NUMERICAL-SIMULATION OF THE VALENTINES DAY STORM DURING THE 1990 WINTER ICING AND STORMS PROJECT

This paper discusses numerical simulation of the first day of the 1990 winter icing and storms project's (WISP) Valentine's day storm (VDS) case with the Purdue mesoscale model(PMM) and comparison of the result s to aircraft, satellite, radiometer and other observations. This situ ation is marked by many observations of supercooled liquid water (SLW) contents of 0.2 g m(-3) to 0.3 g m(-3) by the WISP research aircraft. The PMM was initialized with the ECMWF analysis for 13 February 1990 at 0000 GMT and used to make an 18-h forecast with output every 6 h. T his paper focuses on comparison of the forecast to observations made a t about 1800 GMT since the aircraft encountered significant amounts of SLW during its flight between 1723 GMT and 1928 GMT. Dual channel rad iometer, satellite visible imagery, and other observational data sourc es are also utilized For this study, the PMM includes a new stable clo ud parameterization that is discussed here. In it, cloud water and ice are explicitly calculated while rain and snow are implicitly handled. Between 0 and -40 degrees C, cloud water and cloud ice can coexist; t he conversion of cloud water to cloud ice is governed by the depositio nal growth rate of ice crystals whose concentration is diagnosed on th e basis of temperature. After accounting for precipitation, a saturati on adjustment is done to remove either supersaturated vapor or subsatu ration in the presence of cloud drops. In mixed phase conditions, both saturation vapor pressure and the apportionment of condensate into ic e or liquid are on a mass-weighted basis according to the existing amo unts of ice and liquid or on the basis of temperature if there is neit her.