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ABSTRACT
Current cloud microphysical schemes used in cloud and mesoscale models range from simple one-moment to multimoment, multiclass to explicit bin schemes. This study details the benefits of adding a fourth ice class (frozen drops/hail) to an already improved single-moment three-class ice (cloud ice, snow, graupel) bulk mi- crophysics scheme developed for the Goddard Cumulus Ensemble model. Besides the addition and modification of several hail processes from a bulk three-class hail scheme, further modifications were made to the three-ice processes, including allowing greater ice supersaturation and mitigating spurious evaporation/sublimation in the saturation adjustment scheme, allowing graupel/hail to transition to snow via vapor growth and hail to transition to graupel via riming, wet graupel to become hail, and the inclusion of a rain evaporation correction and vapor diffusivity factor. The improved three-ice snow/graupel size-mapping schemes were adjusted to be more stable at higher mixing ratios and to increase the aggregation effect for snow. A snow density mapping was also added.
The new scheme was applied to an intense continental squall line and a moderate, loosely organized continental case using three different hail intercepts. Peak simulated reflectivities agree well with radar for both the intense and moderate cases and were superior to earlier three-ice versions when using a moderate and large intercept for hail, respectively. Simulated reflectivity distributions versus height were also improved versus radar in both cases compared to earlier three-ice versions. The bin-based rain evaporation correction affected the squall line more but overall the agreement among the reflectivity distributions was unchanged. The new scheme also improved the simulated surface rain-rate histograms.
1. Introduction
Atmospheric cloud modeling has benefited immensely from the continued improvement in computational power. Simulations using explicit spectral bin microphysics (SBM) with large 3D domains in mesoscale models like the Weather Research and Forecasting (WRF) Model (Michalakes et al. 2004; Skamarock et al. 2008) can now be performed (Iguchi et al. 2012a,b). In addition to higher resolution (e.g., Khairoutdinov and Randall 2006)and the advent of multiscale modeling frameworks (MMFs; Randall et al. 2003; Tao et al. 2009), cloud-resolving simulations have also benefited in the form of ever more sophisticated microphysics. Simple bulk liquid (e.g., Kessler 1969) and ice schemes (e.g., Wisner et al. 1972) with only a few categories have grown into...