一次高影响小雨过程预报失误的原因初探

Preliminary Discussion on the Failure Forecasting Related to a High-Impact Light Rain in Shanghai

摘要: 2010年冬春之交的4月，因对8~9日发生在上海地区的一次小雨过程的预报失误，导致9日最高气温的预报与实况相差近7度，市民反响强烈。本文对这一“高影响”小雨过程的预报失误原因，进行了初步分析，结果表明：低层(850 hPa)的中尺度锋生，减缓了低层短波槽系统的过境速度，使短波槽系统在过境时出现“前倾”的异常垂直结构，导致高空(500 hPa)槽线虽已过境，但小雨过程却(因低层低值系统尚未过境)不止的令预报员始料未及的现象；预报员的惯性预报思维在前期预报已出现明显偏差的情况下未及时调整，对数值模式的中尺度预报信息分析不够深入，特别是模式对弱天气过程的要素预报能力把握不准等均是最终造成这次预报失误的主要原因。文章最后还给出了上海地区春季小雨过程预报及数值模式对弱天气过程诊断量改进的几点启示。

Abstract: Due to the failure forecast on a light rain of 8-9 April 2010 in Shanghai, the highest temperature forecast is fault, the error near 7˚C. In this paper, the possible reasons of failure forecast on the high-impact light rain are discussed. The results show that: the lower (850 hPa) mesoscale fron-togenesis slows down the moving speed of low-level shortwave trough, causing the shortwave trough appearing a unusual “forward-lean” vertical structure during the trough movement, lead-ing to the trough line of high-level (500 hPa) passing through Shanghai but the lower-pressure system haven't and the light rain process sustains. Obviously, the unexpected phenomenon is difficult to forecast in operation. Furthermore, the forecast results have not been timely adjusted when the early forecasts have already shown the significant deviations, because of the inertia forecasting thinking. And the mesoscale analysis based on the numerical model products is not in-depth enough. Particularly, the limited capabilities of numerical model forecast on the weak process and its weather elements, are the main reasons for ultimate forecast failure. Finally, some inspirations on the forecast of spring light rain in Shanghai and model using skill on the weak weather process are shown in this paper.