GRAPES-Meso?3km Model Precipitation Forecast Deviation And Its Causes In Southwest Complex Topography Of China

Based on the daily(08:00-08:00 BT)and hourly precipitation observation data in complex terrain area of southwest China in the summer of 2019,the performance and deviation of the short-term precipitation forecast(within 36h)of GRAPES-Meso?3km model has been carefully evaluated in term of the rainfall amount and frequency,in view of different typical landform areas such as the Sichuan Basin(area A),the Yunnan-Guizhou Plateau(northern area B and southern area C)and the highland over the eastern edge of Tibet Plateau(area D).Results show that:(1)The spatial distribution of average daily rainfall amount(frequency)in summer over the complex terrain area of Southwest China and the intra-day scale unimodal/bimodal characteristics of different geomorphic areas are reasonably described by the GRAPES-MESO?3km model,as well as the basic relationship between frequency and intensity of hourly precipitation.(2)The daily rainfall amount(frequency)forecast of the model shows a clear positive deviation on average,and the forecast value is 1.3-2.1 times(1.1-1.3 times)of the observed value in 4 subregions,and the deviation is most prominent in the basin.The positive deviation of daily precipitation in 4 subregions is mainly caused by more rainfall amount forecast in heavy rain days.The positive deviation of frequency in the basin and the highland over the eastern edge of Tibet Plateau is mainly caused by more heavy rain days forecast,however,in the north and south subregions of the Yunnan-Guizhou Plateau,it is caused by more moderate and light rain days in the model.The forecast area of heavy rain(moderate and light rain)has obvious(slight)tendency to be large,and the frequency of larger heavy rain(moderate and light rain)area forecast in 4 subregions is 53.6%-82.8%(22.6%-37.6%).(3)As for daily cycle,the forecast of hourly rainfall amount(frequency)at each time is generally larger,the main positive deviation appears over the observed peak period of night rain.At areas that altitude down of 1200 m,the positive deviation of frequency stretches from the observation night peak to noon,which is consistent with the weaker forecast of heavy rain intensity in the relationship between frequency and intensity of hourly precipitation,indicating that the strong daily rainfall amount forecast is usually manifested as a longer precipitation duration within the day or the hourly precipitation vacant forecast.(4)Diagnostic analysis shows that deviation of terrain height expression is not the main reason of the precipitation forecast deviation in study area,and higher forecast of summer average daily rainfall amount(frequency)in Sichuan Basin is mainly caused by the combination of model's stronger forecast of southwest wind in lower troposphere(south-southeast of Yunnan-Guizhou Plateau)and the special terrain in southwest China.As a result,a vortex and water vapor convergence field are superimposed on the model atmosphere in the lower troposphere in Sichuan Basin,which results in the development of cyclonic vortices and water vapor convergence in the lower layer in Sichuan Basin,and finally leads to the increase of rainfall amount or precipitation events predicted by the model in the basin.