CFSv2对2018年浙江梅雨降水多尺度预测的性能评估

从梅雨预测的业务需求出发,系统开展了CFSv2模式对2018年浙江梅雨期降水预报能力的多时间尺度评估。结果发现3月1日—5月31日的起报结果整体上未能较准确地预测6月浙江大部降水偏少的趋势、仅5月31日的预测结果与实况相符;在延伸期尺度上,CFSv2预测的梅雨期总降水量较实况偏少30%左右;基于相关系数、均方根误差和新定义的综合预报技巧指数等指标分析模式的延伸期预报性能,发现对梅雨期总降水量、逐日区域平均降水量和逐日全省各站降水量的预报技巧有限,对浙江梅雨区的预报水平总体高于浙江全省。评估结果表明CFSv2预报产品表现出显著的系统性干偏差;在延伸期尺度上,随着预报时效的缩短,预报效果并非逐步提升、而是客观存在一个最佳预报时效,各起报日也分别对应着不同的最优预报时段,整体而言梅雨降水的延伸期预测可能对初值并不敏感。

Assessment of multi-scale forecast skill of CFSv2 for Meiyu precipitation over Zhejiang in 2018

Considering the diverse operational need of Meiyu prediction, evaluation of multi-scale forecast skill of CFSv2 for Zhejiang Meiyu precipitation in 2018 was systematically conducted. Despite the forecast results on May 31, which is consistent with the observation, the initial forecast results from March 1 to May 31 could not accurately capture the trend of less precipitation in most regions of Zhejiang in June generally. On the extended-range (ER) scale, the total precipitation predicted by CFSv2 during Meiyu period exhibited approximately 30% less than observation. Based on the correlation coefficient, root mean square error and newly defined integrated forecast skill index, the forecast performance of CFSv2 on ER scale was comprehensively evaluated. The results showed that the forecast skills for total precipitation, daily regional-mean precipitation and daily stational precipitation are very limited during Meiyu period, and the forecast results for Meiyu region in Zhejiang were slightly better than for the entire Zhejiang. The evaluation results indicated that CFSv2 forecast products of Zhejiang show significant systematic bias. On the ER scale, with the decrease of forecast time scale, the improvement of forecast skill was not noticeable, and instead there is an optimal forecast time scale. Also, during the full forecasted period for each ensemble, the most skillful time-interval was commonly not the nearest time frame. To sum up, the CFSv2 forecast for Meiyu precipitation on the ER scale may not be sensitive to the initial field.