Analysis of the characteristics in a strong convective weather process in China

By the combined effect of cold air coming south and southwest inverted trough on April 4, 2001, the inverted groove frontogenesis formed in the middle-line of Hunan province. The local thunder shower and gale, and short-range precipitation weather process had appeared. There were 23 countries and cities in the province appeared gale, 2 stations appeared hail, the largest diameter was 7mm. The maximum sustained wind in this process was 32m/s which was come forth from Changsha, and the temperature was dropped about 14`17°C in a day of Hunan. The main characteristic of the strong convection process was that the speed was very fast. There appeared no severe rainfall in the north of Hunan, and the strong convection only resulted in thunderstorm.

Prophase weather remained warmer, the large quantity unstable energy accumulated, and the vertical wind shear, power, heat and moisture conditions were all conducive to the formation of strong convective weather. It was clear from the analysis of the temperature, humidity, the disturbance of kinetic energy on the ground and high altitude that before and during the process impact, it had positive temperature perturbation; after the process impact, it had negative temperature perturbation and the temperature disturbance on the ground significantly higher than at high altitude levels. This shows that the thermal affect in surface were greater than the height. The humidity in the regions that the strong convective weather occurred was less than the disturbance of surrounding environment; this may be one of the reasons that caused the thunderstorm winds. The levels of humidity disturbance were very large, so the convection was very strong. The disturbance energy occurred in the process significantly higher than before and after it. The largest disturbance energy appeared at 850 hPa, followed by the ground.

According to analysis of upper air sounding data, on the 4th at 8, before the strong convective weather occurred, the wind directions veered with height at three sounding stations in Hunan. It had obvious warm advection and medium to strong vertical wind shear, they accorded with the conditions that the formations of vertical wind shear under the strong convective weather. Until to 5th at 8, from north to south, the direction of upper wind rotated counter-clockwise followed with the height in lower layer, but it rotated clockwise in the high layer. The lower layer had obvious cold advection, and top with warm advection.

Base on comprehensive analysis of the data from automatic weather station and Doppler weather radar, a very good relationship could be found between the surface meso-scale convergence line and the formation of strong convection. The massive strong convection echo presented to the strength graph, the severe center more than 55dBz, echo top height more than 12km, gust front features was very clear. The time of surface meso-scale divergence line appeared ahead of convection echo about half an hour on the velocity diagram. It should be relatively strong echo with that of cyclones, contrary wind front, and other characteristics of Echo.