Bias Attribution of the Diurnal Variation of Regional Precipitation Events over the Middle and Lower Yangtze River Basin in Summer Predicted by the Shanghai Regional Dynamical Forecast System

Using hourly rain gauge observations and ERA5 reanalysis, this study evaluates the performance of the Shanghai Regional Dynamical Forecast System (WRF_CFSv2) at lead times of 13–36 h in forecasting the diurnal variation of summer regional precipitation events (RPEs) over the middle and lower Yangtze River Basin (MLYRB) and investigates the possible causes related to model biases. The results indicate that WRF_CFSv2 can well reproduce the diurnal phase of precipitation amount of RPEs despite the magnitude overestimation throughout the day. The observed occurrence of RPEs and corresponding atmospheric circulation can also be reasonably captured. However, the WRF_CFSv2 exhibits a systematic overestimation of the intensity, occurrence, and spatial coverage of RPEs. Further analysis attributes these biases primarily to the overestimation of dynamic factors, with thermal factors playing a secondary role. Dynamically, the WRF_CFSv2 produces a persistent anomalous low-level cyclone over the MLYRB which leads to the overestimation of the low-level southwesterly winds and water vapour flux convergence (WVFC), acting as the dominant driver for the overestimated nocturnal RPEs. Thermally, a warm bias in the predicted daytime surface conditions leads to a more unstable lower troposphere and strengthened convective available potential energy (CAPE). Rather than acting independently, this overestimated daytime instability combines with strong WVFC and evaporation, functioning as a synergistic factor that culminates in the overestimated daytime RPEs. By evaluating the capability in predicting RPEs and elucidating the sources of model biases, this study provides a scientific basis for optimizing and modifying the physical parameterization schemes of WRF_CFSv2 in future.