Study About A Process Of MCC Converting To MCV On Qinghai-Tibet Plateau

Due to the high altitude and complex topography, the physical properties of the boundary on the Qinghai-Tibet Plateau, such as atmospheric stratification stability on near surface, soil temperature, accumulated snow on the plateau and other property changes have a direct impact on the sensible and latent heat exchange between the plateau earth-atmosphere system, causing intense changes to the three-dimensional thermal structure of the Earth-atmosphere system, making weather change rapidly and MCSs occur frequently, which easily lead to heavy rain, snow, thunderstorms, high winds and other severe convective weather.With WRF (ARW) model and 3DVAR system, using ATOVS satellite data from the National Centers for Environmental Prediction as observation field, a direct assimilation test and numerical simulation is done about the conversion process of MCC to MCV on the plateau in July 22-23,2013. The test results were analyzed. And according to the simulation results, we discussed the mechanism of the transformation process.Comparative analysis of convective clouds and flow filed was done with TBB data that from FY-2E satellites and FNL data that from NCEP. The conversion process happened during 12:00 to 18:00 on July 22. Cyclonic vortex reached maturity and extended downward from 400hPa to the ground at 18:00. The formation of MCC should be related to the center of South Asia High and daily changes. The divergence filed of SAH was in favor of the formation of the MCV. The strengthen of the secondary circulation in the right rear of high-altitude jet is conducive to the development of convective upward motion, may be a major factor of MCC transformed into MCV. Moisture from the Bay of Bengal transported northward to the plateau, providing favorable conditions for the development of mesoscale convective systems and for the occurrence of precipitation.Assimilation test was done with AMSUA, AMSUB and HIRS3 data, and test results showed that:adding satellite data could make up for the lack of conventional observations plateau; different sensor data assimilation have different effects; multiple sensor data assimilation isn’t better than a single data results; cycling assimilation effectively improve the strength and location of the simulated MCV. After comprehensive comparison, we selected the cycle-assimilation AMSUB data scheme as the optimal one. Compared with the control experiment, it can well simulates the position, shape of MCV, situation of convective clouds, and precipitation attribution.Then analysis on the process of MCV with cycling assimilation AMSUB data shows:mesoscale eddies and heavy rainfall occurred in wet isentropic surface subsidence area. Altitude positive MPV1 superimposed lower positive temperature, weakened convective instability, induced cyclonic circulation, making the development of cyclonic vorticity and precipitation. And the latent heat of condensation of water vapor further exacerbated wet isentropic tilt, making vertical vorticity further development. Meanwhile, by the influence of water vapor in the process of heating, the gradient of equivalent potential temperature weakened, negative baroclinic term tends to 0, so the total MPV increases.In MCV occurrence and development period, the conversion of horizontal vorticity to vertical vorticity and positive vorticity transport in the vertical direction both make the low-level positive vorticity increase and cyclonic circulation development. While high-level negative vorticity increases and anticyclonic circulation developments. This arrangement is conducive to the development of convective vortex.There are basically a low-altitude cold anomaly and high-altitude warm anomaly in the generation of MCC and the conversion processes. The warm anomaly between 400hPa-200hPa mainly comes from condensation and evaporation,which can be resolved. And the release of latent heat of precipitation are the main source of disturbance available potential energy. The cold anomaly under 200hPa mainly comes from the vertical movement, including vertical convection and dry adiabatic cooling. Vertical movement make the most contribute to the converting between disturbance kinetic energy is and potential energy efficient. In other words, the main energy source for the development of MCV is latent heat released by convective activity.We compared the east MCV cases and this plateau process. There are no significant regional differences in the horizontal scale between two kinds of MCVs. The MCV on plateau has a shorter life cycle. Due to the higher altitude, the plateau MCV occurred in the middle troposphere, while the bodies of eastern MCVs are located in the lower troposphere. Correspondingly, the convective activity position of plateau MCV is higher than the eastern ones. The positive vorticity of eastern MCVs is mainly from convergence items, while the tilt and vertical transportation items make maximum contribution to the development of plateau MCV. Whether it is eastern MCV or plateau one, the main heat mechanism of its development are the latent heat of water vapor in the release process.