Sensitivity Simulation Study of a Severe Rainfall Event in Sichuan Province under the Influence of Complex Underlying Surfaces

The Land surface of the Sichuan Basin is characterized by complexity and diversity， with frequent occurrences of heavy rainfall.This study utilizes global reanalysis data from the U.S.National Centers for Environmental Prediction and various categories of land use data， including default land use data from the WRF model， MODIS （Moderate Resolution Imaging Spectroradiometer）， and USGS （United States Geological Survey）.Additionally， the 2015 LUCC2015 （Land use datasets in China 2015） datasets and the 2015 GLASS （Global Land Surface Satellite） land use datasets are incorporated.The WRF（Weather Research and Forecasting）model is used to simulate a heavy rainfall event in Sichuan Province.The impact of changes in land surface classification and variations in terrain height on heavy rainfall is discussed through four sets of land use experiments and two sets of terrain sensitivity experiments.The results indicate that the experiments involving different land use types have a significant impact on regions experiencing heavy precipitation.In comparison to the MODIS experiment， the precipitation distribution from the USGS experiment is more concentrated， with a larger coverage area for intense precipitation centers； the LUCC2015 experiment results in a reduction in rainfall intensity in the northeastern part of the Sichuan Basin， accompanied by a more concentrated precipitation distribution； the GLASS experiment simulation， characterized by a relatively uniform land use， results in a reduction in the intensity of both intense precipitation centers.Additionally， the precipitation centers in the northeastern part of Sichuan shift southward.Various land use types also exert influence on near-surface meteorological parameter fields.Through a comprehensive analysis of the various land use experiments， it is evident that a reduction in urban built-up areas results in a decrease in 2 m temperature by 0.5 to 1 ℃； the reduction in vegetation coverage results in an increase in 2 m temperature and an enhancement of 10 m wind speed； the decrease in surface roughness leads to a significant enhancement in 10 m wind speed， with a magnitude of change ranging from 2 to 4 m·s-1.Compared to the default land use types in WRF， the simulated results under the underlying surface types in the LUCC2015 experiment are better.The topography exerts a pronounced influence on heavy rainfall.Following the reduction in elevation of the western mountainous region in the basin， the absence of mountain barriers allows for a more abundant presence of lower-level water vapor and energy.Consequently， moisture and energy can be transported to more northern regions of Sichuan.With the reduction in elevation of the terrain， the low-level airflow intensifies convergence ahead of the mountains， triggering stronger upward motion of air and resulting in enhanced precipitation intensity.This phenomenon leads to a westward shift in the precipitation location and a more concentrated coverage of rainfall.Conversely， with the uplift of the terrain， the mountainous barrier impedes the entry of warm and moist airflow from the south.As a result， energy and moisture become more dispersed， causing a reduction in airflow convergence in the western and eastern parts of the basin.The weakening of low-level airflow ascent leads to a decrease in precipitation intensity and a more dispersed distribution of rainfall.