Observational And Simulation Study On Structure Of Cloud In Qilian Mountains

This paper carries out in-depth analysis of a heavy precipitation process on July 19,in 2007,by using routine observational data,NCEP/NCAR reanalysis,FY-2C satellite data as well as Doppler Radar data.On this basis,th ARPS(Advanced Regional Prediction System) mesoscale model have been utilized to study the influence of the Qilian terrain on clouds and precipitation structure and the microphysical processes,and get the following results:1) 200 hPa westerly trough,500 hPa northwest low vortex and ground cold front was the main weather system affected the course of the heavy precipitation.2)This mixed convective-stratiform cloud precipitation has weak convection.Before rainfall occurred,a body of cold air from 0.5-1km height in the northwest eroded the cloud body,and gradually control lower layer of the cloud body;then,as a result of the increasing southeast wind speed in the lower layer and the northwest wind speed in the middle layer,precipitation began to appear and slowly grow;as the low-level and middle-level wind speed increased and new upper cold air intruded,precipitation went through the heavy precipitation periods.Finally the precipitation cloud system moved out of the valley gradually under the northwest jet.3) Under the influence of Qilian Moutains,the horizontal distribution features of the hydrometeor in the cloud system:In the early stage,content of all the hydrometeor were little and the distribution of them is homogeneous.In the initial stage of development,due to the intrusion of lower cold air in northwest,cloud water,cloud ice,snow,graupel(hail) and rain water content began to increase and the distribution becomes less homogeneous;In the development and prosperity of the period,along with the emergence of the lower shear line and vortex center on the north side of Qilian Mountains,the content of all the hydromeror in the horizontal direction significantly increased and vertical thickness obviously extended. With a gradual eastward shift of cloud system,the scope of the hydrometeor became narrow and its content decreased. 4) Under the influence of Qilian Moutains,the vertical distribution features of the hydrometeor in the orographic cloud:In the early stage,cloud water is mainly distributed in the lower layer,grauple mainly in the freezing level,ice water mainly in the middle and upper level,rain water mainly in the warm zone of lower part close to the ground in the cloud.In the initial stage of development,as a result of enhanced vertical motion,the content of the different hydrometeor increased.In the heavy rainfall period,from the groud to top level,the content of cloud water,ice,snow and graupel(hail) have the highest value.In the weakened period,the content of all the hydrometeor decreased significantly and even disappear.5) Qilian Mountains have a significant effect on the width and intensity of precipitation rainband.The location and intensity of heavy precipitation center in sensitivity experiment deviated up to about 100km from the control experiment,and the 12-h rainfall increases 50%.6) The effect of the mountain topography on the cloud and precipitation system are closely related to the weather system.The ways and degree that the mountain topography affects the construction and distribution of hydrometeor was shown to be related to the position of weather system in relation to the topography and the stage of development of the cloud system.7) Qilian Mountains have an important effect on the intensity and life cycle of the the cloud and precipitation system.Under the influence of Qilian Moutains,the initensity reduced and the life cycle was shorten.Qilian Mountains mainly affect the development and maintenance of the cloud and precipitation system in the mid and late stage.