A Study Of The Regional Model Dynamic Extended Medium-term Forecasting On Persistent Severe Rainfall In Southern China

Persistent severe rainfall(PSR)in southern China is a major category of meteorological disasters in China,posing a considerable threat to human safety and economic stability.As a result,prolonging the forecast range would be of great importance to enhance our disaster prevention and mitigation capabilities.Based on the analyses of the formation mechanisms for PSR events in southern China,this study is aimed at establishing a theoretical framework of dynamic extended forecasting for the regional Weather Research and Forecasting model,developing the theoretical methodology for the model dynamic forecasting,and putting forward a new method and new schemes to extend the forecast lead time for PSR events.The main results are shown as the following four aspects.Firstly,the dynamic features of PSR events in southern China are fully analysed,covering the large-scale circulation,spatiotemporal distribution of waves,and angular momentum transfer characteristics for different types of PSR events.The results indicate that the occurrence of PSR is closely related to an abnormally stable development of the large-scale circulation,which are produced in the context of the specific weather and climate background.The circulation field and spatiotemporal distribution of waves for different types of PSR events have various distributing features:(1)During the pre-flood season(April to June)in South China,the East Asia major trough(3-5 wave numbers)in the middle latitudes strengthens southwards and interacts with the subtropical high(1-2 wave numbers)at 30°N from 3 days prior to the PSR events.(2)During the post-flood season(June to July)in South China,the weather regime transitions occur on 5 days prior to the PSR events.The 40°N trough(2-4 wave numbers)strengthens southwards and interacts with the subtropical high(1-2 wave numbers),which is also affected by the blocking ridge(3 wave number)in the high latitudes.(3)During the Mei-yu period(June to July)over the Yangtze-Huaihe River basin,the transitions of circulation pattern start on 3 days prior to the PSR events.With the northwest development of the subtropical high,there is a transfer process from long to short waves in terms of energy for the trough at 50 °N.(4)The angular momentum transfer value decreases before PSR,and this change propagates from the higher latitudes to the lower latitudes,indicating the function of the blocking situation in the middle and high latitudes.The increases in northward transport during the process show the importance of the northeast-southwest oriented circulation system for the air stream and water vapour convergence during PSR.Secondly,the model error source and mult-scale forecast evaluation are analyzed and discussed.The global model has an advantage in predicting the large-scale atmospheric variation and the regional model is better in terms of simulating small-scale changes,so our efforts in this study are largely put on achieving a more efficient use of large-scale forecasts of the global model,and retaining simultaneously the small-scale features in the regional model domain,which are the theoretical foundation of dynamic extended prediction implementation.Since the initial-boundary error increases continuously with the model integration for PSR forecasting,leading to shortening the model forecast range,the methods of lateral boundary filtering(LBF),update cycle(UIC),and interior spectral nudging(SN)are then employed for the error reduction for boundary conditions,initial conditions and regional model interior forecasting,respectively.Using the LBF,SN and UIC methods in the regional Weather Research and Forecasting(WRF)model,the simulations for the different types of PSR events over southern China are analysed and verified in terms of rainfall and large-scale circulation.The main conclusions can be summarized as follows:(1)The use of SN leads to clear improvements in precipitation forecasts for the categories above light rain,especially for those at lead times of 3-7 days.For the large-scale circulation,the improvement through using the SN is apparent mainly for the lower-value phases of the anomaly correlation coefficient at lead times of 1-5 days.SN also decreases the root-mean-square error(RMSE)for the geopotential height field,temperature field,relative humidity,and latitudinal-and longitudinal-wind fields,at various heights.(2)Using LBF enables better forecasting of precipitation at lead times of 5-7 days,also produces a better forecast for the temperature field at lead times of 7-11 days,as well as the meridional wind field and geopotential height field above 500 hPa.The improvement in precipitation forecasting benefits mainly from the reasonable simulation of the large-scale circulation fields and the integrated water vapour flux convergence distribution.And(3)the UIC improves the prediction of the PSR events,daily precipitation for moderate,heavy and torrential rains(10-100 mm/day).It also improved the simulative forecasts of the two categories of rains with accumulated precipitation above 50 mm and 100 mm at lead times of 5-11 days.Moreover,the longer the forecast lead time is,the larger the decrease in the Brier score.Additionally,the SN+UIC decreased the root-mean-square error for accumulated rainfall(6.2%)and relative humidity(5.67%).In terms of the geopotential height,temperature fields and wind fields,the UIC achieves better results too;moreover,the longer the forecast time is,the larger the decrease in RMSE.Finally,the newly developed forecasting methods,in which the SN,LBF and UIC are incorporated into a new methodology(SN+UIC+LBF),are employed in the WRF model,to investigate the potential for improving the prediction capability for the rainy season during April to August 2016 in southern China.The results were analyzed and verified in terms of precipitation and the related meteorological variables.The use of the SN+UIC+LBF improves the prediction of the accumulated rain band's range(above 100 mm)for PSR over the Yangtze River Valley from 30 June to 6 July 2016.For the batch test,the improvements of precipitation forecasting by using the SN+UIC+LBF are mainly reflected in the 3-9-day forecasts,with more significant improvement for moderate rains.The SN+UIC+LBF improves the prediction of the rain band's range and accumulated rainfall above 50 mm for the two PSR cases,the longer the lead time,the more obvious the improvement by the SN+UIC+LBF.For the anomaly correlation coefficients above 80%and 60%,the SN+UIC+LBF prolongs the forecast range for circulation fields at 500 hPa(0.5 day).The overall outcomes of the SN+UIC+LBF decreases the RMSE for the geopotential height,relative humidity,temperature and wind fields,at all the altitudes.