Study On MCS Of The Heavy Rainfall Event In The Middle Part Of Shaanxi

The rainstorm has been the key to operational weather forecast service,which is still difficult to forecast.And rainstorm predicetion of Shaanxi is failing or dissatisfactory.Due to the influence of specific geographical environment,rainstorms of Shaanxi usually happen in small area,and the atmospheric circulation may also present different situation.Therefore,we must study the particularity besides the universality,in order to improve the rainstorm prediction.A process of the heavy rainstorm on 8-9 August 2007 in the Central Shaanxi Plain,with characterictics a short time,heavy rainfall and abrupt occurrence,is rare for the history.Its prediction is very difficult owing to disadvantageous large-scale environment background. In order to research the law of occurrence and development of this kind rainstorm,to enhances the rainstorm forecast the rate of accuracy,this article take this example as the object of study.Founded in the past research and operational weather forecast,although the NCEP data and so on has good ability for rainstorm forecast,but there is still some problem in obtaining the data.While the T213 data has certain forecast ability to the rainstorm,which is easy to operate in real-time operational weather forecast,and its product has been prompt effective.Therefore,the T213(0.5625×0.5625) data and the observed data with high space and time resolution in recent years(including denser surface data for 1-hour interval,automatic meteorological observing stations precipitation data for 1-minute interval, FY-2C satellite data,Doppler radar data as well as lightning position observation for 1-minute interval and so on),have been used in this research.Through the synoptic meteorology analysis,dynamic and thermodynamic diagnosis as well as numerical simulation,a process of the heavy rainstorm in a short time on 8-9 August 2007 in the Central Shaanxi Plain was analyzed in detail.We have found the favorable large-scale environment background and the possible reason for formation of the strong rainstorm.Evolution and the three-dimensional structure of MCS,especially the slight structural feature of three cloud burst centers M[JCS(M_γCS) in the rainstorm process were revealed.The findings may provide the scientific basis and the forecast objective point for short-term weather forecast. The main conclusion includes:1.The "07.8" strong rainstorm in the Central Shaanxi Plain was the result of interaction among the different weather system,under a favorable large-scale circulation disposition.In the middle level,the break of high-pressured dam which was resulted from the development merge of Qinghai-Tibet Plain high pressure and west the Pacific Ocean subtropical high in the middle Shaanxi is very important.The mutual coordination of the high-pressured dam's break in the middle level,right flank divergence's formation of westerly jet entrance region in the upper level and the shear line's development in the low level had provided the advantageous large-scale environment background for the strong rainstorm formation.2.The vapor accumulation from around the Central Shaanxi Plain had provided sufficient water vapor and potential instability for occurrence of the heavy rainfall.And the vapor accumulation was achieved through the easterly current.The water vapor increased so fast that rainstorm happened suddenly,and the rapid-decreasing of water vapor made the rainfall last short.The occurrence and weakening of the strong precipitation had close relationship with the local change of water vapor,water vapor advection and the change of vapor in surface layer.The formation of intense "pavement pumping" effect and the secondary circulation circle owing to the development of the upper anticyclone vorticity were the dynamic mechanism of rainstorm formation.3.In this event,the zonal distribution from west to east of the rainfall is related with the Qin ling mountain and trumpet-shaped terrain in the Central Shaanxi Plain.And the rainfall was not distributed uniformly in the space,which was mostly centered in the areas of the Qishan,Liquan and Gaoling.It was resulted from the occurrence and development of MαCS, which was also formed by the mergence and development of 2 MβCS.The convective cells merged and developed,and enhanced individually to form the different precipitation centers. The development of the convective cells was resulted from the surface mesoscale convergence system,and the precipitation areas had a good corresponding relationship with the surface mesoscale convergence system.4.The occurrence and sharp growth of the ground lightning has very good indicative significance to the development and strengthens of the rainstorm.The occurrence of the first flash occurs ahead of the strong precipitation happening.The sharp increase of ground lightning has close connection with the sudden increase of the precipitation intensity.And the concentrated regions of the negative ground lightning may indicate the area where strong precipitation happen.5.The simulation using the Weather Research and Forecasting(WRF) model were successful in reproducing the strong rainfall in the Central Shaanxi Plain.The simulation result indicated that,the strong rainfall was related to occurrence of a mesoscale-αvortex,in which the rapid developing MβCS(M_γCS) formed the three strong rainstorm centers.MβCS had different stream field,vertical dynamical and thermal structure.In the daily weather forecast,the numerical forecasting product(NWP) can be used as a forecast reference,by which we may analyze the mesoscale structural feature under the large-scale circulation background.The vertical structure of the stream field:there were different current in different levels in the troposphere that flowed into the ascendant current of theβ-scale precipitation cloud tower,forming different vertical circulations in different levels.And the ascendant current had extend to 200 hPa(or 150 hPa),and flowed out to the southeast and northeast above the level. The vertical dynamical and thermal structure:in Qishan,there was a strong divergence center above 450 hPa,while below 450 hPa the structure over the rainstorm center is opposite from north side and the south.There was weak divergence or convergence on the south side, and convergence or divergence on the north side;The vertical ascent movement inclined to the south,then to the north,and finally to the tropopause;The vorticity column was mutually coupled with double energy center of the potential equivalent temperature,dual unstable stratification columns and two warm centers in middle level,and temperature column with high-low cold center;In the other two precipitation centers,Liquan and Gaoling:the entire column of ascent movement was coupled with thestrong divergence column,as well as the positive vorticity column;In Liquan,the ascent movement column was a saturated vapor column with one strong warm center in the mid-high levels while cold in the high and low levels,which has the unstable stratification structure as "the saddle" field.In Gaoling:there is the strong energy center and frontal zone of temperature deviation below 550 hPa,and below 400 hPa there was a saturated water vapor column.According to the research results above,we may obtain some vantage points for the short-term forecast of this kind of strong rainstorm:when 1) above 500 hPa in the troposphere, when the middle latitude region was controlled by a high-pressure dam,which was formed by the connection between Tibetan Plateau High and the subtropical high,whether does the high-pressure dam break to form the shear line is the key to the rainstorm forecast;the formation of divergence stream field in the northeast of the upper South Asia High has a indicative significance to the occurrence of rainstorm.2) the local change of water vapor, water vapor advection and the change of water vapor in surface layer have a indicative significance to the occurrence and development of the rainstorm,as well as its weakening and extinction.3) according to the time that first fresh occurred,the concentrated regions of the negative ground lightning and the sharp increase of ground lightning,we may predict the occurrence,location and intensity of the strong precipitation in the future.4) comprehensive analysis of the satellite data,radar data and NWP,as well as the steering current and the ground mesoscale system and so on,we may predict the development evolution of MCS and have more time to make short-term forecast of this kind of rainstorm to avoid disaster.Must to investigate a lot of special example of Shaanxi rainstorm and understand law of occurrence and development of heavy rainfall unceasingly in the future,we are able to enhances the rainstorm forecast the rate of accuracy.