Effects of Convection Representation and Model Resolution on Diurnal Precipitation Cycle Over the Indian Monsoon Region: Toward a Convection-Permitting Regional Climate Simulation

摘要

This study investigated the reproducibility of rainfall characteristics of the Indian summer monsoon using a regional climate model (RCM), focusing on the convection representation and model horizontal grid resolution. To understand the importance of convection representation and model horizontal grid resolution, the study employed the Weather Research and Forecasting model WRFv3.8.1 as the RCM. Two series of experiments, that is, cumulus parameterization on/off (CON/COFF), were performed for the monsoon with three different spatial resolutions. Three contrasting monsoon years were selected for the simulations. The simulated rainfalls were evaluated with the India Meteorological Department data set and Tropical Rainfall Measurement Mission 3B42 Version 7. The results revealed that COFF captured fundamental aspects of the rainfall characteristics, such as diurnal cycle, rainfall intensity, and rainfall frequency. Over central India, in COFF, the diurnal precipitation peaks were simulated in afternoon/evening in close agreement with the observations. However, the diurnal precipitation peak in CON appeared 3-6 hr earlier than the observations. The diurnal precipitation variation was more associated with the horizontal grid resolution of the numerical model than representation of convection. Additionally, COFF could simulate diurnally propagating rainfall systems over the Bay of Bengal. Over the Indian land area, COFF simulated less-frequent intense precipitation systems, whereas CON simulated more-frequent widespread weak precipitation. This study demonstrated that convection representation is very important for simulation of diurnal rainfall systems over the Indian monsoon region, although refinement of model horizontal grid resolution is required over mountainous regions, where precipitation is closely related to orography.