一种改进的降水临近外推预报技术方法研究及效果检验

An Improved Method of Precipitation Nowcasting Extrapolation Forecasting and Its Verification

中尺度模式对于0~2h的预报存在起转问题,因此外推预报成为0~2h临近预报中不可或缺的技术方法。目前INCA(Integrated Nowcasting through Comprehensive Analysis System)降水外推预报中,外推矢量扰动由外推矢量随时间的变化确定,即相邻时次外推矢量差确定,本文统计2017年7月江淮地区(29°~35.5°N,114°~120°E)INCA系统相邻10min外推矢量偏差来代表外推矢量随时间变化。若降水向东北方向移动,则外推矢量方向规定为东北方向。统计结果表明:①对于江淮地区,外推矢量以东北方向和西南方向偏差为主,外推矢量越大,其偏差的大小越大;②从外推矢量方向看,东北方向外推矢量占绝大多数(73%),可能是因为绝大多数降水发生在槽前形势下,引导气流为西南风。基于外推矢量偏差统计样本,随机生成多个符合外推矢量偏差分布的外推矢量扰动,得到多个不确定的外推矢量,将INCA确定性外推预报变为0~2h降水外推集合预报,采用均方误差(MSE)、TS评分、BIAS评分和Brier技巧评分等方法,对集合预报结果检验表明:随着预报时效增加,集合预报比确定性预报的优势更明显。因此,考虑外推矢量的不确定性可以提高降水外推预报准确率。

Mesoscale models have the spin-up problem in nowcasting(0 to 2 hour forecast),so extrapolation forecast has become an indispensable technical method for nowcasting.At present,the extrapolation vectors in INCA(Integrated Nowcasting through Comprehensive Analysis system)precipitation extrapolation prediction are deterministic and do not change with forecast time,which results in that the velocity and direction of precipitation movement do not change with the forecast time,which is inconsistent with the reality.It is necessary to consider the variations of extrapolation vectors with time and to make them as the disturbances of extrapolation vectors adding to deterministic extrapolation vectors.Extrapolated vector perturbations are determined by the variations of extrapolated vector with time,which is the deviation of extrapolated vectors at adjacent times.The adjacent 10-minute extrapolated vector deviation during July 2017 of the INCA system is statistically calculated to represent the variation of extrapolated vectors with time.Extrapolation vectors are the current and future precipitation moving speed and direction.For easy understanding,if precipitation moves northeast,this paper stipulates that the direction of extrapolation vectors is northeast.The statistical results over the Changjiang-Huaihe Region demonstrate that the deviations are mainly in southwest and northeast.The larger the extrapolation velocities are,the greater the deviations velocities will be.Vast majority of precipitation occurs in front of troughs,so northeast extrapolation vectors account for 73%.Based on the statistical samples of extrapolated vector deviation,several extrapolated vector perturbations are generated randomly based on the distribution of extrapolated vector deviation,and several uncertain extrapolated vectors are obtained.The deterministic extrapolated prediction of INCA is changed to 0-to-2 hour extrapolated ensemble precipitation prediction.The ensemble forecast effects are verified by the Mean Square Error(MSE),TS score,BIAS score and Brier Skill score.The results demonstrate that the results of ensemble forecast are better than that of deterministic forecast with increasing forecast time.Therefore,the accuracy of precipitation extrapolation prediction can be improved by considering the variations of extrapolation vectors with time.