2016年9月4日下午16时(北京时)左右,发生在杭州市区和西湖及周边区域的一场突发的短时阵雨天气对“杭州G20峰会”相关活动的准备工作造成了极大的影响.本文分析了该次阵雨天气的成因,讨论了定点和定时短时期近预报的局限性和短时临近预报难点.本文分析表明,当时重点监视的杭州东部宁波至绍兴一带的主要对流系统并未直接影响到杭州市区,东移的天气尺度高空槽系统也尚未影响到该区域,该次阵雨天气是在弱的静力不稳定条件下,由午后形成的海风锋与干线在杭州湾西北岸共同触发的浅层对流系统向西南快速移入杭州西湖及其周边区域形成.由于该对流天气系统具有空间尺度小、生命史短、移动快速、发展高度低、反射率因子强度低、短时雨强较大等特点,加之当天杭州及周边区域上空存在大量在静止卫星云图上难以同积云区分的高层卷云,使得天气雷达和静止气象卫星对该系统的监测能力受到显著的影响,以致于仅依赖这两类资料对其做出较长时效的短时临近预报也非常困难,因此使用高时空分辨率的加密自动站资料分析中尺度环境场的要素变化对于此浅对流天气系统的短时临近预报至关重要.%At about 16:00 BT 4 September 2016 the urban area of Hangzhou,West Lake and the surrounding area received a sudden convective shower,which caused great impacts on preparation related to "Hangzhou G20 Summit".This paper analyzes the mechanism of the shower,and discusses limitations of shorttime forecasting and difficulties of nowcasting.The results indicate that the mainly focusing convection system at that time over Ningbo,eastern Hangzhou area,and Shaoxing did not directly affect Hangzhou City,and eastward-moving synoptic-scale upper trough system did not affected this area either.However,under the weakly statically unstable conditions,the shower was produced by a shallow convective system which was triggered by an afternoon sea breeze front with a dry line in the northwest Hangzhou Bay.As this convective system had some characteristics such as small spatial scale,short life,rapid movement,low cloud top,weak reflectivity and intensive rainfall intensity,together with a large number of high-level cirrus clouds which cannot effectively be discriminated from cumulus over Hangzhou and the surrounding area in the satellite images,the ability of weather radar and stationary meteorological satellite monitoring the system was weakened significantly,so that nowcasting the shower is also very difficult if only using these two types of observations.Therefore,it is crucial to use automatic weather station data with high spatiotemporal resolutions to analyze the environmental conditions for nowcasting the type of shallow convective weather systems.
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