The Precipitation Characteristics Of Rain Belt And Evaluation Of The Operational Precipitation Forecast Model Over The Yangtze-Huaihe River Valley

Continuous heavy precipitation often occurs in the Yangtze-Huaihe River Valley,and flood disasters are easy to form during Meiyu period.Affected by various underlying surface,monsoon circulation and the interaction of multi-scale systems,the Meiyu precipitation process is extremely complex and the Yangtze-Huaihe River Valley is one of the regions with prominent uncertainty of numerical forecast.Strengthening the understanding of temporal–spatial characteristic of precipitation and forecast deviation can be helpful not only the development and evolution of the application of operational numerical weather forecast models,but also the improvement of physical process.Based on the observation analysis and the evaluation of forecast,the performance of the operational numerical forecasting models for the spatial location and temporal evolution of precipitation in rain belt have been analyzed and the correlation of mode prediction deviation with large-scale circulation have been discussed.The main conclusions are summarized as follows:1)Based on observation datasets the precipitation characteristics of large-scale rain belt have been analyzed.The rain belt is distributed quasi–east–west,and the large precipitation center is distributed over the middle-lower reaches of the Yangtze River Valley.The analysis show that the rain belt is quasi-east-west and its large precipitation center is located in Dabie Mountain and southern Anhui Mountain.The precipitation has two diurnal peaks,one is in the early morning and the other in the late afternoon.The diurnal peak of precipitation frequency occur in the afternoon,and that of precipitation intensity occur in the early morning.According to the morphology,rain belts were divided into two types: zonal and meridional.It can be seen that there are obvious differences in precipitation characteristics between two types of rain belts.The days of the zonal rain belt accounted for 81 % of the total precipitation process,which mainly occurred from mid-to-late June to mid-to-early July.The precipitation center of the zonal rain belt is consistent with the characteristics of total precipitation.The diurnal peaks of precipitation amount,frequency,and intensity all appear in the early morning.The meridional rain belt accounts for about 19%.Moreover,the precipitation center is mainly located in the eastern coastal area,and diurnal peaks of precipitation amount,frequency and intensity are all in the afternoon.The warm anomaly presents a baroclinic structure of "warm upper and cold lower" in the entire troposphere.The circulation structure of the zonal rain belt presents a "cold–warm–cold" temperature structure.The position of the subtropical high is more western than climate.The lower southwest wind converges over the Yangtze-Huaihe River Valley.In contrast,the circulation structure of meridional rain belt shows a "cold–warm" structure.The low-level circulation shows the abnormal northerly wind in the east and southern wind in the west over the Yangtze-Huaihe River Valley.2)The performance of the ECMWF global high-resolution products(ECMWF)in reproducing rain belt has been analyzed.The zonal rain belt is quasi–southwest–northeast,and the center of large precipitation is located in southern Anhui Mountain in observation.The ECMWF forecast the rain belt is quasi–east–west,and the center of large precipitation accurately which is consistent with observation.The ECMWF reproduces the northward movement of the zonal rain belt but overestimates precipitation in every month during warm season.The forecast shows a large bias in diurnal cycle of precipitation frequency,while the bias is small in precipitation intensity.The results show that the ECMWF tends to have a northward and westward shift.The ECMWF has a larger rainfall area and a steeper slope than that in the observation.The ECMWF can reasonably reproduce the baroclinic structure of "warm upper and cold lower",but it slightly overestimates the high-level abnormal warm center and underestimates the low-level cold center.The 500 hPa anticyclone circulation is abnormally stronger and the low-level southwest wind is stronger,water vapor can be transported to the northern,corresponding to the north of the rain belt.3)Compared the performances of the ECMWF and the regional forecast product(GRAPES?3km)in simulating the extreme heavy precipitation proces.The ECMWF overestimates precipitation amount and precipitation frequency with the largest bias in the afternoon.However,it underestimates the precipitation intensity.The ECMWF overestimates the frequency of weak precipitation and underestimates the frequency of heavy precipitation.The GRAPES?3km forecast reasonably reproduce the structure of intensity–frequency and diurnal variation of precipitation,and the forecast overestimation deviation in the magnitude.The ECMWF forecast the position of the rain belt is more northern and western while the GRAPES?3km forecast towards south.Further analysis reveal that the zonal location of the rain belt is highly correlated with the intensity of low-level jet.The monsoonal rain belt moves more northward when the low-level jet is strengthened,and visa-versa.Forecast deviation of south wind is mainly occur at night.