Impact of horizontal resolution on the regional climate simulations of the summer 1998 extreme rainfall along the Yangtze River Basin

In this study, the effects of varying horizontal grid resolutions on the regional climate model (RCM) simulation of the summer 1998 extreme rainfall events along the middle to lower reaches of the Yangtze-River Basin (YRB-ML) are examined using the Pennsylvania State University-National Center for Atmospheric Research mesoscale model (i.e., MM5). Results show that the MM5 simulation with the finest grid size of 4 km reproduces reasonably well the rainfall intensity and rainbelt distribution over the YRB-ML during all the Meiyu phases, whereas lower-resolution simulations tend to shift the east-west-oriented major rainbelt southwestward as a result of the generated local storms and excessive rainfall during the second Meiyu rainy phase in which vertical (horizontal) wind shears are weak (strong). The excessive rainfall occurs during the evening hours when the low-level jet is relatively strong and in the Meiyu frontal zone where convergence of warm and moist air is pronounced. Results reveal the importance of incorporating convective parameterizations in removing potential instability in heavy rain-producing storms even for RCMs with 3-5 km horizontal resolutions. It is highly desirable to use higher-resolution models with realistic parameterized and grid-scale cloud physics schemes to study the regional climate of heavy rainfall events.