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ABSTRACT
This is the fourth in a series of papers exploring the effects of the number of predicted moments in bulk microphysics schemes. In Part III, the three-moment version of a new multimoment scheme was used to simulate a severe hailstorm. The model successfully reproduced many of the observed gross characteristics, including the reflectivity structure and the maximum hail sixes at the ground. In this paper, the authors compare a series of sensitivity experiments using various one- and two-moment versions of the scheme with the three-moment version to explore the effects of predicting additional moments on the simulated hydrometeor fields, precipitation, and storm dynamics.
Six sensitivity runs were performed. They varied in their ability to reproduce the precipitation pattern, storm structure, and peak values of microphysical fields of the control simulation. The two-moment simulations, which used diagnostic relations to prescribe the relative dispersion parameter, α, closely reproduced the spatial pattern, quantity, and phase of the precipitation at the surface as well as the overall storm structure, propagation speed, and peak values of several hydrometeor fields. The two-moment simulations, which used fixed values of α, on the other hand, differed more from the control. The runs using one-moment versions of the scheme were considerably different from each other and were poor at reproducing the control simulation.
The results suggest that there is a dramatic improvement in the simulation moving from one- to two-moment schemes. For the case studied, it was found that if maximum particle size is not of concern, a two-moment scheme with a diagnostic dispersion parameter can reproduce most of the important aspects in a hailstorm simulation with a three-moment scheme.
(ProQuest Information and Learning: ... denotes formulae omitted.)
1. Introduction
With continuous increase in computational power, operational numerical weather prediction (NWP) models are quickly moving toward convective-scale resolutions. Bulk microphysics schemes, which are employed when clouds are fully resolved, are thus playing an increasingly important role in both operational NWP and in research. It has been shown that in general, there is an increase in simulation skill as the complexity of the microphysics scheme increases, particularly for the inclusion of the ice phase (Cotton and Anthes 1989). For example. McCumber et al. (1991) showed an improvement in simulating a tropical squall line...