An Evaluation of the Influence of Several Convective Parameterization Schemes on a Real Time Turbulence Model (RTTM) in Weakly-Forced Environments

Abstract

The purpose of this study is to improve the convective forecasts of weakly forced environments within the Real Time Turbulence Model (RTTM). Focusing on the southeastern portions of the United States, due to the amount of NASA flight missions conducted in that region, four cases are simulated using the Kain-Fritsch, Grell, and Kain-Fritsch 2 cumulus parameterization schemes. Three of the four cases have weak synoptic forcing with limited, sparse precipitation coverage and one case has strong synoptic forcing with concentrated, heavy precipitation coverage. It is usually within the weak synoptic forcing environments that the currently used Kain-Fritsch scheme greatly overestimates the precipitation amounts. Verification of model precipitation against satellite, radar and surface observations was performed in order to statistically assess the impact of the Kain-Fritsch, Grell, and Kain-Fritsch 2 schemes for the four cases. Then, a comparison of the performance of the schemes and an examination of their influence on the local environment is conducted. We found that the Kain-Fritsch 2 scheme more accurately depicts the precipitation coverage in the weak forcing environments due to modifications made to the cloud model and trigger function, which involve alterations to the CAPE calculation and precipitation efficiency relationship and the inclusion of a minimum environmental entrainment rate and variable cloud radius. By strengthening the cloud model's dependence on deep layer relative humidity the modifications reduce the amount of precipitation produced in a weakly forced environment while having a minimal effect on a strongly forced environment.