Abstract
Observational and numerical studies have consistently shown the importance of latent heat release associated with frontal precipitation in the development of a Mei-Yu front. However, a systematic evaluation of precipitation parameterization in the simulation of a Mei-Yu front has been rare in the literature. In order to enhance our understanding on precipitation parameterization of Mei-Yu fronts, this study conducts numerical experiments to evaluate the performance of subgrid-scale cumulus schemes and resolvable-scale microphysics schemes to simulate the Mei-Yu frontal system on 4-5 June 1998 at grid resolutions of 45 km and 15 km, using the Penn State/NCAR mesoscale model MM5. Principal findings are summarized here. The horizontal extent and intensity of precipitation, the partitioning off precipitation into grid-resolvable and subgrid-scale portions, the vertical thermodynamic profile in the precipitation region and the embedded mesoscale structure are extremely sensitive to the choice of cumulus parameterization schemes. This is true for both the 45- and 15-km grids. The partitioning of precipitation into subgrid scale and resolvable scale is sensitive to the particular cumulus parameterization that is used in the model, but it is nearly the same on both the 4S- and IS-km grids for a given cumulus parameterization. The detailed ice-phase microphysical processes do not have a significant impact on the rainfall pattern on either the 45- and 15-km grids. However, the inclusion of cloud ice-snow-graupel microphysical processes increases the total surface precipitation amount by 30% compared to the amount with only warm raid processes. Variations in the subgrid-scale cumulus parameterization have a much larger impact on the distribution and amount of Mei-Yu frontal precipitation than do variations in the resolvable-scale microphysics parameterization at mesoscale grid resolutions of 10-50 km.
Highlights
Precipitation is one of the most difficult parameters to forecast in numerical weather pre diction
A systematic evaluation and the improvement in subgrid-scale parameterization across a wide range of grid sizes, in the mesoscale modeling range of 1 0-50 km, represent areas for further research. Both the observational and modeling studies show that deep convection and its associated latent heat release within precipitating systems play important roles in the frontogenetical processes of a Mei-Yu front
The proper treatment of convection and precipitation pro cesses which are responsible for latent heat release is crucial for the success of a numerical simulation of a M ei-Yu front
Summary
Precipitation is one of the most difficult parameters to forecast in numerical weather pre diction. Successful applica tions in mesoscale models have been reported in both semi-prognostic tests (Grell et al, 1991) and fully prognostic tests (Zhang et al, 1989; Kuo et al, 1996) Most of these studies have evaluated one or several CPSs for convective phenomena in specific synoptic environments, mainly midlatitude cases in the continental United States. Kuo et al (1996) assessed the performance of various subgrid-scale cumulus schemes and resolvable-scale microphysics schemes in the simulation of an explosive oceanic cyclone, using the Penn State/NCAR mesoscale model MM5. A series of numerical simulations of the Mei-Yu frontal system on 4-5 June 1998 are conducted using the Penn State/NCAR nonhydrostatic mesoscale model MM5 at grid resolutions of 45 km and 15. We are interested in understanding how different schemes affect the development of a Mei-Yu front, the precipitation distribution and total amount as well as the simulated mesoscale structures
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