基于天气雷达资料的外推预报是灾害天气临近预报的基础， 选取4次强降水过程分析了外推预报的误差。主要分析方法包括3个步骤： （1）采用多尺度回波跟踪方法确定雷达回波的运动场； （2）采用半拉格朗日平流方案对雷达回波进行外推； （3）预报结果和观测结果进行对比。利用去相关时间方法分析了雷达回波的可预报性， 利用预报技巧评分和相对绝对误差对外推预报的误差进行了定量分析。此外， 还分析了外推预报的误差与尺度之间的关系， 以及外推预报中的不确定因素——回波强度变化和回波运动场变化在预报误差中的相对重要性。这4次强降水过程的误差分析表明， 预报误差随预报时效的变化基本上是以指数规律递减的， 大尺度的降水系统对应较长的持续性， 对于发展演变较快、 尺度较小的风暴， 其持续性较短。
At present, the extrapolation forecast based on radar echoes is the mainstay of the disaster weather nowcasting. Four heavy precipitation events were selected to analyze the errors of radar echo extrapolation technique by a three-step procedure. Firstly, the motion field of precipitation is determined by multi-scale tracking radar echoes by cross-correlation (MTREC) scheme. Secondly, radar reflectivity is adverted by means of a semi-Lagrangian advection scheme assuming stationary motion. Thirdly, the forecasts are compared to observations to calculate the lifetime and other measures of predictability. The procedure is repeated with images that have been decomposed according to scales to describe the scale dependence of predictability. Based on the four heavy precipitation events, it is showed that forecast skill decreases with lead time by an approximate exponential law. Large-scale precipitation corresponds to the long persistence, for the evolution of faster, smaller scale storm, its persistence is shorter.
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