Study On Characteristics And Formation Mechanism Of Strong Convective Weather Over Tibet Plateau

Using kinds of conventional and unconventional data, the characteristics and formation mechanism of severe convective storms have been analyzed in Tibet. The main work have been done by statistically analyzed 108 thunderstorms from June to September in 2010 and 2011,compared characteristics of radar echo and stratification of thunderstorms over Tibetan plateau with low altitude, numerical simulation of thermodynamic thunderstorms which is very common in summer in Tibet.some results and conclusions as follows:There are kinds of thunderstorms over Tibet plateau, which have asymmetry distribution in spatial and temporal and shorter lifetime compared with the thunderstorms over the flatland; There is a certain regularity of source and path of the thunderstorm radar echo over Tibet; Echo of thunderstorms over Tibetan areas shows bulk shape more and then isolated convection; the intensity of most radar echo of thermodynamic thunderstorms is 40-50dBz in Xigaze, followed by the level of 50-70dBz, and the intensity of the echo of the dynamic thunderstorms mainly remains in 40-70dBz; There is an average echo top of 5.07km which thermodynamic thunderstorms could reach over Xigaze, while the dynamic thunderstorms it is 7.43km. The vertical liquid water (VIL) level is lower in both type of thunderstorms,mostly less than 30 kgm-2 in Xigaze; The intensity of thunderstorm echo of thermodynamic and dynamic is mainly in the level of 30-50dBz over Lhasa. There is an average echo top of 4.79km which thermodynamic thunderstorms could reach over Lhasa, while the dynamic thunderstorms it is 6.19km. The vertical liquid water (VIL) level is lower in both type of thunderstorms, mostly less than 30 kgm-2 over Lhasa. This statistical data is the most important information for the short-term forecasting.Lower atmospheric stratification is unstable, and senior is stable when thunderstorm happens in Lhasa, Chengdu and Nanjing; Characteristics of relative humidity are wet in the lower and dry in the Senior, and the relative humidity at low altitudes is much higher than in Tibet; The higher the altitude,the lower CAPE values; the lower the altitude, the greater the maximum echo intensity thunderstorms, and Tibet’s echo intensity is weakest, the average is about 40dBz, Chengdu slightly higher, with an average of about 53dBz, Nanjing highest, with an average of about 60dBz; The height of echo top, echo bottom and the strongest echo in Tibet is the lowest, Chengdu is slightly higher, Nanjing is highest; and the higher the altitude, the lower the thunderstorm’s vertical liquid water content. Because convective cells is relatively weak on the plateau, and water vapor is not enough, the height of the convective cells is low, which is opposite in Chengdu and Nanjing.The mechanism of thermodynamic thunderstorm formation over Tibet on 15 July of 2010 was studied using the three dimensional hailstorm numerical model with fully elastic primitive equations, which is established by Institute of Atmospheric Physics, Chinese Academy of Sciences. Results show that:thermodynamic thunderstorm’s occurrence, mature and demise structural features in the Lhasa areas was successfully simulated by this model. The updraft is relatively weak when the thermodynamic thunderstorm happened, the maximum total water vapor mixing ratio occurred in the ripe stage of a thermodynamic thunderstorm, and also the ascending velocity reached the maximum value, and then thermodynamic thunderstorm cell falling into extinction phase.the majority of thermodynamic thunderstorm’s type of hydrometeors (ice crystal, snow, graupel and hail particles) is ice crystal particles with highly volume of generated, But in the early stages of thermodynamic thunderstorm, mainly for graupel particle, the water vapor content of thermodynamic thunderstorm is less than normal, appearing after the daily maximum temperature with the unique characteristics of thermodynamic thunderstorms over the plateau.the structure of thermodynamic thunderstorm wind field characterized by low-level convergence and upper divergence. Thermal disturbance is the main trigger mechanism of thermodynamic thunderstorm,and thermodynamic thunderstorm can also happens in the case of smaller cape value over the plateau.A thermodynamic thunderstorm process occurring in Lhasa area over the Tibetan Plateau has been investigate using the Weather Research Forecast (WRF) model with three cloud microphysical schemes, which took place on 15 July of 2010.The results show that the radar echoes, water vapor, and other characteristics of the wind of thermodynamic thunderstorm over the plateau is Successfully simulated, there is a convergence and updraft over Lhasa area. The surface temperature is high in Lhasa in the afternoon, which is the location of heat source, and lower atmospheric stratification is unstable, and the low level water vapor is enough over there;Purdue Lin microphysical processes and WSM6 microphysical processes are more sensitive than Firerr microphysical processes, especially Purdue Lin microphysical processes is most gifted over the plateau; The heat in the afternoon over the plateau and thermodynamic thunderstorm there are closely related.