A Study On Rainstorm And Its Forecasting Methods In Baishan City

Rainstorm is one of the major catastrophic weather in northeastern China.Baishan City,located in the hinterland of Changbai Mountains,has the most rainstorm in summer season due to its terrain and the global precipitation system,comparing with other cities in Jilin province.heavy rainstorms cause not only floods,serious soil erosion,but also landslides,mud-rock flow,levee breach and the destruction of agriculture crops.In order to improve precision of rainstorm forecast in cities like Baishan,where the topography is way too complex,we carried out the study and reached the following conclusions:(1)The rainstorms in Baishan City have some specific climatic characteristics.Looking from the spatial variation characteristics together with the annual variation,rainstorm areas seem quite predictable;Baishan City,Linjiang City and Jingyu County were the major areas affected.The rainstorms are more and more rarely seen if we move northeastward,while they seldomly appeared in Changbai Country.According to the analysis into variation of historic rainstorms,the amount was arising annually.Beside this according to the time distribution into months for the years 1981 to 2000,rainstorms were reported in June to September with the most reported in July.After 2000,rainstorms focused in late July and early August.In a word,rainstorms are more likely to appear in July or August than in June or September.(2)By analyzing historical 34 years of weather data,and considering the 3 weather types specified above,3 weather synoptic models are created correspondingly.Typhoon with northwestward arc is the main precipitation system.West wind through rainstorm is closely related with intensity and position of westerly through.The through line orientation is southwest to northeast.The intensity of through is 564-570 dagpm.The location of through is at about 42-45°E and 122-125°E.Rainstorm occurred on the western side of WPSH is closely related with location of shear line and jet.The shape of WPSH is block and 588 dagpm counter's latitudinal extent is 36-38°N.The west end of the ridge of WPSH locates between110-120°E.The intensity of WPSH reaches 592 dagpm.The falling area of rainstorm in our city is between 580-584 dagpm counter.The shape of WPSH is banded and 588 dagpm counter's longitudinal extent is 95-165°E.The falling area of rainstorm in our city is between 578-582 dagpm counter near the northern ridge of WPSH.We choose different physical quantities as every rainstorm system has varied threshold value of physical quantities.(3)By studying of torrential rainstorm events,there are several conclusion: Typhoon rainstorm is closely related with typhoon tracks.This type of rainstorm is filled with vapor and the high rainfall gradient value is degressive from typhoon center.West wind through rainstorm always occurs in front of the 500 hPa through and 850 hPa jet has vapor convergence.Warm and wet air from jet and cold air from west wind through meet and result in heavy rain.The region rainstorm need good collaboration of water vapor,energy and dynamic conditions.Most of the rainstorm occurred on the western side of WPSH is mixed rainfall with convective precipitation as a primary type.This rainfall type have high precipitation intensity,tight precipitation period and much of instable energy.K index and ?se of 850 hPa are instable energy indicator.Satellite cloud pictures and radar imagines are kinds of a prediction tool.‘Train effects' in satellite pictures and 35-45 dbz of radar echo are the indicators of rainstorm prediction.(4)Based on the three types of rainstorms,this paper creates three types of weather prediction models along with the corresponding prediction equations.Several historic rainstorm events are used to test the equations;test result shows 15-30 mm prediction bias for typhoon rainstorm,-10-10 mm prediction bias for west wind trough rainstorm and 10-20 mm prediction bias for rainstorm occurred on the western side of WPSH,which means the equations predict a bigger rainfall volume than actual historic data;therefore,in realistic rainfall forecast,it is suggested to revise the volume to be smaller.Tested from the perspective of rainstorm forecast grading,the model and equations have 50% accuracy in typhoon rainstorm forecast,80% accuracy in west wind trough,and 77.8% accuracy in edge of subtropical high shear type rainstorm forecast.There is no omission but it does have a 15% empty quote rate on average.The model-equation forecast approach is scientific,quantitive and objective.In realistic forecast work,we can correct the rainfall predicted volume after analyzing the type of rainstorm,and then verify the forecast with the equations.