Abstract:

 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 11 weather stations in Shanghai, cycles of precipitation were analyzed and low frequency information was extracted. Types of precipitation were classified by an empirical orthogonal function method of vector field. The results show that precipitation has a significant 30-50 days cycle 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 always appear near the ??Okhotsk Sea. Cold air in middle and high latitudes mainly occurs in these two areas. Low frequency 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 stagnating 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.

Key words: Heavy rainfall during Meiyu season, Low frequency weather map, Extended forecast model