Comparison of Characteristics and Environmental Factors of Thunderstorm Gales over the Sichuan-Tibet Region

Abstract

Characteristics, environmental factors and synoptic situations of thunderstorm gales over the Sichuan-Tibet Region from 2010 to 2017 are analyzed based on significant weather report, surface observations and sounding data from China Meteorological Administration and ERA-Interim reanalysis data from European Centre for Medium-Range Weather Forecasts(ECMWF). Distinct properties are revealed through comparison of characteristics and environmental parameters of thunderstorm gales over highland(1 km above sea level) and basin(1 km below sea level). Results show that thunderstorm gales occur over the highland during a full year except winter, with two peaks in May-June and September, respectively. Their diurnal variation shows a major peak at 2000 BT. However, thunderstorm gales over the basin are active both in the afternoon and in the evening mainly in summer. The annual station-averaged frequency of thunderstorm gales over the highland is about 2 times per station, proportions of which to thunderstorms and gales are about 4.5% and 8%, respectively. It is only 0.4 times per station for thunderstorm gales over the basin, which account for 1.5% of the thunderstorms but 60% of gales. The atmospheric water vapor content, convective available potential energy and downdraft convective potential energy over the highland are significantly lower than those over the basin. The mean vertical temperature lapse rate in the middle and lower troposphere over the highland is larger than that over the basin. Usually, there is a shallow moist layer in the middle troposphere overlaid on a drier air layer over the highland. However, there is usually significant dry air in the middle troposphere and a moist layer at low level over the basin. Synoptic situations of thunderstorm gales over the Sichuan-Tibet Region are composited during two peaks in May-June and September, respectively. During May and June, the vertical wind shear of the environment is strong, with the middle level affected by a westerly trough transporting weak cold advection at 500 hPa, and the upper level located on the right side of a jet entrance at 200 hPa. However, in September, the middle level over the Sichuan-Tibet Region is at the north edge of subtropical high pressure at 500 hPa, with significant dry air in the mid-upper troposphere and remarkable warm moist air flow at low level. Though synoptic situations are different in two seasons, both of them can provide favorable condition to the formation of thunderstorm gales.

Keywords

• thunderstorm gales
• spatial and temporal distributions
• environmental factors