Study On The Physical Mechanisms And MCSs Of The Sustained Heavy Rainfall Event In South China In June 2005

The evolution characteristics and environment conditions of "05.6" heavy rainfall process in South China is studied in detail. NCEP/NCAR re-analysis data with the resolution of 1°×1° four times a day is used for this research. Benefited from the applying of "973" research Program, abundant new observation can be available for analysis include adding surface observation for 3-hour interval, auto weather station observation for 1-hour interval, FY-2C satellite infrared image and the temperature of brightness blackbody on cloud-top (TBB) data with the resolution of 0.05°×0.05° for 1-hour, wind/RASS profiler data, Doppler radar observation, lightning position observation for 1-hour interval, satellite cloud drift wind observation for 6-hour interval and Outgoing Long-wave Radiation(OLR) observation for 1-hour interval with the resolution of 0.5°×0.5°. The key points that explained the maintaining of the heavy rainstorm is given out through the research of interaction between mid-latitude and low-latitude synoptic systems, the basic synoptic characteristics, dynamical and thermal structure, and the occurrence mechanism of "05.6" South China quasi-stationary front. On the basis of work above, mesoscale assimilation analysis and numerical simulation is applied by use of high-resolution observation data include new remote sensing data for revealing more information of mesoscale synoptic systems, especially of the fine features of meso-P scale systems. Finally, the developing conditions and structure features of three different examples of Mesoscale Convective Systems that happened in South China, District between Yangzi River and Huaihe River and North China were introduced, and the diversity characteristics of MCS in the East China is discussed. The main conclusions are as follows:1. During the "05.6 " heavy rainfall, the sub-tropical monsoon in South China Sea took activities near the coast of South China, and the ITCZ was weak. The convective convergence zone is maintaining in South China with a nearly zonal strong convective activities characterized with low OLR. The strength of belt-shaped Sub-tropical High is maintaining and the position of its ridge is stable at 16°N. With the west-stretching and north-advancing of Sub-tropical High, the main rain belt was moving to the North China and the persistent rainfall process in South China was interrupted After 25 of June.2. The lasting of positive vorticity in the mid-level of troposphere explains the impact of synoptic systems in westerly belt on the process of "05.6" South China heavy rainfall. The south-invading of 700hP cold air before happening of the process in the mid-latitude had important impact on this event, and activities of cold air was limited below 850hpa during this heavy rainfall, and the invading of cold air in the planet boundary layer may be the most important feature for South China heavy raining. The sub-tropical jet in high level moved eastward and southward during the raining process, which made the high level jet and low level jet (LLJ) close and folded, and the positive feedback of coupling mechanism between high level and low level jet may be the important reason for the sustaining rainfall in South China.3. The quasi-stationary front in South China in the low level of troposphere is mainly taking the form of cyclonic shear formed by easterly or northerly cool air flow and strong warm wet air SW flow, which the weak 6se front zone and equivalent barotropic structure existed below 700hPa. The somewhat weak south wind and the more active north wind blow toward each other from the two sides of the stationary front. A meridian vertical circulation across the low level jet is existed related with the stationary front, and front zone situated in the ascending flow region. The divergence in high level is much stronger than the convergence in the low level for the quasi-stationary front in South China.4. The calculation of large-scale water-vapor transportation characteristic indicated that the "05.6" heavy rainfall process mainly came from the water-vapor transport of subtropics monsoon air in South China Sea, and the water vapor convergence area mainly located in the planet boundary layer. The net deficit of water vapor flux is that the net inflow in crosswise (north and south direction), and the net outflow in the east and west direction.5. The convergence and metamorphosis of horizontal wind is one of the main reasons of the formation of stationary front;the frontogenesis produced by vertical shear is approximately one magnitude smaller than the horizontal one;The contribution of diabatic heating item to the frontogenesis mainly displays in the intermediate troposphere. Moreover, ageostrophic force effect also plays vital roles in the production and maintenance of stationary front.6. The source area of mesoscale convective cloud cluster concentrated in the open region between the Darning and Dayao Mountain in middle part of Guangxi, the mountainous area in northwest Fujian and the branch range of Guangdong Lianhua Mountain, the open valley region between Jiulian and Luofu Mountain, the occurrence and development of rain cluster had close relation with mesoscale convective system.7. The analysis of radar PPI pictures demonstrated that the three strong rainstorm centers occurred in middle of Guangxi, middle of Guangdong and the northeast of Fujian were generated by the strong convective cloud echo, the mesoscale convergence area reflected in the convective echo zone and the radial velocity field had close relationship with the evolution of the mesoscale convective system. During the period of strong precipitation, the pressure reduced, the south wind enlarged, the air humidity increased, etc in the surface layer.8. The simulative results indicated that the MCSs in strong rainstorm centers in Guangdong, Guangxi, Fujian had their own different occurrence circumstances and structure features, the MCSs in Guangxi and Fujian occurred in the maximum low-level convergence area on the south of quasi-stationary frontal zone, while the high 6se channel in the left side of LLJ was the source region of warm sector MCS in Guangdong. The main performance of warm sector MCS is the wind speed convergence line of the mesoscale southern wind, the low level jet stream perturbation and the mesoscale ageostrophic force in its left side may be its main trigger mechanisms. While the MCSs on stationary front mainly correspond with ground vortex or convergence center.9. In the occurring and developing process of mesoscale convective system, the low-level atmospheric layer is in a weak static instability environment, at the same time, moist symmetry instability or convective instability may be one of the forming conditions of MCS in South China;The simulative results confirmed that the mesoscale positive vorticity perturbation in the intermediate troposphere has close relation with the occurrence and development of MCS in South China, the mesoscale wind speed perturbation above the low level jet stream or the mesoscale divergence center in the upper troposphere also has certain triggering action to MCS.10. The research on three typical examples of MCS strong precipitation events in South China, Yangtze River and Huaihe and North China area indicated that the mesoscale convective system rainstorm process in East China in summer has the characteristics of diversity, so each possibility should be considered in the operational forecasting. MCSs in different areas have common characteristic of universality, but also display different characteristics of themselves in such aspects as environmental condition, convective extension level, structure characteristic, and trigger mechanism and so on.