Improving Precipitation Forecasts by the Operational Nonhydrostatic Mesoscale Model with the Kain-Fritsch Convective Parameterization and Cloud Microphysics

The Japan Meteorological Agency has operated a nonhydrostatic mesoscale model (MSM) of 5-km horizontal grid spacing since March 2006. The development of the moist process scheme to improve precipitation forecasts is continued because the major purpose of the operational MSM is to prevent natural disasters such as torrential rain. Since the horizontal grid spacing of 5 km is too coarse to resolve cumulus convection, the pre-operational MSM with only cloud microphysics sometimes produced too narrow and too intensified precipitation. To eliminate this weakness, the Kain-Fritsch (KF) convective parameterization is adopted simultaneously with cloud microphysics to the operational MSM. The KF scheme has been originally developed for the Weather Research and Forecast modeling system and implemented to the MSM with Dr. Kain's consent in April 2002. The original KF scheme applied to the humid climate area of Japan and surrounding sea sometimes failed to initiate its parameterized convection when the lowest atmosphere was wet and dynamical forcing was weak. Some modifications to calculation of precipitation and triggering of convection were added and parameters were tuned to improve precipitation forecasts by the MSM. As a result, the forecast skill was quite improved compared to the pre-operational MSM from the viewpoint of quantitative precipitation forecasts. This talk presents the treatments and characteristics of precipitation forecasts by the MSM and the results of some modifications of the KF scheme.