上海梅汛期强降水的低频特征及延伸期预报

Low-frequency characteristics of heavy rain and its extended forecast during Meiyu season in Shanghai

利用2007—2013年NCEP/NCAR的700 h Pa经、纬向风场及水汽场逐日再分析资料和上海市11个气象站逐日降水资料对上海梅汛期强降水进行周期分析,提取低频信息,并利用向量场的经验正交函数方法对其进行分型。结果表明:上海地区梅汛期降水存在30—50 d的显著周期。在强降水发生期,低频系统存在4个主要聚集区。贝加尔湖以西至河套地区存在并维持低频反气旋,鄂霍次克海附近多为低频气旋,这两个地区是中高纬冷空气的主要活动区域;孟加拉湾附近的低频反气旋及热带洋面的低频气旋是水汽的两大源地。这些区域的显著低频系统的生消是延伸预报的主要依据。上海入梅首场强降水发生前,多为偏北气流控制。南北低频气流辐合区向北移至30°N附近,上海地区梅汛期强降水发生。低频风场及水汽场的北传与梅雨带的移动有较好的对应,当低纬低频水汽稳定北传至30°N附近时,江南北部入梅,随后偏南水汽或继续北进或滞留,对应梅雨带的持续北抬或间歇性停滞。低频经向风及水汽输送的特征是梅汛期延伸期强降水的前兆信号。跟踪监测低频偏南气流的北传进程有助于预报入梅强降水过程。

Based on wind field and water vapor field on 700 hPa from daily NECP/NCAR reanalysis data during 2007 to 2013,as well as daily precipitation data at 1 1 weather stations in Shanghai,cycles of precipitation were an-alyzed and low frequency information was extracted.Types of precipitation were classified by an empirical orthog-onal function method of vector field.The results show that precipitation has a significant cycle with 30-50 days during Meiyu season in Shanghai.There exist 4 main accumulation areas for low frequency system during heavy rainfall.Low frequency anticyclones lie in the west of Lake Baikal to Hetao Area,and low frequency cyclones al-ways appear near the Okhotsk Sea.Cold air in middle and high latitudes mainly occurs in these two areas.Low fre-quency anticyclones near the Bay of Bengal and on surface of tropical ocean are two larger source areas of water vapor.The main references of extended forecast are generation and dissipation of significant low frequency system in the areas mentioned above.North air flow controls over Shanghai before the first heavy rain happens during Meiyu season.When low frequency water vapor moves north to near 30°N,the rainy season begins in Shanghai. Low frequency wind field and water vapor field moves to north,which is corresponding with moving of Meiyu belt.When low frequency water vapor is stable near 30°N,the rainy season begins in the north of study area,and then water vapor continues moving to north or remaining,it corresponds with Meiyu belt continuing north or stag-nating intermittently.Low frequency meridianal wind and water vapor transport are an indicative signal of extended heavy rain.Therefore,tracking low frequency southerly flow can contribute to forecast strong precipitation in rainy season.