting cycle starts with a report of the daily reanalysis index on about the 4th of the month. The next post is this, the TempLS report, usually about the 8th. Then when the GISS result comes out, usually about the 15th, I discuss it and compare with TempLS. The TempLS graph uses a spherical harmonics to the TempLS mesh residuals; the residuals are displayed more directly using a triangular grid in a better resolved WebGL plot <a href="http://www.moyhu.blogspot.com.au/p/blog-page_24.html">here</a>. </div><br /><br /><br /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/08/giss-july-up-015-from-june.htmlnoreply@blogger.com (Nick Stokes)6tag:blogger.com,1999:blog-7729093380675162051.post-7958179108676220696Mon, 07 Aug 2017 18:04:00 +00002017-08-08T16:30:29.786+10:00July global surface temperature up 0.11°C<a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">TempLS mesh</a> anomaly (1961-90 base) was up from 0.568°C in June to 0.679°C in July. This follows the <a href="http://moyhu.blogspot.com/2017/08/july-ncepncar-up-0058.html"> smaller rise</a> of 0.06°C in the NCEP/NCAR index, and a <a href="http://www.drroyspencer.com/2017/08/uah-global-temperature-update-for-july-2017-0-28-deg-c/">similar rise</a> (0.07) in the UAH LT satellite index. The July value is just a whisker short of July 2016, which was a record warm month. With results for Mexico and Peru still to come, that could change.. <br /><br />Again the dominant change was in Antarctica, from very cold in June to just above average in July. On this basis, I'd expect GISS to also rise; NOAA and HADCRUT not so much. Otherwise as with the <a href="http://moyhu.blogspot.com/2017/08/july-ncepncar-up-0058.html"> reanalysis</a>, Middle East and around Mongolia were warm, also Australia and Western USA. Nowhere very hot or cold. Here is the map:<br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/08/map.png" /><br /><br /><a name='more'></a><div style="color: #aa0000;">This post is part of a series that has now run for six years. The TempLS mesh data is reported <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">here</a>, and the recent history of monthly readings is <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#L1">here</a>. Unadjusted GHCN is normally used, but if you click the TempLS button there, it will show data with adjusted, and also with different integration methods. There is an interactive graph using 1981-2010 base period <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#Drag">here</a> which you can use to show different periods, or compare with other indices. There is a general guide to TempLS <a href="https://moyhu.blogspot.com.au/p/a-guide-to-global-temperature-program.html">here</a>. <br /><br />The reporting cycle starts with a report of the daily reanalysis index on about the 4th of the month. The next post is this, the TempLS report, usually about the 8th. Then when the GISS result comes out, usually about the 15th, I discuss it and compare with TempLS. The TempLS graph uses a spherical harmonics to the TempLS mesh residuals; the residuals are displayed more directly using a triangular grid in a better resolved WebGL plot <a href="http://www.moyhu.blogspot.com.au/p/blog-page_24.html">here</a>. </div><br /><br />http://moyhu.blogspot.com/2017/08/july-global-surface-temperature-up-011.htmlnoreply@blogger.com (Nick Stokes)3tag:blogger.com,1999:blog-7729093380675162051.post-6699168728897782550Wed, 02 Aug 2017 18:40:00 +00002017-08-03T04:40:13.259+10:00July NCEP/NCAR up 0.058°CIn the <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">Moyhu NCEP/NCAR index</a>, the monthly reanalysis average rose from 0.241°C in June to 0.299°C in July, 2017. This is lower than July 2016 but considerably higher than July 2015. The interesting point was a sudden rise on about July 24, which is responsible for all the increase since June. It may be tapering off now. <br /><br />It was generally warm in temperate Asia and the Middle East, and even Australia. Antarctica was mixed, not as cold as June. The Arctic has been fairly cool. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/data/freq/days.png" /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/08/july-ncepncar-up-0058.htmlnoreply@blogger.com (Nick Stokes)0tag:blogger.com,1999:blog-7729093380675162051.post-2643262054063327908Fri, 21 Jul 2017 16:32:00 +00002017-07-22T02:32:27.690+10:00NOAA's new ERSST V5 Sea surface temperature and TempLSThe paper describing the new version V5 of ERSST has been <a href="https://doi.org/10.1175/JCLI-D-16-0836.1">published</a> in the Journal of Climate. The data is posted, and there is a NOAA descriptive page <a href="https://www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst-v5">here</a>. From the abstract of the (paywalled) paper, by Huang et al: <br /><blockquote>This update incorporates a new release of ICOADS R3.0, a decade of near-surface data from Argo floats, and a new estimate of centennial sea-ice from HadISST2. A number of choices in aspects of quality control, bias adjustment and interpolation have been substantively revised. The resulting ERSST estimates have more realistic spatio-temporal variations, better representation of high latitude SSTs, and ship SST biases are now calculated relative to more accurate buoy measurements, while the global long-term trend remains about the same. </blockquote>A lot of people have asked about including ARGO data, but it may be less significant than it seems. ARGO floats only come to the surface once every ten days, while the more numerous drifter buoys are returning data all the time. There was a clamor for the biases to be calculated relative to the more accurate buoys, but as I frequently argued, as a matter of simple arithmetic it makes absolutely no difference to the anomaly result. And sure enough, they report that it just reduces all readings by 0.077°C. That can't affect trends, spatial patterns etc. <br /><br />The new data was not used for the June NOAA global index, nor for any other indices that I know of. But I'm sure it will be soon. So I have downloaded it and tried it out in TempLS. I have incorporated it in place of the old V3b. So how much difference does it make? The abstract says<br /><blockquote>Furthermore, high latitude SSTs are decreased by 0.1°–0.2°C by using sea-ice concentration from HadISST2 over HadISST1. Changes arising from remaining innovations are mostly important at small space and time scales, primarily having an impact where and when input observations are sparse. Cross-validations and verifications with independent modern observations show that the updates incorporated in ERSSTv5 have improved the representation of spatial variability over the global oceans, the magnitude of El Niño and La Niña events, and the decadal nature of SST changes over 1930s–40s when observation instruments changed rapidly. Both long (1900–2015) and short (2000–2015) term SST trends in ERSSTv5 remain significant as in ERSSTv4. </blockquote>The sea ice difference may matter most - this is a long standing problem area in incorporating SST in global measures. On the <a href="https://www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst-v5">NOAA page</a>, they show a comparison graph: <br /><br /><img src="https://www.ncdc.noaa.gov/sites/default/files/styles/716px_width/public/Globally_annually_avg_SSTA_0.jpg?itok=c855stMq" /><br /><br />There are no obvious systematic trend differences. The most noticeable change is around WWII, which is a bit of a black spot for SST data. A marked and often suspected peak around 1944 has diminished, with a deeper dip around 1942. <br /><br />TempLS would be expected to reflect this, since most of its data is SST. Here is the corresponding series for TempLS mesh plotted: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/v4v5.png" /><br /><br />Global trends (in °C/century) are barely affected. Reduced slightly in recent decades, increased slightly since 1900:<br /><br /><table><tbody><tr><td>start year</td><td>end year</td><td>TempLS with V4</td><td>TempLS with V5 </td></tr><tr><td>1900</td><td>2016</td><td>0.769</td><td>0.791 </td></tr><tr><td>1940</td><td>2016</td><td>0.978</td><td>0.974 </td></tr><tr><td>1960</td><td>2016</td><td>1.489</td><td>1.465 </td></tr><tr><td>1980</td><td>2016</td><td>1.631</td><td>1.607 </td></tr></tbody></table><br />Almost identical behaviour is seen with TempLS grid. <br /><br /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/07/noaas-new-ersst-v5-sea-surface.htmlnoreply@blogger.com (Nick Stokes)17tag:blogger.com,1999:blog-7729093380675162051.post-2298513683238497716Thu, 20 Jul 2017 02:29:00 +00002017-07-20T19:59:00.135+10:00NOAA global surface temperature down just 0.01°CDown from 0.83°C in May to 0.82C in June (report <a href="https://www.ncdc.noaa.gov/sotc/global/201706">here</a>). I don't normally post separately about NOAA, but here I think the striking difference from GISS/TempLS mesh is significant. GISS <a href="https://moyhu.blogspot.com.au/2017/07/giss-june-down-019-from-may.html">went down</a> 0.19°C, and <a href="https://www.blogger.com/went%20down">TempLS mesh</a> by 0.12°C. But TempLS grid actually rose, very slightly. I have <a href="http://www.moyhu.blogspot.com.au/2014/08/templs-and-noaa-are-converging.html">often noted</a> the close correspondence between NOAA and TempLS grid (and the looser one between TempLS mesh and GISS) and attributed the difference to GISS etc better coverage of the poles. <br /><br />This month, the cause of that difference is clear, as is the relative coolness of June in GISS. With <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">TempLS reports</a>, I post a breakdown of the regional contributions. These are actual contributions, not just average temperature. So in the following: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/break.png" /><br /><br />you see that the total dropped by about 0.12°C, while Antarctica dropped from conributing 0.07C to -0.07C, a difference that slightly exceeded the global total drop of 0.12C. <br /><br />That doesn't mean that, but for Antarctica, there would have been no cooling. May had been held up by the relative Antarctic warmth. But it is a further illustration of the difference between the interpolative procedures of GISS and TempLS and the cruder grid-based processes of NOAA and TempLS grid. I would probably have abandoned TempLS grid, or at least replaced it with a more interpolative version (post coming soon), if it were not for the correspondence with NOAA and HADCRUT. <br /><br /><span style="color: red;"><b>Update:</b> I see that the <a href="http://journals.ametsoc.org/doi/10.1175/JCLI-D-16-0836.1">paper for ERSST V5</a> has just been published in J Climate. I'll post about that very soon, and also, maybe separately, give an analysis of its effect in TempLS. I see also that NOAA was still using V4 for June; I assume they will use V5 for July, as I expect I will. The NOAA ERSST V5 page is <a href="https://www.ncdc.noaa.gov/data-access/marineocean-data/extended-reconstructed-sea-surface-temperature-ersst-v5">here</a>. </span><br /><br />Here is the NOAA map for the month. You can see how the poles are missing.<br /><br /><img border="0" src="https://www.ncdc.noaa.gov/sotc/service/global/map-blended-mntp/201706.gif" width="700" /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/07/noaa-global-surface-temperature-down.htmlnoreply@blogger.com (Nick Stokes)8tag:blogger.com,1999:blog-7729093380675162051.post-2261019006945802693Sat, 15 Jul 2017 00:12:00 +00002017-07-15T10:12:17.446+10:00GISS June down 0.19°C from May. GISS was down from 0.88°C in May to 0.69°C in June.The GISS report is <a href="https://data.giss.nasa.gov/gistemp/news/20170714/">here</a>; they say it was the fourth warmest June on record. The drop was somewhat more than the <a href="http://moyhu.blogspot.com/2017/07/june-global-surface-temperature-down-012.html">0.12°C in TempLS</a>. The most recent month that was cooler than that was November 2014. <br><br>The overall pattern was similar to that in TempLS. The big feature was cold in Antarctica, to which both GISS and TempLS msh are sensitive, more so than HADCRUT or NOAA. Otherwise, as with TempLS, it was warm in Europe, extending through Africa and the Middle East, and also through the Americas. Apart from Antarctica, the main cold spot was NW Russia. <br><br>So far, July is <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">also cold</a>, although with some signs of warming a little from June. As usual, I will compare the GISS and previous TempLS plots below the jump. <br><a name='more'></a><br>Here is GISS<br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/GISSjun.jpg"></img><br><br>And here is the TempLS spherical harmonics plot <br><br><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/map.png"></img><br><br><br><br><br><br><br><br> http://moyhu.blogspot.com/2017/07/giss-june-down-019-from-may.htmlnoreply@blogger.com (Nick Stokes)7tag:blogger.com,1999:blog-7729093380675162051.post-737515107919153906Fri, 07 Jul 2017 23:10:00 +00002017-07-08T09:10:14.202+10:00June global surface temperature down 0.12°C<a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">TempLS mesh</a> was down from 0.704°C in May to 0.586°C in June. This follows the <a href="https://moyhu.blogspot.com.au/2017/07/june-ncepncar-down-016.html"> slightly larger fall</a> of 0.16°C in the NCEP/NCAR index, and falls in the satellite indices, which had risen in May. The June anomaly (1961-90 base) is now a little below mid-2015 values, and is the coolest month since Nov 2014. In fact, it is similar to the 2014 annual average, which was still a record in its day. <br /><br />The big turnaround was in the Antarctic, which went from quite warm to very cool. This is reflected in the TempLS grid values, which are less sensitive to the poles; T grid actually warmed. This pattern tends to be reflected in the main indices, with GISS generally picking up the polar changes; NOAA and HADCRUT less so. Otherwise as with the <a href="https://moyhu.blogspot.com.au/2017/07/june-ncepncar-down-016.html"> reanalysis</a>, Europe was warm, NW Russia cold, Arctic neutral, warm spots in the Americas. Here is the map, and I'll show below that the breakdown, which emphasises the Antarctic turnaround. : <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/map.png" /><br /><br />Breakdown plot: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/break.png" /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/07/june-global-surface-temperature-down-012.htmlnoreply@blogger.com (Nick Stokes)14tag:blogger.com,1999:blog-7729093380675162051.post-6722387100029657504Tue, 04 Jul 2017 02:24:00 +00002017-07-05T07:57:06.382+10:00New RSS TLT V4 - comparisonsAs mentioned in my <a href="https://moyhu.blogspot.com.au/2017/07/june-ncepncar-down-016.html">previous post</a>, RSS has a new V4 TLT out - announcement <a href="http://www.remss.com/node/5206">here</a>. I'm now using it in place of V3.3. The J Climate paper describing it is <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0768.1">here</a>: <br /><br /><b><span style="color: #38761d;">A satellite-derived lower tropospheric atmospheric temperature dataset using an optimized adjustment for diurnal effects</span></b><br /><span style="color: #38761d;"><br /></span><span style="color: #38761d;">Carl A. Mears and Frank J. Wentz</span><br /><span style="color: #38761d;">Remote Sensing Systems, 444 Tenth Street, Santa Rosa, CA, 95401</span><br /><br />I quoted from the abstract in my previous post. <br /><br />The changes are described in those links, and are not surprising, given the previous datasets (eg TMT, TTT) that have come out in V4. I thought here I would just show a comparison of recent changes in both UAH and RSS - they are rather complementary. In the graph below, I have converted RSS from 1979-1999 to the UAH base of 1981-2010. I use reddish for UAH, bluish for RSS (12 month running mean): <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/tlt1.png" /><br /><br />The effect of the change is clearer if a common measure is subtracted - I use the average of the four sets here for that: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/tlt2.png" /><br /><br />Now you can see what has happened. RSS TLT V4 is close to UAH V5.6, and UAH V6 is close to the old RSS V3.3 (which RSS described as having a known cooling bias). As they noted, the new RSS V4 shows more uniformity over time. The overall picture is that TLT measures are not stable; much less so than surface measures, as I noted <a href="https://moyhu.blogspot.com.au/2015/12/big-uah-adjustments.html">here</a>. <br /><br />Contrary to some (mainly sceptic) opinion, satellite measures are not naturally superior. Measuring the temperature at various levels of the troposphere is a worthwhile endeavour, but it is not a substitute for surface. In fact, I think TLT has had undeserved prominence, and I rather thought RSS should drop it altogether. It is an attempt to get as close to surface as possible, but it isn't very close, and sacrifices much reliability in trying to get there. I notice the John Christy now usually quotes UAH TMT. <br /><br />The reason for loss of reliability is that the MSU is trying to make deductions from a microwave signal which is a mix of various layers in the troposphere, with a large background noise generated at the surface. It is hard to discriminate, and harder as you try to see closer to the surface. They try to get around this by taking two measures designed for higher levels (TMT and, for UAH, a tropopause level TP), and forming a linear combination which is designed to subtract out the higher troposphere and stratosphere levels. But as with any such differencing, errors increase. <br /><br />People have the idea that satellites just have to be better, because they can survey the whole Earth with one instrument. But that is far from true. The downsides are described in this <a href="http://www.drroyspencer.com/2015/04/version-6-0-of-the-uah-temperature-dataset-released-new-lt-trend-0-11-cdecade/">UAH overview</a> and the various RSS papers, and include: <br /><ul><li> There is only one instrument, or at most a few, while at the surface there are thousands, creating lots of redundancy. One consequence is that with satellites there is a big problem with the inevitable changeovers. Surface stations needd some adjustment when the instruments or environments change but that is minor compared with changing the whole instrument base every few years. </li><li>The instrument doesn't read a thermometer at every level. It has to resolve a mixed incoming microwave beam, confounded with surface noise. You can get some resolution with frequency bands, and a little more with differing angles of view. But it is really squinting, and in the end you have to solve an inverse problem, which takes adventurous mathematics. </li><li>The instrument gives a snapshot just twice a day. At surface, even the old min/max thermometers, though read only once, continuously monitored the minn and max for 24 hours, and of course now we have thousands of stations recording at high frequency. A problem with twice a day is that you have to make adjustments for what time of day it is, because of diurnal variation. And that diurnal pattern depends on the level (not clearly known), season etc. A hard enough problem, but the big one is </li><li>diurnal drift. It isn't the same time every day, due to orbit changes, and they seem to have trouble deciding exactly what time it is. Roy Spencer says of V6:<br /><i><span style="color: purple;">For example, years ago we could use certain AMSU-carrying satellites which minimized the effect of diurnal drift, which we did not explicitly correct for. That is no longer possible, and an explicit correction for diurnal drift is now necessary. The correction for diurnal drift is difficult to do well, and we have been committed to it being empirically–based, partly to provide an alternative to the RSS satellite dataset which uses a climate model for the diurnal drift adjustment.</span></i></li><li> It is a long standing bugbear, and much of the RSS change also seems to be in the drift correction. From their paper abstract: <br /><i><span style="color: purple;">Previous versions of this dataset used general circulation model output to remove the effects of drifting local measurement time on the measured temperatures. In this paper, we present a method to optimize these adjustments using information from the satellite measurements themselves. The new method finds a global-mean land diurnal cycle that peaks later in the afternoon, leading to improved agreement between measurements made by co-orbiting satellites.</span></i><br /> </li></ul><br />Those are just some of the problems which lead to such large version changes.<br /><br /><span style="color: red;"><b>Update</b>: From a tweet from Carl Mears, <a href="http://www.remss.com/blog/faq-about-v40-tlt-update">here</a> is a useful FAQ on the changes. </span><br /><br /><br /><span style="color: purple;"><b>Further:</b> David asked below about comparison with radiosondes. That FAQ has a diagram showing the comparison:</span><br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/07/sonde.png" /><br /><br /><span style="color: purple;">It is sat - sondes, so when you see in this century that the plot goes down, it means that radiosondes are showing more warming that satellites. With UAHV6.0 it is a lot more; with RSS TLT V4 it is closer, but sondes still show more warming. As the FAQ says: </span><br /><span style="color: purple;"><br /></span><i><span style="color: purple;">"Note that all satellite data warm relative to radiosondes before about 2000, and then cool after about 2000. We don't know if this overall pattern is due to problems with the radiosonde data, with the satellite data or (most likely) both."</span></i><br /><br /><br />http://moyhu.blogspot.com/2017/07/new-rss-tlt-v4-comparisons.htmlnoreply@blogger.com (Nick Stokes)45tag:blogger.com,1999:blog-7729093380675162051.post-9059826309587040600Sun, 02 Jul 2017 15:54:00 +00002017-07-03T02:08:09.446+10:00June NCEP/NCAR down 0.16°CIn the <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">Moyhu NCEP/NCAR index</a>, the monthly reanalysis average fell from 0.40°C in May to 0.241°C in June, 2017. This makes it the coolest month for nearly two years - since 0.164°C in July 2015. Even so, it was still the third warmest in the record for that index, though I comment caution in compare values decades, because of lack of homogeneity. It was only just behind 2013 (0.249) for second place. It's the first time for nearly two years that a month fell behind an earlier corresponding month other than 2016. <br><br>The main cool spot was Antarctica, and the main reason for the drop was that, as well, the Arctic dropped back to average, with Siberia mixed. Europe was warm. <br><br>in other (tropospheric) news, RSS has brought out a V4 version of TLT, described in a J Climate paper by Wentz and Mears <a href="http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0768.1">here</a>. I'll start using it for this month's reporting. I was actually wondering whether they would, since the trend seems to be more toward quoting TMT and TTT. AS has been the pattern with V4, the low trend that RSS V3.3 showed until recently, which gave rise to umpteen pause stories, has come closer to other records, mainly, they say, due to a revised diurnal correction. Here is their abstract: <br><br><blockquote><h4>A satellite-derived lower tropospheric atmospheric temperature dataset using an optimized adjustment for diurnal effects</h4> <i>Carl A. Mears and Frank J. Wentz<br>Remote Sensing Systems, 444 Tenth Street, Santa Rosa, CA, 95401</i><br> Temperature sounding microwave radiometers flown on polar-orbiting weather satellites provide a long-term, global-scale record of upper-atmosphere temperatures, beginning in late 1978 and continuing to the present. The focus of this paper is a lower-tropospheric temperature product constructed using measurements made by the Microwave Sounding Unit channel 2, and the Advanced Microwave Sounding Unit channel 5. The temperature weighting functions for these channels peak in the mid to upper troposphere. By using a weighted average of measurements made at different Earth incidence angles, the effective weighting function can be lowered so that it peaks in the lower troposphere. Previous versions of this dataset used general circulation model output to remove the effects of drifting local measurement time on the measured temperatures. In this paper, we present a method to optimize these adjustments using information from the satellite measurements themselves. The new method finds a global-mean land diurnal cycle that peaks later in the afternoon, leading to improved agreement between measurements made by co-orbiting satellites. The changes result in global-scale warming (global trend (70S-80N, 1979-2016) = °0.174 C/decade), ~30% larger than our previous version of the dataset (global trend, (70S-80N, 1979-2016) = 0.134C/decade). This change is primarily due to the changes in the adjustment for drifting local measurement time. The new dataset shows more warming than most similar datasets constructed from satellites or radiosonde data. However, comparisons with total column water vapor over the oceans suggest that the new dataset may not show enough warming in the tropics. </blockquote><br><br>I have updated the data link in the source table. <br><br><br><br><br><br> http://moyhu.blogspot.com/2017/07/june-ncepncar-down-016.htmlnoreply@blogger.com (Nick Stokes)6tag:blogger.com,1999:blog-7729093380675162051.post-236664489086832071Tue, 27 Jun 2017 10:34:00 +00002017-06-27T20:34:25.976+10:00Temperature station distribution - equal area plotI have been <a href="https://moyhu.blogspot.com.au/2017/06/world-map-equal-area-projection-more.html">experimenting</a> with maps that are a byproduct of my systematising a <a href="https://moyhu.blogspot.com.au/2017/06/cubing-sphere.html">cubed sphere grid</a>. I thought it would give a better perspective on the distribution of surface stations and their gaps, especially with the poles. So here are plots of the stations, land and sea, which have reported April 2017 data, as used in TempLS. The ERSST data has already undergone some <a href="https://moyhu.blogspot.com.au/2017/03/making-even-sst-mesh-on-globe.html">culling</a>. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/flattemp.png" /><br /><br />It shows the areas in proportion. However, it shows multiple Antarctica's etc, which exaggerates the impression of bare spots, so you have to allow for that. One could try a different projection - here is one focussing on a strip including the America's: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/rottemp.png" /><br /><br />So now there are too many Africa's. However, between them you get a picture of coverage good and bad. Of course, then the question is to quantify the effect of the gaps. <br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/temperature-station-distribution-equal.htmlnoreply@blogger.com (Nick Stokes)6tag:blogger.com,1999:blog-7729093380675162051.post-8535692318760809980Fri, 23 Jun 2017 04:04:00 +00002017-06-23T14:05:02.996+10:00World map equal area projection - moreIn my <a href="https://moyhu.blogspot.com.au/2017/06/world-map-projection-using-cubed-sphere.html">last post</a>, I showed an equal area world map projection that was a by-product of the cubed sphere gridding of the Earth's surface. It was an outline plot, which makes it a bit harder to read. Producing a colored plot was tricky, because the coloring process in R requires an intact loop, which ends where it started, and the process of unfolding the cube onto which the map is initially projected makes cuts. <br /><br />So I fiddled more with that, and eventually got it working. I'll show the result below. You'll notice more clearly the local distortion near California and Victoria. And it clarifies how stuff gets split up by the cuts marked by blue lines. I haven't shown the lat/lon lines this time; they are much as before. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/worldmap.png" width="1000px"/><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/world-map-equal-area-projection-more.htmlnoreply@blogger.com (Nick Stokes)0tag:blogger.com,1999:blog-7729093380675162051.post-5486214141562648743Sun, 18 Jun 2017 23:13:00 +00002017-06-19T09:15:00.215+10:00World map projection using cubed sphereThis post follows on from the <a href="http://moyhu.blogspot.com/2017/06/cubing-sphere.html">previous post</a>, which described the cubed sphere mapping which preserves areas in taking a surface grid from cube to sphere. I should apologise here for messing up the links for the associated WebGL plot for that post. I had linked to a local file version of the master JS file, so while it worked for me, I now realise that it wouldn't work elsewhere. I've fixed that. <br /><br />If you have an area preserving plot onto the flat surfaces of a (paper) cube, then you only have to unfold the cube to get an equal-area map of the world on a page. It necessarily has distortion, and of course the cuts you make in taking apart the cube. But the area preserving aspect is interesting. So I'll show here how it works. <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/profmap.png" width=1000 /><br /><br />I've repeated the top and bottom of the cube, so you see multiple poles. Red lines are latitudes, green longitudes. The blue lines indicate the cuts in unfolding the cube, and you should try to not let your eye wander across them, because there is confusing duplication. And there is necessarily distortion near the ends of the lines. But it is an equal area map. <br /><br />Well, almost. I'm using the single parameter tan() mapping from the previous post. I have been spending far too much time developing almost perfectly 1:1 area mappings. But I doubt they would make a noticeable difference. I may write about that soon, but it is rather geekish stuff. <br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/world-map-projection-using-cubed-sphere.htmlnoreply@blogger.com (Nick Stokes)2tag:blogger.com,1999:blog-7729093380675162051.post-2129730517698235496Fri, 16 Jun 2017 19:26:00 +00002017-06-19T08:40:37.041+10:00Cubing the sphereI wrote <a href="https://moyhu.blogspot.com.au/2015/10/optimised-gridding-for-temperature.html">back in 2015</a> about an improvement on standard latitude/longitude gridding for fields on Earth. That is essentially representing the earth on a cylinder, with big problems at the poles. It is much better to look to a more sphere-like shape, like a platonic solid. I described there a mesh derived from a cube. Even more promising is the <a href="https://en.wikipedia.org/wiki/Icosahedron">icosahedron</a>, and I wrote about that more recently, <a href="https://moyhu.blogspot.com.au/2017/04/icosahedral-earth.html">here</a> and <a href="https://moyhu.blogspot.com.au/2017/04/a-magical-easter-egg.html">here</a>. <br><br>I should review why and when gridding is needed. The original use was in mapping, so you could refer to a square where some feature might be found. The uniform lat/lon grid has a big merit - it is easy to decide which cell a place belongs in (just rounding). That needs to be preserved in any other scheme. Another use is in graphics, where shading or contouring is done. This is a variant of interpolation. If you know some values in a grid cell, you can estimate other places in the cell. <br><br>A variant of interpolation is averaging, or integration. You calculate cell averages, then add up to get the global. For this, the cell should be small enough that behaviour within it can be regarded as homogeneous. One sample point is reasonably representative of the whole. Then they are added according to area. Of course, the problem is that "small enough" may mean that many cells have no data. <br><br>A more demanding use still is in solution of partial differential equations, as in structural engineering or CFD, including climate GCMs. For that, you need to not only know about the cell, but its neighbors. <br><br><table><tr><td>A cubed sphere is just a regular rectangular grid (think Rubik) on the cube projected, maybe after re-mapping on the cube, onto the sphere. I was interested to see that this is now catching on in the world of GCMs. <a href="https://www.gfdl.noaa.gov/bibliography/related_files/wmp0701.pdf">Here</a> is one paper written to support its use in the GFDL model. <a href="http://www.sciencedirect.com/science/article/pii/S0021999196900479">Here</a> is an early and explanatory paper. The cube grid has all the required merits. It's easy enough to find the cell that a given place belongs in, provided you have the mapping. And the regularity means that, with some fiddly bits, you can pick out the neighbors. That supported the application that I wrote about in 2015, which resolved empty cells by using neighboring information. As described there, the resulting scheme is one of the best, giving results closely comparable with the triangular mesh and spherical harmonics methods. I called it enhanced infilling. <br><br>I say "easy enough", but I want to make it my routine basis (instead of lat/lon), so that needs support. Fortunately, the grids are generic; they don't depend on problem type. So I decided to make an R structure for standard meshes made by bisection. First the undivided cube, then 4 squares on each face, then 16, and so on. I stopped at 64, which gives 24576 cells. That is the same number of cells as in a 1.6° square mesh, but the lat/lon grid has some cells larger. You have to go to 1.4° to get equatorial cells of the same size. <br><br>I'll give more details in an appendix, with a link to where I have posted it. It has a unique cell numbering, with an area of each cell (for weighting), coordinated of the corners on the sphere, a neighbor structure, and I also give the cell numbers of all the measurement points that TempLS uses. There are also functions for doing the various conversions, from 3d coordinates on sphere to cube, and to cell numbering. </td><td><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/cubed.png"></img></td></tr></table><br><br>There is also a WebGL depiction of the tesselated sphere, with outline world map, and the underlying cube with and without remapping. <br><a name='more'></a><br>The basic process of mapping from sphere surface to cube goes as following (with options) starting from the lat/lon data <ol><li> You may want to rotate around the axis relative to the cube; add something to the longitudes <li> Convert lat/lon to radians θ,φ by miltiplying by π/180. <li> Convert to z (3D on unit sphere) by z=(cos(θ)*sin(φ),sin(θ), -cos(θ)*cos(φ)). This gives a conventional start view with 0N,0E as 0,0 in the x-y plane. <li> Project to u on unit cube. For each z, divide by max(abs(z)), abs being abs of coordinates. <li> Remap on cube. I use v = tan(b*u)/tan(b), applied to each coordinate, with b=0.86. </ol>The reason for the last mapping is that the projection concentrates points towardd the face centers. As you go away from the center, the area ratio (sphere to cube) is (1/cos(α))^3, where α is angle from line through face center. So it reaches a maximum of sqrt(27) at corners. The last mapping counters this, reducing the discrepancy to about 20%. Any function with cubic-like behaviour will do; I use tan() because it is easily reversed, which is often needed. <br><br>So I'll show here a WebGL active plot of how it works. This is a 16x16 mesh on each face. The plot initially looks overly crowded, but you can hide things with the checkboxes top right. It shows the grid on the surface, with a map and dots for the stations. Underneath there is the cube that is projected onto the surface. It shows in mauve the logical regular grid, and in green the result of the local mapping. It is the green grid that is projected, but the regular grid is what the process seeks to match in area to the cells on the sphere surface. <br><br><div ></div><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyJSlib.js"> </script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/Map.js"> </script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/mesh.js"> </script><script type="text/javascript" src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/pages/webgl/MoyGLV2.1.js"> </script><h4>Appendix - structure details.</h4>The structure (list) as a 1 MB R data file, is <a href="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/cubedata.sav">here</a>. At top level it has a readme string and 6 lists that are called cube$n1, ..n2 up to ..n64, or you can call them cube[[1]] etc. Each has material for a grid of the nominated degree of face division (1x1, 2x2 etc). I normally start use by writing, say, p=cube$n16, and thereafter dea only with p. <br><br>Most of the data is in the form of a "score". This is derived from numbering and underlying cubic grid of twice the division. Nodes and cells have scores, the latter derived from the face center. You don't need to know how it is derived, just that nodes and faces have them. <br><br>The main data for the cube are p$inodes and p$ifaces, which are the scores for the logical (regular) grid. There is also a triangular mesh, pointing to the nodes in score order. There is a neighbor list p$nb, which lists each cell and the scores of its 4 neighbors. There is also a list p$stns of the cells, by score, that each station in the TempLS database is mapped into. That is just what you need for gridded integration, along with p$wt, which are the areas of the cells on the sphere. There is also a convenience function p$lines, which gives the nodes in the order you need to draw the gridlines. <br><br>The main functionality is from the conversion programs, with names like c2s etc. The ones provided are c2s and s2c, c2m and m2c, s2i, i2s, c2i, i2c, c2m, m2c. The symbols stand for <ul><li> s are 3d coordinates on the unit sphere <li> i are the scores. So s2i tells what cell score a surface point belongs in, and i2s gives the 3D coords of the face score i. Every function involving i must have the list p as a second argument. <li> c are the 3D coordinated of the regular grid on the unit cube <li> m are the mapped points (using that tan() function. You won't often need them. </ul><br><br>You can of course nest the conversions. Each of these sublists such as cube$n16 has a $readme string. <br><br>The dataset is located as an R datafile <a href="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/cubedata.sav">here</a>. <br><br><br><br> <br><br> http://moyhu.blogspot.com/2017/06/cubing-sphere.htmlnoreply@blogger.com (Nick Stokes)3tag:blogger.com,1999:blog-7729093380675162051.post-970925508003441434Thu, 15 Jun 2017 20:30:00 +00002017-06-16T06:30:50.327+10:00GISS May unchanged from April - second warmest May on record.As with <a href="https://moyhu.blogspot.com.au/2017/06/may-global-temperature-unchanged-from.html">TempLS</a>, GISS showed May unchanged from April, at 0.88°C. Although that is down from the extreme warmth of Feb-Mar, it is still very warm historically. In fact, it isn't far behind the 0.93°C of May 2016. June looks like being cooler, which reduces the likelihood of 2017 exceeding 2016 overall. <br /><br />The overall pattern was similar to that in TempLS. A big warm band from N of China to Morocco (hot), with warmth in Europe, and cold in NW Russia. Wark Alaska, coolish Arctic and Antarctica mixed. <br /><br />As usual, I will compare the GISS and previous TempLS plots below the jump. <br /><a name='more'></a><br />Here is GISS<br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/GISSmay.png" /><br /><br />And here is the TempLS spherical harmonics plot <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/map.png" /><br /><br /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/giss-may-unchanged-from-april-second.htmlnoreply@blogger.com (Nick Stokes)5tag:blogger.com,1999:blog-7729093380675162051.post-3511854750659678620Tue, 13 Jun 2017 00:16:00 +00002017-06-13T21:43:57.719+10:00Integrating temperature on sparse subgridsI've been intermittently commenting on a <a href="https://climateaudit.org/2017/05/18/how-dependent-are-gistemp-trends-on-the-gridding-radius-used/">thread</a> on the long-quiet Climate Audit site. Nic Lewis was showing some interesting analysis on the effect of interpolation length in GISS, using the Python version of GISS code that he has running. So the talk turned to numerical integration, with the usual grumblers saying that it is all too complicated to be done by any but a trusted few (who actually don't seem to know how it is done). Never enough data etc. <br /><br />So Olof chipped in with an interesting observation that with the published UAH 2.5x2.5° grid data (lower troposphere), an 18 point subset was sufficient to give quite good results. I must say that I was surprised at so few, but he gave this convincing plot: <br /><br /><img src="https://i0.wp.com/postmyimage.com/img2/581_image.png?zoom=2" /><br /><br />He made it last year, so it runs to 2015. There was much scepticism there, and some aspersions, so I set out to emulate it, and of course, it was right. My plots and code are <a href="https://climateaudit.org/2017/05/18/how-dependent-are-gistemp-trends-on-the-gridding-radius-used/#comment-772588">here</a>, and the graph alone is <a href="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/olof.png">here</a>. <br /><br />So I wondered how this would work with GISS. It isn't as smooth as UAH, and the 250 km less smooth than 1200km interpolation. So while 18 nodes (6x3) isn't quite enough, 108 nodes (12x9) is pretty good. Here are the plots: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/giss250.png" /><br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/giss1200.png" /><br /><br />I should add that this is the very simplest grid integration, with no use of enlightened infilling, which would help considerably. The code is <a href="https://climateaudit.org/2017/05/18/how-dependent-are-gistemp-trends-on-the-gridding-radius-used/#comment-772642">here</a>. <br /><br />Of course, when you look at a statistic over a longer period, even this small noise fades. Here are the GISS trends over 50 years: <br /><br /> <table><tbody><tr><td>1967-2016 trend C/Cen</td><td></td><td width="80">Full mesh </td><td width="80">108 points </td><td>18 points </td></tr><tr><td>250km</td><td></td><td>1.658</td><td>1.703</td><td>1.754 </td></tr><tr><td>1200km</td><td></td><td>1.754</td><td>1.743</td><td>1.768 </td></tr></tbody></table><br /><br />This is a somewhat different problem from my intermittent search for a 60-station subset. There has already been smoothing in gridding. But it shows that the spatial and temporal fluctuations that we focus on in individual maps are much diminished when aggregated over time or space. <br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/integrating-temperature-on-sparse.htmlnoreply@blogger.com (Nick Stokes)15tag:blogger.com,1999:blog-7729093380675162051.post-8433295822134830785Wed, 07 Jun 2017 14:52:00 +00002017-06-08T00:52:34.060+10:00May global temperature unchanged from April<a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#mesh">TempLS mesh</a> was virtually unchanged , from 0.722°C to 0.725°C. This follows the <a href="https://moyhu.blogspot.com.au/2017/06/may-ncepncar-up-006.html">smallish rise</a> of 0.06°C in the NCEP/NCAR index, and larger rises in the satellite indices. The May temperature is still warm, in fact, not much less than May 2016 (0.763°C). But it puts 2017 to date now a little below the annual average for 2016. <br /><br />The main interest is at the poles, where Antarctica was warm, and the Arctic rather cold, which may help retain the ice. There was a band of warmth running from Mongolia to Morocco, and cold in NW Russia.. Here is the map: <br /><br /><img src="https://s3-us-west-1.amazonaws.com/www.moyhu.org/2017/06/map.png" /><br /><br /><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/may-global-temperature-unchanged-from.htmlnoreply@blogger.com (Nick Stokes)16tag:blogger.com,1999:blog-7729093380675162051.post-5563712276232809460Fri, 02 Jun 2017 18:06:00 +00002017-06-03T09:53:45.078+10:00May NCEP/NCAR up 0.06°CSo far in 2017, in the <a href="https://moyhu.blogspot.com.au/p/latest-ice-and-temperature-data.html#NCAR">Moyhu NCEP/NCAR index</a>, January to March were very warm, but April was a lot cooler. May recovered a little, rising from 0.34 to 0.4°C, on the 1994-2013 anomaly base. This is still warm by historic standards, ahead of all annual averages before 2016, but it diminishes the likelihood that 2017 will be warmer than 2016. <br /><br />There were few notable patterns of hot and cold - cold in central Russia and US, but warm in western US, etc. The Arctic was fairly neutral, which may explain the fairly slow melting of the ice.. <br /><br /><span style="color: blue;"><b>Update</b> - UAH lower troposphere V6 <a href="http://www.drroyspencer.com/">;rose considerably</a>, from 0.27°C to 0.45°C in May.</span><br /><br /><br /><br />http://moyhu.blogspot.com/2017/06/may-ncepncar-up-006.htmlnoreply@blogger.com (Nick Stokes)24