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Message: analysis problem in RUC and fix

Posted by Stan Benjamin on 1 Apr 99, 15:43 MT

Happy April Fools' Day

This news isn't so funny -- we've uncovered a significant bug in the RUC's use of significant level temp/dewpoint rawinsonde data (the TTBBs). Basically, it has NOT been getting used, and worse, the RUC has been forcing in linearly interpolated temperatures and dewpoints between mandatory levels. This doesn't hurt too much if the sounding does not have much structure, but if there are significant inflection points in temperature or dewpoint away from mandatory levels, then this bug has put in some ugly structures. You can see a painful example of this at http://maps.fsl.noaa.gov/hybfront_prob, which contains gif files of raob, RUC2 (wrong), and MAPS40 (correct) soundings for Del Rio, Texas for yesterday at 1200 UTC. There is no problem with the significant wind levels -- they have been getting assimilated correctly.

The fix is very simple, commenting out half a line of code. It has been tested at FSL and is being tested at NCEP. Because of the proximity to the weekend and programmer schedule at NCEP, it looks like this change will not be implemented until next Tuesday April 6 (by coincidence, the 1-year anniversary of the RUC-2 implementation). For what it's worth, this bug is not in the 40km MAPS running at FSL, since it is associated with the expected order of raob levels, which is different at NCEP and FSL.

We'll post another message about the exact time of the fix when it is set, but the best guess now is 1200 UTC on Tuesday.

We at FSL apologize for not finding this before. We knew for a few weeks that there was some problem with significant level data but had thought that it was intermittent -- it wasn't. It makes us cringe to think of the effects this bug must have sometimes had on operational forecasts. On the other hand, the overall behavior of the RUC2 has still been fairly good, and will now improve in some situations.

Stan Benjamin et al.

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