Saturday, April 12, 2014

Significant GHCN errors in March 2014

Every month I run my least squares program TempLS on the data from GHCN V3 TAVG QCU and ERSST, when enough stations have reported, to estimate the month's temperature. A few days ago I posted for March. I found a substantial rise of 0.27°C.

As luck would have it, I had recently upgraded my program for showing the spatial pattern using spherical harmonics. So when the picture looked a bit strange, I checked and rechecked my revisions.

But in trying to analyse a spurious-looking cool spot in central Africa, I discovered that FORT-ARCHAMBAULT (now Sarh) in Chad had reported a very surprising -31.6°C. That was clearly a sign error, and fixing it removed the cool spot and raised the global average to 0.31°C. I traced the sign error to the CLIMAT form submitted by Chad.

The reason it had such a large effect is that it is an oasis in a data desert, and is weighted to represent a large area.

There was also an oddity with the map which was a very hot spot over N Greenland. When I looked into that, I found the March average at NORD ADB (81°N) was 4.3°C. It's usually about -30. This time I found that OGIMET says that no forms for March had been submitted.

So I looked further, and it appears that the data from September 2013 has been re-entered in the GHCN V3 QCU file for March 2014. Naturally this has a huge effect. Just omitting NORD ADB brought the global back to 0.23°C. But there were 5 Greenland stations, all with March averages of more than 4°C.

So I don't now have a reliable March estimate. I may calculate one without Greenland, but I think I'll wait for the errors to be fixed.

FWIW, with Greenland removed, the March average anomaly is 0.559°C, a rise of about 0.25°C. Almost back to where I started. But Greenland is important here.
Denmark has the same problem. 19.3°C in Copenhagen, March 2014! This time the March data is from July 2013.


Post a Comment