Study On Cloud System Over The Tibetan Plateau From Satellite Observation And Numerical Simulation

By using 7-year(May 18,2006-May 18,2013)CloudSat Satellite observations,occurrence frequencies of 8 cloud types(Cirrus,Altostratus,Altocumulus,Stratus,Stratocumulus,Cumulus,Nimbostratus,Cumulus congestus)for different levels and seasons over the Tibetan Plateau are constructed to examine characteristics of their horizontal and vertical distributions.Compared occurrence frequencies and structural characteristics of the cloud system with heating field distribution,the heating effect of cloud systems activities on the atmosphere is discussed over the Plateau.Moreover,apparent heat source(Q1)and apparent moisture sink(Q2)are calculated for the same period in the Plateau region,the East Asia monsoon region and the northern pacific region by making use of the ERA-Interim reanalysis dataset of European Center.With the heat sources(Q1)and moisture sinks(Q2)profiles over the 3 regions,heating mechanism associated with cloud frequency are discussed.Finally,a typical case of rainstorm near the Tibetan Plateau was simulated by making use of the SMS-WARMS(Shanghai Meteorological Service-WRF ADAS Real-time Modeling System)model,CloudSat Satellite observations were directly used to evaluate the comparison made between simulated cloud water and cloud ice distribution,testing the prediction for model to cloud vertical structure qualitatively.The results were summarized as follows:(1)The frequency of total cloud occurrence is about 35%.Among low,middle and high clouds,the highest frequency is low cloud near 21%,followed by middle clouds about 14%,and the lowest one is high cloud which is less than 1%.In horizon structure,high frequency of total cloud occurrence is located in the Southeast and Northwest parts of the Tibetan Plateau forming two centers of relatively plentiful clouds.The spatial distribution of low cloud frequency is consistent with that of total clouds,middle cloud frequency is high in the north and middle Plateau and low in the South Plateau which is obviously different with low cloud,and,on the other hand,high cloud frequently occurs in the South Plateau.In terms of vertical structure,level of the highest frequency is 5-6 km above sea level for low clouds,7-8 km for middle clouds and 11-12 km for high clouds.For seasonal variation,in regard to high frequency of occurrence,during winter low cloud is over the west Plateau;during spring middle cloud over the middle north part and low clouds over the west and southeast;during summer,high and low clouds over the south of the Plateau.There is very low occurrence frequency for all kinds of clouds during fall.(2)The horizonal distribution of positive values of moisture sinks(Q2)are well corresponding to the horizonal distribution of cloud frequency in the Tibetan Plateau,especially for low cloud.Large positive values of moisture sinks(Q2)are located in the Southeast and Northwest parts of the Tibetan Plateau which are collocating with high frequency of low cloud indicating obvious latent heating release of condensation.For seasonal variation,Q2 is positive all year round with larger values in spring and summer.Correspondingly,the highest frequency of cloud occurs in summer,followed by spring,and the lowest are fall and winter.In general,one of the main factors that the Tibetan Plateau is considered as an atmospheric heat source,is latent heat release of water vapor condensation in cloud formation.In summer,this feature is more prominent.(3)With regard to comparison among different regions,low clouds occur most frequently in the Tibetan Plateau and the East Asian monsoon region(EA),followed by middle clouds.In the Northwestern Pacific(NWP)frequency of high and low clouds is higher than that of middle clouds.In terms of cloud type,there is a large amount of nimbostratus and altostratus over the TP,and significant amount of cirrus in the EA with frequent occurrence of summer deep convective clouds.On the other hand,cirrus,deep convective cloud and Altostratus are dominant in the NWP.The vertical distribution and seasonal variation of moisture sinks(Q2)are well corresponding to cloud occurrence frequency in the three regions.Over the TP,heating atmosphere by latent heat release of condensing water vapor is mainly due to Nimbostratus and Altostratus.In the EA,deep convection and Altostratus are making much more contribution to the heating than the others.In the NWP,atmospheric heating is largely coming from deep convective cloud which is condensing water vapor and releasing latent heat with manifestation of vertical transport of eddy moist static energy.(4)Based on CloudSat observations,the characteristics of cloud system structure in a rainstorm case taking place in Sichuan were revealed.It is shown that the observed cloud structure is consistent with the development and evolution of the weather processes associated with the rainstorm,its center is dominated by the cloud type of nimbostratus,with a mixture of stratocumulus and deep convective clouds.(5)A regional rainstorm in Sichuan was simulated by using the SMS-WARMS model,and the result was very well.At the same time,the simulation of cloud water and cloud ice distribution was compared with the qualitative evaluation,by using the CloudSat satellite data,the results show that the simulation of the mixing area and cloud water,cloud ice(large melting layer corresponding to the big range),corresponding to the cloud reflectivity of satellite observed is stronger,and the observed cloud type is dominated by nimbostratus,stratocumulus and deep convective cloud.However,because the CloudSat satellite is a polar orbiting satellite,the time resolution of the rainstorm observation is not high,and there are some limitations in the evaluation of the model.