Stratiform Precipitation Rates and Resulting Surface Visbility Forecasts using the MM5

The Integrated Meteorological System (IMETS) is an automated weather data system utilized by Air Force weather forecasters in support of Army operations. The U.S. Army Research Laboratory (ARL) is supporting the forecasters to make battlefield weather forecasts by using mesoscale models. Model outputs are used by the ARL's tactical decision aids which have a need for precipitation, precipitation rates, and visibility. In this study, the Mesoscale Model Version 5 (MM5) provided by the Air Force Weather Agency was used. The MM5 has a resolution of 15 km with 41 vertical levels and uses the Reisner mixed-phase explicit moisture microphysics package. This project, which uses model runs over a variety of terrain and atmospheric conditions, focuses on the model's short-term stratiform precipitation rates, precipitation type, and resulting visibility forecast based on software developed by ARL. Results from cold-season studies during 2003 and 2004 indicate that the MM5 correctly forecasts precipitation occurrence 82 percent of the time, but tends to underforecast precipitation rates early in the model forecast and overforecast precipitation rates beyond the 15-h forecast period. Additionally, the model forecasts much weaker precipitation intensity for snowfall than for what is observed. The visibility forecasts show their best skill level for rain forecasts but huge errors are noted for snow forecasts. A discussion on how these model biases relate to the model physics will conclude this paper and presentation.