Characteristics of GPS Vapor in a Persistent Heavy Rainfall Related to Southwest Vortex

Using GPS Precipitable Water Vapor (GPS-PWV) observations from the ground-based GPS network in Chengdu Plain, NCEP reanalysis data, observed precipitation from automatic weather stations (AWS) and the specific humidity of single radiosonde station, the variations of water vapor in a persistent heavy rainfall process over the northeastern Sichuan Basin from 15 July to 18 July in 2010 is analyzed, focusing on the changes of GPS-PWV under the influencing system (southwest vortex) and its relationship with precipitation. Usually, there is a sharp rise of GPS-PWV when the precipitation occurs, and the altitude or terrain of each GPS station has an obvious impact on the value of GPS-PWV, precipitation and its peak. The GPS-PWV is not simple proportional related with the actual rainfall, the occurrence time and value of the precipitation have a certain relationship with the observatory sites and the movement of the southwest vortex. When the vortex circulation begins to develop, the instability of water vapor transport makes the GPS-PWV fluctuant increases. A large surge of GPS-PWV increase can be observed 13—17 h before the formation of the southwest vortex, reaching the maximum about 0—4 h before the vortex formation. The increase ends when it is fully formed, and the GPS-PWV gradually drops to the original level or even lower with the eastward shift of the southwest vortex. The positive area of the vertical flux of moisture divergence is well correspond with the area of precipitation, and it can better describe the strong rise, the movement of the convergence and divergence and water vapor transport than moisture flux divergence in the process, and the change of GPS-PWV is consistent with the vertical flux of moisture divergence. In addition, the trend of the water transport and convergence in the development process of the vortex circulation is consistent with the change of GPS-PWV, the surge and drop of GPS-PWV have some indicating significance in the formation or weakening of heavy rainfall. Compared with the radiosonde stations, AWS can well reflect the changes of water vapor near the ground, and radiosonde stations can provide the vertical distribution of the specific humidity. The regional ground-based GPS station network may capture the overall changes of the whole layer of atmospheric water vapor with the high time resolution. Therefore, more attention should be paid to the comprehensive application of water vapor information that obtained by these different observations means. Also, the southwest vortex as one of the major systems that induce the rainstorm process of the Sichuan Basin and the Yangtze Valley, adding to the important significance of GPS-PWV to discuss the variation of water vapor field before and after the generation of the southwest vortex.