Studies On The Features Of Meiyu Frontal Cloud Systems (MFCS)Based On Cloudsat Observation And Simulation

Vertical statistical characteristics such as horizontal/vertical scale, radar reflectivity, water content of Meiyu Front Cloud System(MFCS) in Jianghuai areas during the year2006to2010are analyzed using Cloudsat satellite data, FY geostationary satellite data and conventional ground observation data. MFCS evolution process is divided into development, maturity and dissipation stage. Typical MFCS precipitation cases detected by Cloudsat in different stages are studied for exploring characteristics of macro-and-micro physical variables and summarizing cloud structure of each stage. Finally, cloud water change and latent heating distribution are studied base on The Weather Research&Forecasting (WRF) model, and the feedback mechanism of latent heating to rainstorm is summarized. The results show as follows:Cloudiness is more than60%during Meiyu period in2006to2010.The single-layer cloud frequency (53.2%) is greater than the frequency of the multi-layer cloud (36.4%). Double-layer cloud has the highest frequency among multi-layer cloud. High cloud has the highest cloud frequency (39.5%), followed by middle-level cloud (27.1%), and low cloud frequency is significantly less than the other two kind clouds. As with cloud thickness, low cloud is the thickest one, with an average thickness of5kilometers; middle-level cloud ranks the second one, with an average thickness of3to3.5kilometers; the thinnest cloud is high cloud, with an average thickness of2to3kilometers.A water layer which is uniform from south to north and has distinct upper and lower bounds locates at the height of1-15km over Jianghuai areas. Average droplet number concentration of this layer ranges from15cm-3to30cm-3and maximum liquid water content frequency center on the height of12kilometers with the value of20.3%. Solid water usually located at the height of5to15km, the water frequency has little change from north to south, and the maximum ice number concentration are located at11kilometer(nearby-40℃).MFCS has multi-scale structure features. The developing convective cells in middle and lower reaches of Yangtze River eventually connected from east to west, evolving as mixed cloud with several hundreds of kilometers. In the development stage, the cloud species is mainly deep convective cloud with little cirrus, altostratus and altocumulus. In the maturity stage, cloud species show systematic distribution in frontal areas. Clouds present as cirrus, altostratus, deep convective cloud, altocumulus and cirrus successively from south to north of frontal zone. In the dissipation stage, several convective precipitation clouds imbedded in the center of the convective and stratiform mixed cloud. Radar reflectivity is associated with the droplet effective radius, radar reflectivity of deep convective cloud is larger than the radar reflectivity of other kinds of cloud. There is a bright band near0℃layer in MFCS. The relative humidity in prefrontal is significantly great than post-frontal.Large particles mainly concentrate on super-cooled cloud areas with temperature-20℃to0℃. Solid particle number concentration with maximum value occurring in the upper layer of-40℃is1to2orders of magnitude smaller than the number concentration of liquid particles. Liquid/ice water path changes dramatically in horizontal direction. There are rarely liquid droplets in cirrus as its low temperature. In development stage, liquid/ice droplet effective radius, number concentration and water content are larger than those in the other two stages. In the maturity stage, particle effective radius, number concentration and water content of warm layer in mesoscale convective cloud increase monotonically with height, reaching extreme value at the height of5kilometer and-dRe/dT maintaining a large constant value. Ice crystal effective radius is trimodal distribution in mixed layer due to the complex micro-physical process. Ice crystal number concentration increases monotonically with height which is200L-1in-40℃layer. There are only ice crystals in the upper layer of-40℃. Solid water content and effective radius change consistently, decreasing with height. But the number concentration increases with height first and decreases slightly.The WRF simulation results show that maximum radar reflectivity is greater than50dBz in the strong echo center. In the maturity stage, potential pseudo-equivalent temperature intensive belt lies between latitude32°N to34°N, and the cloud belt lies on the high side, paralleling with the potential pseudo-equivalent temperature intensive belt. There are convective instability zones in the south and north side of Meiyu front. Vorticity, divergence and water vapor flux cohere with each other from high and low levels providing dynamic and thermodynamic conditions for Meiyu-front rainfall. Thick atmosphere in storm center is heated by latent heating, mass air outflow from upper frontal zone which make upper level divergence strengthen, the low level air decompress and convergent at the same time. Vertical circulation intensified consequently. In addition, Barometric gradient increase and horizontal wind speed increase as a result of low level depression, making the shear line maintain and strengthen. Low level convergence couple with strong vertical circulation can produce condensation which leads to further heating, making heavy precipitation on the ground. As the updraft weakened in upper troposphere and downdraft strengthened in middle and lower troposphere, snow and graupel fall down and melt, forming to large cloud droplets and raindrops, being the seeder cloud. As abundant cloud water in feeder cloud, coagulation by gravity and turbulence happened which translate cloud droplet to precipitation.