The influence of cumulus parameterization on model forecasts of rapid oceanic cyclogenesis

Abstract

Numerous studies have left little doubt that latent heat release (LHR) can significantly influence many features of extratropical cyclone systems. Recent experiments with the NCAR/PSU mesoscale model indicated that not only did different moisture parameterizations significantly affect model forecasts, but that forecasts for different cyclonic systems responded very differently to the parameterizations. Model output from the NCAR/PSU model is examined for two cyclonic systems, with four different parameterization experiments used to produce four forecasts for each system. The output was then examined in three and four dimensions to qualitatively and quantitatively determine the direct and indirect effects of latent heat release on model output. The results clearly showed the difference in the general dynamics of the two cyclones. One demonstrated a very strong reliance on diabatic processes for its early development, becoming more adiabatic late in its history, while the other initially developed very adiabatically and became more diabatic after about the mid-point of the forecast period. The cyclonic system that started out diabatically was more sensitive to moisture parameterization. The models clearly showed the differences between precipitation fields generated by the different parameterizations. In particular, allowing evaporation of precipitation in non- saturated layers greatly decreased the areal extent of light precipitation, while having minimal effect in areas of heavy precipitation.