Features Of Intensifying Thunderstorms Moving Down From The Mountains And Case Study In Beijing

Beijing often subject to the influence of thunderstorms moved from the western ornorthern mountains, due to the sophisticated topography and surface environment of the vicinityareas of BeiJing, it presents significant challenges to nowcast whether these thunderstormsstrengthen or weaken when they move towards the plain. Therefore, understanding of theevolution and characteristics of these thunderstorms and revealing the mechanism of theatmospheric thermal dynamic interaction have very important scientific value to improve theaccuracy of thunderstorm nowcasting. Although some case studies had been carried out to studythe mechanism of thunderstorms enhanced when moved down from the mountains, it still has todo more cases studies on synoptic-scale background, especially the local thermodynamic andkinetics conditions when thunderstorms strengthen. In addition, statistical analysis onconvective weather mainly based on the synoptic-scale circulation systems (such as vortex orupper trough) in the past. Although mesoscale meteorology and radar meteorology have theclassification of "linear thunderstorm" and "thunderstorms group", categorized statisticalanalysis of thunderstorms based on the classification is relatively rare. Detailed analysis of theintensified process of thunderstorms moving down from mountains is still need to bestrengthened, especially the formation mechanismt of the maximum rainfall center.In order to slove these problems above,18thunderstorms cases which significantlyenhanced after moved down from mountains were selected from2008to2011. These18caseswere divided into two types of "linear thunderstorm" and "thunderstorm group" according totheir radar echo shape characteristics, including8"linear thunderstorm" and10"thunderstormgroup". Synoptic-scale background and local physical conditions were first analyzed by usingNCEP/NACR Reanalysis data and local sounding data. Then statistical analysis of Meso-scalecharacteristics were studied using radar echo data,VDRAS data and automatic weatherstation data. Based on the features of statistical analysis, this paper studied the enhancingmechanisms of two typical significantly enhanced thunderstorms. One "thunderstorm group"occurred on August9,2011and another one "linear thunderstorm" occurred on June23,2011.The main conclusions of this paper are as follows: (1) Enhanced thunderstorms often occurred at favorable Synoptic-scale background, withCAPE between1000～2000J㎏-1.Well organized "Linear thunderstorm" usually hadfavorable weather systems both in upper layer and low level, such as low eddy horizontal slot orshort-wave trough in500hPa and convergence line850hPa. Poor organized "thunderstormgroup" didn't have co-ordination weather systems in upper layer and low level, there just hadconvergence line in850hPa or a weak cold front on the surface. Particularly well organized"linear thunderstorm" usually had a strong vertical wind shear environmental(vertical windshear≥15m s-1from surface to6km height),and poor organized "thunderstorm group" usuallyhad small strong vertical wind shear environment. It demonstrated that thunderstorms couldintensify with large CAPE in weak vertical wind shear environment.(2) Before the enhanced thunderstorms enter the plain, the critical region in the plain had asurface convergence line or convergence zone exists. The strength of cold pool of "linearthunderstorm" was stronger than that of "thunderstorm group". AT the187.5meters height ofboundary layer, there was also had boundary layer convergence line exists, mainly formed byoutflow boundary of the earlier thunderstorm (usually westerly wind) and the environmentaleast wind. The intensity and range of cold pools of "linear thunderstorm" usually stronger than"thunderstorm group" and lie northeast southwest direction as a belt; the intensity and range ofcold pools of "thunderstorm group" often weaker and smaller and spotty distributied.(3)Detailed analysis of intensified case of "thunderstorm group" taken place on August9,2011demonstrates:(a) Before thunderstorms occured,local atmosphere had strong unstableenergy(CAPE2798J㎏-1), but environmental vertical wind shear was weak, that leaded to therelative weak organized thunderstorms.(b) Terrain elevation raiseed the height of thunderstormscold pool outflow, it overlaid on the existing surface heat circulation, increased instabilitytriggered thunderstorms occurred, resulted in thunderstorms spread in space with the "jump".(c)When gust front formed by outflow boundary of the earlier thunderstorm reached to the existingthunderstorm in HaiDian district, its strong temperature gradient and convergence upliftmovement made the thunderstorm cell intensified rapidly. At the same time,thunderstormmoved from the western mountains merged into thunderstorm cells in HaiDian district, causeduplift movement continue and then formed heavy rain and lightning center in HaiDian district.(4) On the June23,2011typical "Linear thunderstorm" case,before the thunderstormmoved into the plain, although the CAPE was not large(354J㎏-1),but the environmentalvertical wind shear was very strong(20.3m s-1from surface to6㎞height),which wasconducive to supercell form.(b)Before the thunderstorms moved into the Beijing urban areas,pre-existing southeast wind transported warm and moist air to the plain,water vaporaccumalated at low levels with specific humidity＞16g kg-1on surface.(b)Due to the"inversion layer" at the height of800hPa, the warm and moist air were separation from the cold and dry air above, so the atmosphere unstable energy was accumulated in the lower level.(c)Atthe same time, due to terrain height of the Beijing area, temperature, water vapor content,relative humidity conditions were significantly different in the plains and mountains.(d) Whenthe well organized linear thunderstorms moved to the southeast plain, they breaked theinversion layer and intensified along the foot of the mountains. A supercell formed at right end(relative to the direction of thunderstorms movement) of linear thunderstorms, when it gotadequate supply of water vapor near foot of the mountains, so it violently enhanced and formedthe heavy rain center in ShiJingShan district.