Prediction of the Diurnal Variation of Summertime Precipitation Over the Sichuan Basin by a Regional Model

The nocturnal precipitation over the Sichuan Basin (SCB) exhibits obvious spatial variation, most of the existing numerical research of precipitation event over the SCB is based on case studies. In this study, the long-term climatological evaluation of the prediction of diurnal spatio-temporal evolution of precipitation and relevant physical mechanism over the SCB by 9 and 3 km operational numerical weather prediction model is carried out. It is found that the SWC-WARMS model with 9 km grid spacing overestimates the summer precipitation, especially over the western slopes of the SCB. The northeastward timing delay of the precipitation peak over the SCB is well reproduced in the model, with predicted precipitation peaking at midnight over the southwestern SCB and in the early morning over the northeastern SCB. However, the predicted diurnal peak of precipitation appears 2-3 hr earlier than the observed peak. Increasing horizontal resolution to 3 km reduces the wet bias and improves the diurnal phase of precipitation. Compared to reanalysis, the predicted easterly anomalous wind appears earlier and the anomalous wind speed is stronger. Moreover, the predicted atmosphere is too unstable, which is associated with "warm below and cold above" vertical biases in temperature and wet biases in specific humidity. Thus, model precipitation peaks too early in the day over the SCB, and is overestimated. The weakening of the predicted southwesterly flow in the middle troposphere weakens the northeastward propagation of precipitation after midnight over the SCB. The most significant improvements are found for the diurnal variation of low-level anomalous wind speed in the 3 km resolution model, contributing to a more realistic representation of precipitation over the SCB. Plain Language Summary Although it is known that the nocturnal precipitation is dominated over the Sichuan Basin (SCB), the diurnal spatial variation of precipitation and the associated atmospheric circulation based on hourly long-term climatological prediction are not well understood. Our results show that the SWC-WARMS model can reasonably reproduce the precipitation on both daily and hourly time scales in summer, although the model overestimates the precipitation and peaks earlier than observation over the SCB. In general, the model with higher resolution produces a more realistic representation of precipitation than the coarse resolution. Further analysis suggests that the overestimation of precipitation over the SCB is attributed to the stronger low-level easterly anomalous wind, much more moisture supply, and intensified atmospheric instability in the model than in ERA5. In addition, the predicted earlier easterly anomalous wind contributes to the earlier peak of precipitation in the day over the SCB. Findings of these basic model biases may be helpful to further understanding of the main cause of nocturnal peak of precipitation over the SCB, and provide a promising way forward for improvement of the model physical processes.