Improved Light Precipitation Forecasts in New Eta-12: Reduction in spurious light precipitation in moist marine air masses

Abstract

This is a brief on an improvement in precipitation forecasts that has, in general, continued to be noticed since the new grid-scale precipitation scheme went into effect when the new Eta-12 became operational on 27 November 2001. One of the elements in the new scheme is the addition of vertical diffusion of cloud water between model layers. The main purpose for this change was to improve erroneous forecasts of 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 under the marine stratus regime.

A few examples of improvements for each region are shown.

Learning objectives

1. The Eta-12 improved grid-scale precipitation scheme will result in fewer false alarms for light precipitation.
2. This improvement is the result of reducing cloud water mixing ratios (which is critical for precipitation formation in the Eta grid-scale precipitation parameterization) in model layers.

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-12 model data
 

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