Introduction

Reduction in spurious light precipitation in moist marine air masses

The Eta model has long been known for having a high bias for light precipitation amounts. This has been particularly noticeable in warm-season light precipitation in the mid-Atlantic and southeastern U.S. during periods of moist onshore flow, and in the coastal and offshore areas of the West Coast when the marine layer predominates.

Clouds result in precipitation in the old version of the Eta grid-scale precipitation scheme when the mixing ratio of cloud condensate reaches a critical level through a process called "autoconversion." Excessive spatial coverage and occurrences of light precipitation thus result from incorrectly high cloud water mixing ratios. Possible reasons for the high cloud water mixing ratios include

• Unrealistically low RH thresholds for cloud condensation, given the relatively high horizontal resolution of the Eta
• For onshore flow, lower RH thresholds for cloud condensation over land (75%) than over water (80%)
• Lack of vertical diffusion, resulting in too little cloud water evaporation, and high cloud water concentrations in individual model layers.

In the latest Eta implementation, horizontal and vertical resolution is improved from 22- to 12-km and from 50 to 60 levels, respectively. More important for this discussion, however, is the change in the Eta grid-scale precipitation scheme and the addition of vertical diffusion between model layers. The impacts of the latter changes on light precipitation in particular are significant. In the rest of this case, we will show examples of the impact on light precipitation distribution in both the eastern and western U.S.

Credits

By Dr. Bill Bua, UCAR/COMET

Thanks to:

• Brad Ferrier and Stephen Jascourt for helpful discussions and review of this case
• Ying Lin for radar rainfall estimates
• Eric Rogers for help with obtaining Eta-22 and Eta-10 model data