Reykjavik and GHCN adjustments.

There was a WUWT guest post recently by Paul Homewood on GHCN and GISS adjustments, and their effect on Iceland. It was somewhat along the lines of an earlier Eschenbach post on Darwin.

Then there were the usual thoughtful WUWT responses
When are the legal people going to be brought into this?
Someone should go to jail if this is tampering as it appears.
we’ve still a bunch of scumbags on this earth pretending that a dynamic history is OK
etc. So what happened

No, history was not rewritten. What the folks there don't seem to want to acknowledge is that GHCN circulates two files, described here. The file everyone there wants to focus on is the adjusted file (QCA). This, as explained, has been homogenized. This is a preparatory step for its use in compiling a global index. It tries to put all stations on the same basis, and also adjust them, if necessary, to be representative of the region. It is not an attempt to modify the historical record.

That record is contained on the other data file distributed - the unadjusted QCU file. This contains records as they were reported initially. It is generally free of any climatological adjustments. For the last 15 or so years, Met stations have submitted monthly CLIMAT forms. You can inspect these online. Data goes straight from these to the QCU file, and will not change unless the Met organisation submits an amended CLIMAT file. This is the history, and no-one is tampering with it.

The adjusted file does change, as the name suggests it may. Recently, it has been modified to use an improved pairwise comparison homogenization algorithm due to Menne and Williams. It is now (as of Dec 15 2011) used by GISS instead of their own homogenization algorithm, which makes the QCA file much more significant.

Update. I have a new post which looks at the GHCN adjustments more generally, with visualization.

The Iceland Met Office record

So Paul raised this with the Icelandic Met Office, with the following loaded questions:
a) Were the Iceland Met Office aware that these adjustments are being made?
No we were not aware of this.
b) Has the Met Office been advised of the reasons for them?
No, but we are asking for the reasons
c) Does the Met Office accept that their own temperature data is in error, and that the corrections applied by GHCN are both valid and of the correct value? If so, why?
The GHCN “corrections” are grossly in error in the case of Reykjavik but not quite as bad for the other stations. But we will have a better look. We do not accept these “corrections”.
d) Does the Met Office intend to modify their own temperature records in line with GHCN?
No.

And the Met sent their own version of the Reykjavik data. But what is missing from this dialog is that GHCN was never altering their QCU record, and is not suggesting that the Iceland records should be changed. I'll show the QCU data for the period most complained of, post 1939. Units are .01°C. Unfortunately, the Dec numbers got lost in formatting.

Year	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov
1937	80	-160	-130	500	690	960	1160	1040	860	400	360
1938	10	180	200	470	600	920	1130	1060	940	490	170
1939	-150	130	360	500	870	1080	1300	1230	1180	740	150
1940	160	170	-20	300	760	970	1120	1010	710	630	130
1941	-30	-150	210	540	880	1150	1230	1150	1150	710	480
1942	160	190	270	360	770	970	1140	1130	790	260	410
1943	40	-90	150	290	500	990	1140	990	810	450	200
1944	-170	60	140	410	660	990	1300	1180	800	460	10
1945	-260	10	410	440	760	980	1190	1200	960	720	650
1946	240	-20	280	320	840	870	1060	1060	800	760	140
1947	320	-200	-280	170	830	990	1090	1100	770	560	30
1948	70	200	320	120	460	940	1060	1100	620	310	250
1949	-270	0	10	0	360	980	1050	1010	900	450	250
1950	280	-70	130	220	710	930	1240	1220	720	450	70
1951	-150	-90	-300	0	690	980	1060	1140	970	570	90
1952	-290	-30	100	340	620	830	1010	1010	760	570	260
1953	60	180	270	10	680	1020	1190	1150	970	420	200
1954	200	10	160	450	760	1020	1060	1100	610	370	270
1955	-200	-270	140	560	570	990	1050	1010	810	390	430
1956	-320	270	360	360	660	840	1070	990	930	470	500
1957	110	-150	40	500	700	990	1200	1130	710	470	300

Despite all the invocation of the Iceland data, there is no electronically readable form supplied. You can compare the link above, though, and you will see that the GHCN QCU data is almost identical. There are changes of a fraction of a degree - the Iceland Met do say that they have also made some adjustments.

Actually there are isolated differences. These seem to be sign differences, where GHCN has a negative sign. The sign may have disappeared in scanning the Iceland numbers.

The story of the Reykjavik adjustments was taken up by Ole Humlun, with again the same disdain for the record contained in the GHCN QCU file and the purpose of the homogenization adjustment. He refers to the Iceland data, and to Rimfrost, but does not mention the almost identical QCU.

The V3.1 adjustments

It is true that the adjustments have changed recently. V3.1 was released 4 November 2011. I have a QCA file from 14 July 2011, and this is, for Reykjavik, almost identical to the QCU file. I have a v2.mean file from Dec 2009, which is the V2 unadjusted file, and it is essentially identical to the current QCU file. In v2 style, it had duplicates. there were four for Reykjavik, but they had little overlap and where they did, were consistent. The unadjusted file has not changed, but the QCA file has.

I also have an adjusted v2 file from Dec 2009. The adjustments are substantial, but less than the current ones.

So here is the plot of the current QCU file vs the adjusted QCA file:



And here is the plot of differences



As you see, the adjustments are substantial. I have done a repeat of the GHCN analysis that we did for Darwin, with a histogram of the effect on trends. I will blog about this shortly. Current estimate is that this adjustment is in the top 10%.