The Mesoscale Convection System of Torrential Rain in Warm Sector over East Side of the Tibet Plateau

The northwest China is located at the northeast of the Tibet Plateau, with a broad zone and complex terrain. The torrential rain occurred occasionally in the region. The formation of torrential rain and defensive ability of human beings are different due to the complex terrain. The storms occurred simultaneously with mountain torrents and debris flows, resulting in major casualties and economic losses. Studies have shown that most of the heavy rain occurred in the front of upper trough under the background of warm and wet southwest flow and near the shear line formed by both northerly and southerly at low level. A heavy rain occurred at the east side of the Tibet Plateau is completely different from previous features of heavy rain in the same region. It happened under the control of warm high ridge and south wind flow field in synoptic scale. Heavy precipitation has emerged in the warm region before large scale rain belt arrived. The torrential rain occurred in warm region mostly appeared in south China and rarely in north area. It has the feature of severe convective precipitation with weak disturbance in synoptic scale. The NWP model is capacity-constrained to forecast it.

A torrential rain in warm sector occurred at east side of Tibet Plateau, with the maximum hourly rainfall of 65mm, along with thunder and lightning. The evolution of mesoscale convective system was analyzed focusing on the development and propagation at by using the data of satellite, CINRAD, automatic weather stations, the conventional observation, and NCEP/NCAR reanalysis data. The results show that, due to the bell-like terrain of the east of Tibet Plateau and the block of Liupanshan mountain, a low-level jet formed as long as 200-300 km on 700 hPa. The low level jet triggered the development of convective cloud band. The forward propagation of Meso-β-scale convective cloud cluster (MCS) was the major cause of Torrential rain. The radar echoes showed obvious characteristics of low center of mass warm cloud precipitation, the zonal distribution in north and south of strong echo monomer greater than 35 dBz, the movement of convective cells with 1time/h along the low-level Jet from south to north. The significant train effect formed zonal torrential rain at east side of Tibet Plateau.

In the environmental conditions of high temperature and humidity, extreme instability of the atmosphere and a potential for severe convective weather, more attention should be paid to the formation and maintain of southwest low-level Jet. It is significant to the formation and development of the convective system in warm sector. In order to improve the forecast ability of NWP model, it is necessary to investigate the mechanics of the formation of torrential rain in the warm sector.

Key words: East side of the Tibet Plateau; Low level Jet; Convective cloud band; Convective cells propagation; Torrential rain in Warm sector