Cold Air Activities in July 2004 and Its Impact on Intense Rainfalls over Southwest China

The severe rainfall events in the mid-summer of July 2004 and the roles of cold air in the formation of heavy precipitation are investigated by using daily observational precipitation data of China and NCEP/NCAR reanalysis. The results show that the severe rainfalls in Southwest China are closely related to the cold air activities from the mid-high latitudes, and the events take place under the cooperative effects of mid-high latitude circulation and low latitude synoptic regimes. It is the merging of a cold vortex over mid-latitudes with the northward landing typhoon and eastward Southwest China Vortex, as well as the abrupt transformation from a transversal trough into an upright one that causes three large alterations of mid-high atmospheric circulation respectively in the early and middle ten days of this month. Then, the amplitude of long waves soon magnifies, leading to the unusual intrusion of cold air to low-latitude areas in the mid-summer. Meanwhile, the warm and humid southwest summer monsoon is quite active. The strong interactions of cold air and summer monsoon over Southwest China result in the large-scale convective rainfalls on the southern side of cold air. With regard to the activities of cold air, it can influence rainfalls in three prominent ways. Firstly, the incursion of upper-level cold air is often accompanied by partial southerly upper-level jet. The ascending branch of the corresponding secondary circulation, which is on the left front side of the jet center, provides the favorite dynamic upward motion for the rainfalls. Secondly, the southward movement of cold air contributes to the establishment of atmospheric baroclinic structure, which would lead to baroclinic disturbances. The atmospheric disturbances associated with the intrusion of cold air can destroy the potential instability stratification, release the convective available potential energy （CAPE） and finally cause convective activities. In addition, the advection processes of dry and cold air at the upper level along with the advection of humid and warm air at the lower level are rather significant for the reestablishment of potential instability in the precipitation area, which is one of the crucial factors contributing to persistent rainfalls.

A Probabilistic Prediction Model for Heavy Rain-Caused Landslides in the Chongqing Region

Landslide （or rockslide） is a geological disaster that is mainly induced by strong precipitation, among a number of other natural inducing factors. Based on 1615 landslide cases, a statistical analysis is performed to find the relationship among the landslide occurrence time, rainfall 0 10 days ahead, and probability of landslides over the Chongqing region. The results show that 1） strong rainfall-caused landslides occur mainly on the day it rains or ] 2 days after the heavy rain, and as time goes on, the likelihood of the disaster reduces rapidly; 2） the heavier the rainfall, the closer the landslide time is to the precipitation time. A concept of ＂effective precipitation＂ is thus developed, and a categorical prediction model for heavy raincaused landslides is established. Tests show that for categories Ⅲ, Ⅳ, and Ⅴ landslides, the model forecast accuracy arrives at 29.9%, 75%, and 100%, respectively. This indicates that the categorized probabilistic prediction can serve as a warning for the landslide prevention and mitigation.