EXPLICIT FORECASTING OF SUPERCOOLED LIQUID WATER IN WINTER STORMS USING THE MM5 MESOSCALE MODEL

An explicit microphysical parametrization including ice physics was de veloped for use in the NCAR/Penn State Mesoscale Model Version 5 (MM5) . This scheme includes three options of increasing complexity to repre sent the hydrometeor species. The scheme is evaluated by comparing mod el simulations with two well observed winter storms that occurred duri ng the Winter Icing and Storms Project. The evaluation focused on the prediction of supercooled liquid water (SLW), which is of particular i mportance to aircraft icing. The intercomparisons showed that: 1. The double-moment microphysical scheme, in which both ice mixing ratios an d number concentrations were predicted, performed best, with close agr eement to the observed fields. 2. The single-moment schemes, in which the mixing ratio of ice species are predicted and number concentration specified, performed reasonably well if a diagnostic equation for N-o ,N-s, the Y-intercept of the assumed exponential snow distribution, is allowed to vary with snow mixing ratio. 3. Accurate microphysical sim ulations of SLW in shallow upslope clouds and cyclonic storms required accurate simulations of the kinematic and thermodynamic structure and evolution of the storms. Though the two storms were dynamically diffe rent, the SLW formed through a balance of the condensational growth of cloud water and the depletion of cloud water by deposition and riming of snow and/or graupel for both storms. The results of this study sug gest that accurate prediction of SLW over limited areas of the country may be possible using the current microphysical parametrization and h igh-resolution grids (delta x < 10 km).