Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model

Abstract

An explicit microphysical parametrization including ice physics was developed for use in the NCARPenn State Mesoscale Model Version 5 (MM5). This scheme includes three options of increasing complexity to represent the hydrometeor species. The scheme is evaluated by comparing model simulations with two well observed winter storms that occurred during the Winter Icing and Storms Project. The evaluation focused on the prediction of supercooled liquid water (SLW), which is of particular importance to aircraft icing. The intercomparisons showed that:1. The double-moment microphysical scheme, in which both ice mixing ratios and number concentrations were predicted, performed best, with close agreement 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 No,,, the Y-intercept of the assumed exponential snow distribution, is allowed to vary with snow mixing ratio.3. Accurate microphysical simulations 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 different, 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 andlor graupel for both storms. The results of this study suggest that accurate prediction of SLW over limited areas of the country may be possible using the current microphysical parametrization and high-resolution grids (6x < 10 km).A more direct approach to forecasting SLW with a forecast model would use the model-generated vertical motion field to produce SLW and the ice crystal fields to deplete the SLW. This approach has been extensively used in cloud models based on explicit microphysical schemes (Lin et al. 1983), with relatively good success for convective, summertime storms (Murakami 1990). Winter storms, however, typically require largerscale models for proper simulation, including the incorporation of time-dependent lateral boundary conditions and synoptic-scale forcing. The recent studies by Kuo and Low-Nam (1 990), Stewart (1992), Bruintjes et al. (1994), Modica et al. (1 994), Stewart et al. ( 1 995), Rasmussen et al. (1993, and Tremblay et al. (1996) have shown good success in simulating the dynamics of winter storms using mesoscale models. The simulation of precipitation and SLW using explicit microphysical schemes in mesoscale models, however, has received less attention.In this paper we present a microphysical parametrization with three options of hydrometeor representation based on previous schemes and some new aspects. Using this parametrization, we evaluate the performance of the National Center for Atmospheric Research (NCAR)Penn State Mesoscale Model Version 5 (MM5) in simulating SLW in winter storms over the continental US, using data from two well-studied storms during the Winter Icing and Storms Project (WISP, R...

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