Study On Extreme Heavy Precipitation Forecast In China Based On The Ensemble Forecast System

As extreme heavy precipitation is a small probability event and occurs with agreat deal of uncertainties, it is very difficult for accurate prediction. On the basis ofdaily precipitation data of756stations in china, the extreme precipitation thresholdvalue were defined in different stations according to percentile method, variationcharacteristics of extreme precipitation events in different8areas of china werestudied.Then, according to the principle of the Anderson-Darling test, a method forextreme heavy precipitation forecast based on the ensemble forecast data wasstudied,and the mathematical model(Extreme precipitation Forecast Index) based onthe Continuous differences of the cumulative distribution functions (CDF) betweenmodel climate and the EPS forecast was established. Extreme precipitation ForecastIndex （EPFI）was applied to forecast experiment for the several extreme heavyprecipitation events happened during17-31July2011, the effect of forecast wasevaluated. Finally, the method for continuous heavy rain forecast was studied accordingto the principle of EPFI forecast research.Then, five continuous heavy rain processhapped in jianghuai area were assessed, and the results were compared with T213control forecast, the main conclusions are as follows:(1)The extreme precipitation frequency, intensity and extreme precipitation in the totalprecipitation show increase trends in the last50years. However, it is found that thetrend change of EPE showed remarkable regional differences. The extremeprecipitation frequency and intensity show decreased trends In Northeast China andnorth China. However, there is an opposite trend in western part of Northwestchina, the Mid-low Reaches of the Yangtze river south china and Tibet plateau. Theprecipitation extremes and extreme precipitation in the total precipitation showdecreased trend in eastern part of northwest china while the extreme precipitationfrequency and intensity increased, the extreme precipitation changed little insouthwest china.The extream precipitation of Beijing and Wuhan was stronger thanthe regional average ones.(2) The extreme heavy precipitation distribution of model is very close to the observational one, while the model extreme heavy precipitation is weaker,and theextreme heavy precipitation decreases with the extension of forecasting time. Therewas a certain deviation to the observational one: the high of extreme heavyprecipitation of Guangdong was partial to the Jiangsu, Zhejiang and Fujian;Southern Tibet had a false value center.(3) The result shows that EPFI can take advantage of the tail information of the CDFbased on the EPS, providing scientific and rational forecast for extreme heavyprecipitation. The EPFI based on CMA T213EPS can give early warning onextreme heavy precipitation3to7days in advance. With the forecast time extending,EPFI was less skillful.(4) By comparing the CDF of two model climate datasets, we find that the forecast skillof the CDF that contains historical model precipitation information from June toAugust is better than the CDF that contains only the July precipitation information.China’s rainy season has certain continuity (mainly distributed in June-August),Thus, it is better for the model climatic CDF to include objective precipitationinformation as much as possible.(5) The evaluation criteria of the regional extremely heavy rainfall in Jianghuai wasgiven by Comprehensiving variety of continuous heavy rain definition. the resultssuggest that T213control forecast can forecast continuous heavy rain the2-7days inadvance, the average forecast ability was5.4days. But forecasting ability isassociated with different processes. the forecast is skillful in high rainfall process,vice versa.(6) EPFI prediction ability is significantly higher than the control forecast, forecastability have improved the process of five times, especially for weaker in the processof continuous heavy rain forecast to improve more. On June1,2011-30, persistentheavy rain forecast experiments, four inspection index assessment shows that EPFIforecast skill is higher than T213control of extreme heavy precipitation forecast,but EPFI forecast has a higher false alarm rate.