The fundamental features of the extreme severe rain events in the recent 10 years in the Beijing area

This paper analyses the fundamental properties of 10 extreme severe rain events (the event is defined as its precipitation≥ 100 mm/(24 h) at any representative weather station among the 14 Districts or Counties with the rain intensity≥ 40 mm/h over at least one automatic weather station) during 2006-2013 in the Beijing region, based on the rainfall data per 5 min of automatic weather stations, the data series of SA radar, TBB (Temperature of Black Body) of FY-2, routine sounding and 1°×1° NCEP/NCAR final analysis. The investigation results show that the severe rain centers, formed from long periodic convective cells or MCS organized/amalgamated by multi cells (typeⅠ), are usually located before the mountain or central urban areas, and the distribution feature is related to the terrain forcing caused by easterlies airflow under the lower layer or the urban forcing. However, the extreme severe rain events which are dominated by the Train Effect associated with MCS of multi cells (typeⅡ) are connected with two different properties of raining process: The convective activity happening on the warming area and the other near a front, so the rainfall distributions are frequently parallel to the axis of low level jets or fronts. The precipitation distributions of the extreme rain events caused by MCS of typeⅠare more localized, and each of their average precipitation of the whole administrative area is much lower than 50 mm. One of them is led by mixed convection, and its extreme rainfall is usually caused by long periodic or hardly moving convective cells to be generated and dissipated repeatedly,with the convection height relatively low. However, the extreme rain events led by deep convection are usually caused by multi cells to develop, organize and combine, and the lowest TBB of this typical MCS is usually lower than -55℃ since convective cloud top develops high. Short-time strong rainfall of these extreme events is intermittent: the initial rainfall corresponds with convective cells developing, and the second period is connected to the process of their organizing and combining. This study indicates that typeⅡMCS (Train Effect) shows often the two independent convection zones in the first stage: One is connected with the front and the other is a warm convection zone related to a low level jet. When the front moves toward the warm convection zone, the cold air in the low layer passes into the warm convection area gradually, the two convection zones begin to amalgamate slowly into single one while convective activity becomes to flourish, or since convection in the warm side prevents inflow of vapor, the front convection zone becomes weaker and weaker, meanwhile, the warm convection develops with more blossoming and organizing. The MCS , unlike MCS of typeⅠ, is usually to give rise to the city wide torrential rain (≥ 50 mm) even heavy rainstorm (≥ 100 mm). The large scale vapor conditions are obviously different among the various typical convective severe precipitation processes. In most cases the vapor sources of the severe rain to be connected with Train Effect are related to two warmly moist monsoon flows: one originates from the Bay of Bengal and the other is from the western Pacific Ocean, so the large scale vapor supplies are abundant. However, the vapor of the severe precipitation led by mixed convection of the typeⅠMCS is connected principally with offshore southeasterly wind in the low level; the vapor of the typeⅠMCS dominated by deep convection exists mostly in the lower troposphere even only near the surface, and in most cases the total vapor content of large scale is much less than others. No matter which extreme severe rain events, the strongest radar echo heights are rarely higher than 5 km, in most cases are lower than 4 km during the heaviest rainfall. The CAPE levels of the ambient atmosphere have often good relationship with convective heights (Top of Radar reflectivity), but correspondence among the ambient CAPE, momentary convection intensity and level of the strongest reflectivity or the maximum rainfall intensity is completely heterogeneous.