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HOME > Monitoring_and_Data > Oceanic and Atmospheric Data > Reanalysis: Atmospheric Data

Problem with 8/2004+ Ice Analyses

In August 2004, the old sea-ice analysis programs were upgraded. The new analyses had a different land-sea mask and more importantly changed the land values from 0 to 1.5. The sea-ice concentration over water remained in the range of 0 to 1. Examination of the code for CDAS, Reanalysis-2 and the operational GFS indicated that all these codes would limit the read-in values to lie between 0 and 1. Consequently one might expect that interpolating the sea ice from a land point (value=1) would produce coastal sea ice which would be obvious in August. At least that was the assumption of two people who examined the potential effects of the new sea ice analyses. (Yes, I was one of them.) No unusual sea ice was noticed in the various analyses during August 2004 and people thought there was no problem in using the new sea ice analyses.

In Februrary 2005, an email from Europe asked when the 20C surface temperatures off Denmark was going to be fixed in the operational forecasts. Looking at a global map one easily miss the problem but if you zoomed in, one could see a few of grid point with obviously warm temperatures. The problem was traced back to the new sea-ice analyses. The 3 assimilation systems thought certain coastal grid points had sea ice. However, the code then thought the sea ice was inconsistent with the current SST (so true) and promptly rejected both the SST and sea-ice analyses. (At one time, both analyses were produced by the same system.) When you reject both analyses, the SST remains constant in time. You wouldn't notice it after a few weeks but in 6 months, it can become pretty obvious.

The Fix

The idea behind reanalysis is to keep a fixed assimilation system; however, one needs to change the code to keep up with the changing data sources. 20C water in coastal Denmark during February is not an observation but an incompatibility between the ice analyses and the software. The software (cycle/sfc) was changed to (1) interpolate sea-ice from the the nearest water point rather than the nearest point and to (2) believe the SST analyses even when the sea-ice analyses is inconsistent with the SST analyses. Consequently the changes will make the surface conditions better reflect the input sst/sea-ice analyses.


These changes give the assimilation systems the correct sea ice. However, we are under no illusions that the sea-ice climatology will remain the same after 8/2004. First, there will be some changes caused by the differing sea-ice analyses schemes. We've already seen changes in the snow climatology caused by upgrades in the snow analyses. I wouldn't be surprised by changes in the SST caused by improvements in the observations. Second, the old scheme's interpolation could have produced a ice-free coastal water point if the nearest point were land. The new scheme will put in sea ice if appropriate. Duplicating this error would have been difficult given that the land-sea mask and resolution (not sure) have and are expected to change in the future.


The problem caused certain grid points to ignore the new SST analyses. Consequently at these points, the SST remains constant in time and the problem become very noticable when the SSTs are a minimum (late winter). In this plot, three coastal points had the incorrect surface temperature. tmp plot of N. Europe The following is a global plot of the difference between the surface temperatures in the original and fixed analyses. You can see that August SSTs were used in a number of coastal grid points. You will also notice a 1-2 C warming in the Artic relative to the new analyses. plot of difference in sfc tmp The Arctic warming in the old analyses was unexpected and not present in the January and December mean temperatures. Detailed examination indicated that the snow depth was less over the Arctic sea ice which caused a greater heat flux through the sea ice.

Differences in tabular format

R2 flx files monthly average 200501
R2 flx files monthly average 200502

R1 pgb files monthly average 200408
R1 pgb files monthly average 200409
R1 pgb files monthly average 200410
R1 pgb files monthly average 200411
R1 pgb files monthly average 200412

R1 flux files monthly average 200408
R1 flux files monthly average 200409
R1 flux files monthly average 200410
R1 flux files monthly average 200411
R1 flux files monthly average 200412

Affected Files

The files from 00Z August 1, 2004 to March, 2005 were affected. The new (v1.1) files have the following codes:
  • NCEP/NCAR Reanalysis (CDAS): subcenter=1, process=181
    00Z August 1, 2004 to 12Z March 22, 2005.
  • NCEP/DOE Reanalysis (R2):subcenter=1, process=196
    00Z August 1, 2004 to 12Z March 28, 2005.

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Page last modified: February 22, 2005
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