Precipitation parameterization in a simulated Mei-Yu front

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 of 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, bat it is nearly the same on both the 45- and 15-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 rain 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.