A Study On The Classification And Forecast Of Mesoscale Rainstorms In Jilin Province

The rainstorm is the key facter that causes floods,landslides,debris flows and other disasters.The forming mechanism,exact rainstorm forecast and alert are still worldwide problems.Mastering the regularity and mechanism,exploring objective forecast methods of mesoscale rainstorms in Jilin province,and establishing an objective automatic system have important scientific value and practical significance to improve rainstorm forecast and alert,and to further improve the effectiveness of disaster prevention and reduction.In this paper,a detailed study is made on the synoptic types and forecast of mesoscale rainstorms in Jilin province by using conventional meteorological data and regional automatic station data.First of all,the spatial and pemporal distribution characteristics of mesoscale rainstorms in Jilin province during 2010 to 2012 are statistical analyzed.Secondly,4 types and characteristics of each type of mesoscale rainstorms in Jilin province are given by analyzing data with high resolution outputed from WRF model which simulates mesoscale rainstorm cases.Thirdly,according to the forming mechanism,thresholds of predictors of each type are given by using 14 major physical quantities as predictors.Fourthly,on the basis of these studies,the operational forecasting system of mesoscale rainstorm indices of Jilin province is build and tested.The main results are as follows:1.According to the spatial scale,mesoscale rainstorms in Jilin province are divided into two categories,which are respectively the ?-? mesoscale rainstorms and the meso-?-scale rainstorms.Most of the ?-? mesoscale rainstorms occurred in the central-southern part of Jilin,then in the eastern,and a few occurred in the western,while the distribution of the meso-?-scale rainstorms is stochastic and locally.Most of both two mesoscales of rainstorms occurred in Tonghua area of Jilin province affected by the terrain.2.Mesoscale rainstorms in Jilin province occurred during May to September generally,concentrated in July-August,frequently appeared in July(51.4%),then in August(30.6%).Mesecale rainstorms appeared early at 5th May(2010),latest at 28 th September(2012).The meso-? scale rainstorms are in the majority in May-June,while the ?-? mesoscale rainstorms are in the majority from July to September.The temporal change characteristics of mesoscale rainstorms in Jilin province during 2010-2012 shows that,most of mesoscale rainstorms appeared in 2010(44.4%),then in 2012(31.9%)and the few occurred in 2011(23.6%)as the annual variation.3.68% of the ?-? mesoscale rainstorms occur within the large-scale background of the interaction between the subtropical anticyclone and the westerly systems,while 22.2% rainstorms are only arising from the westerly systems and few(8.9%)are caused by typhoon northward.The majority(55.6%)of the meso-?-scale rainstorms occurs under the influence of the westerly systems,while the minority(40.7%)occurs when the subtropical anticyclone and the westerly systems interplay.4.Based on the analysis of mesoscale rainstorms in Jilin province by using simulated data of WRF3.3.1,according to characteristics of mesoscale weather systems,the mesoscale rainstorms are classified into four types,which are respectively the shear-type,the exit-region-type,the convergence-line-type and the convergence-jet-type which including the meridional convergence type and the latitudinal convergence type.The analysis of influence systems,the spatial structure,characteristics,etc.of rainstorm area of the 4 types shows that,the shear-type of mesoscale rainstorms appears near and on the south side of the low-level shear line,while the convergence-jet-type appearing in the convergence area,the exit-region-type appearing on the left exit region of the jet,and the convergence-line-type appearing near the convergence lines in the surface layer.5.The key predictors of each type are given according to the statistics of thresholds of 14 predictors,considering the ingredients-based methodology,the cooperate of dynamical and thermodynamic conditions,and the cooperate of systems on the high and low level.(1)The shear-type requires the excellent dynamic and vapour conditions,meaning that the jet at 850 h Pa should reaches 22 m/s in average,the temperature(T)and dew-point temperature(Td)and specific humidity(q)at 850 h Pa should reaches 15 ? and 13 ? and 12 g/kg around generaly.(2)The convergence-jet-type also requires good dynamic condition,such as the jet at 850 h Pa should reaches 22 m/s around in average.Rainstorms also occur when the jet is weak but the vapour and other conditions are strong.(3)The exit-region-type requires minimum dynamic condition among 4 types,meanwhile excellent convergence,vapour and thermodynamic conditions.For instance the T and Td and q at 850 h Pa should reaches 17 ? and 15 ? and 13 g/kg around generaly.(4)The convergence-line-type usually need high temperature and humidity in a wide range.The convergence of wind direction or wind speed in the surface layer should be concerned primarily when making the rainstorm warning.6.The operational forecasting system of mesoscale rainstorms indices of Jilin province with reasonable structural design,clear physical basis,advanced methods,high automaticity and simple interfaces supports the mesoscale rainstorm forecast by using conventional meteorological data,regional automatic station data and simulated data of WRF3.3.1.The forecasting results are good according to the test based on several rainstorms occured during 2013-2014.The results of this research can improve the understanding of characteristics of mesoscale rainstorms in Jilin province.The thresholds of predictors and the system provide a new technical support to the heavy rain forecast and alert in Jilin province,an important basis of decision to defense and mitigate severe weather such as rainstorms,promoting the economic and social development of Jilin province.The results also have certain value for promotion and application.