Abstract: Traditionally,the subtropical highs are considered to be dominated by downward motion,not conducive to precipitation in these areas.But there have been a heavy precipitation within the subtropical high every few years in Gansu Province,China.It is very difficult to forecast heavy rainfall inside the subtropical high.An extreme heavy precipitation process under the control of the subtropical high occurred in Gansu are analyzed by a variety of conventional and unconventional observations to study its forming mechanism.From the prediction results of models,it is found that the large scale model behaved poorly in predicting this kind of heavy rainfall.Through the observation data,the overlay of the area of divergence in northeastern Qinghai-Tibetan Plateau with high pressure and the mesoscale convergence line near surface layer contributed to form a strong upward movement,which constituted the weather condition of the heavy precipitation.Against this background,abundant water vapor,extremely thick warm clouds,and weak vertical wind shear created a high probability of strong precipitation.By analyzing various meso-scale observation data,some factors that have indicative effect in predicting such heavy rainfall have been found as follows: (1) The maximum gradient of TBB (Temperature of Black-Body) in front of the movement direction of the cold desk center in the convective cloud cluster was the place with the strongest precipitation. (2) The strongest precipitation occurred about 1-2 hours after the total number of lightning reached its peak and with the lightning places were well positioned. (3) Radar echoes of the convective bodies which caused heavy rainfall have echo characteristics of warm cloud precipitation with low mass center. (4) The strongest rain intensity appeared about 30-40 minutes after the maximums of CR and VIL were reached.

Key words: extreme heavy rainfall, subtropical high, predictability, precipitation efficiency