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Message: bug in moisture fields for RUC/MAPS boundary conditions

Posted by Stan Benjamin on 16 Jul 99, 15:22 MT

Most of you have probably noticed problems with very excessive precipitation in RUC and MAPS runs near the boundary over tropical oceans, particularly in the last couple of months, and perhaps also excessive areal coverage over the same regions.

Two bugs were found yesterday in the program that interpolates Eta fields to RUC/MAPS coordinates for use as lateral boundary conditions. The worst of these bugs was the use of virtual temperature instead of temperature for calculation of saturation vapor pressure. This is resulting in overestimates of moisture at lowest levels at the boundary by as much as 4 g/kg or more over the Gulf of Mexico and Caribbean. The second bug is that boundary mixing ratios where the temperature is less than -15 deg C is being underestimated by the ratio of saturation vapor pressure over ice to that of water.

These bugs has been fixed in MAPS runs as of 12z this morning. We are talking to NCEP about when we can get it fixed there for the operational RUC. You can compare the precip near the boundaries between MAPS runs and RUC runs using this home page. If you have any comments about problems you've seen in the RUC that appear to be related to this, please post them. The improvement from the fix will probably be most apparent to users along the Gulf Coast and south Atlantic coast.

Stan Benjamin


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