18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Wednesday, 1 August 2001: 2:15 PM
The sensitivity of modeled supercell storm dynamics to several cloud microphysical parameters
Susan C. Van den Heever, Colorado State University, Fort Collins, CO; and W. R. Cotton
Poster PDF (106.1 kB)
Mesoscale numerical models have been successfully used to enhance our understanding of supercell storm dynamics, however, the impact of cloud microphysics on severe storm dynamics has received relatively little attention. Simulations of classic type supercells appear regularly in the literature, whereas simulations of high-precipitation and low-precipitation type supercells, where microphysical processes may play a more important role, appear much less frequently. The difficulties encountered in the latter cases may, in part, be due to the microphysical parameterization schemes being utilized. Many severe storm simulations have been performed using simplified microphysical schemes, generally as a result of computer constraints. For example, ice processes, which may have a significant impact on the thermodynamics of a developing supercell, have often been completely excluded, even though the implications of this exclusion are not well understood. The aim of the work presented here is to examine the sensitivity of supercell storm dynamics to several microphysical parameterization options.

The mesoscale model chosen to conduct this study was the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University. A single grid with 1 km grid spacing in the horizontal and variable spacing in the vertical was used. The model was horizontally homogeneously initialized using a sounding that is representative of severe storm days over Oklahoma. A warm, moist bubble was used to initiate convection. The bulk microphysical species include vapor, cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail. The simulations were run out for two hours.

Several tests have been conducted to evaluate the sensitivity of supercell storm dynamics to the type of microphysical scheme used, to variations in the mean hail and rain diameters, and to the exclusion of ice processes. Within most microphysical parameterization schemes, several parameters are assigned values somewhat arbitrarily as a result of the lack of microphysical measurements for many storm systems. One such parameter is the mean diameter of a species. In the first series of tests, simulations were performed in which the mean hail diameter was varied from 3mm to 4cm, and the mean rain diameter from 1mm to 5mm. In the second series of tests, all ice processes were turned off in order to evaluate the effect of excluding ice species. Finally, it is sometimes argued that the more complicated a parameterization scheme, the better the scheme performs. The two-moment microphysical scheme was used in place of the single-moment scheme in the third series of tests to assess the validity of this hypothesis. The results of these sensitivity tests show significant effects on numerous aspects of supercell storm dynamics. These results will be presented.

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