Researchers Reveal Cloud Microphysical Processes of Orographic Torrential Rainfall in Sichuan

Aug 11, 2014     Email"> PrintText Size

Orographic torrential rainfall can cause floods, landslides, debris flows and other natural disasters, which is a great threat to the lives and properties of people, and is a great challenge to mountain weather forecasters. Sichuan Basin, located east of the Tibetan Plateau, north of the Yunnan-Guizhou Plateau, south of the Qinling Heights, is of complex terrain and it is a storm-prone place, where debris flows, landslides and other secondary geological disasters occur frequently. Especially, after two serious earthquakes, Wenchuan earthquake (2008) and Ya'an earthquake (2013), soil of Sichuan complex terrain surrounding these earthquake areas become much looser, so frequent torrential rainfall is more likely to lead to debris flows and other disasters. Therefore, understanding of physical processes responsible for the development of orographic torrential rainfall is crucial for the improvement of operational forecasts in this area. 

There are three important factors for orographic rainfall: larger-scale atmospheric circulation, the interaction of the ambient flow with terrain, and cloud microphysical processes. Many studies on orographic torrential rainfall in Sichuan addressed the first two factors, while studies on the cloud microphysical processes of orographic torrential rainfall in Sichuan have been seldom conducted. The dominant cloud microphysical processes of torrential rainfall in Sichuan are still not known. 

By using a high-resolution numerical simulation data of an orographic torrential rainfall triggering debris flow in Sichuan, researches with the Institute of Atmospheric Physics studied the dominant cloud microphysical processes responsible for the development of the torrential rainfall. Their results show that in the strong precipitation period, particle sizes of all hydrometeors increase, and mean-mass diameters of graupel increase the most significantly, compared with those in the weak precipitation period. The terminal velocity of raindrops is the strongest among all hydrometeors, and the graupel’s is in the second place, but much smaller than the raindrops’. Differences between various hydrometeors’ terminal velocities in the strong precipitation period are larger than those in the weak precipitation period, which favors the relative motion, collection interaction and transformation between the particles. Orders of magnitudes of various hydrometeors’ sources and sinks in the strong precipitation period are larger than those in the weak precipitation period, causing a difference in the intensity of precipitation. And water vapor, cloud water, raindrop, graupel and their exchange processes play a major role in the production of the torrential rainfall. 

This type of orographic torrential rainfall discussed in this study is common in summer in Sichuan region. The results may be of great significance for the in-depth and comprehensive understanding of orographic torrential rainfall’s microphysical mechanism in Sichuan. The study was published early online in Advances in Atmospheric Sciences in August 2014 at http://159.226.119.58/aas/EN/abstract/abstract2533.shtml. 

Microphysical flowchart for the Milbrandt 2-mom scheme. The rectangles represent the various water species (water vapor, cloud water, rain water, cloud ice, snow, graupel, hail), and the arrows are the processes that link the species. (Image by IAP)