Research On Dynamic And Vapor Structures Of Two Rainstorms In Ningxia

Ningxia located at the northern edge of the summer monsoon region in China. Rainstorm is one of the major natural disasters, and it is an important contributor for providing water resource. Since the end of20century, the heavy precipitation occurred frequently, the days of heavy precipitation and precipitation intensity reach the highest values in past decades in the Ningxia urban along the Yellow River. In order to improve and enhance prediction ability of rainstorm, the mechanism of heavy rainstorm occurred needs to study urgently.To explore the dynamic and spatial structures at the different stages of development of heavy rainfall in Ningxia, this paper chosen two typical rainstorm cases happened in Ningxia in2012, using data of conventional and high-density observations from regional weather stations, together with the multi-source high resolution measurements and retrieval data products from satellites and Doppler radars, rainstorm of the large-scale circulation background, affecting synoptic systems, small and meso-scale features of weather systems, cloud physical processes and the dynamic and vapor structures are analyzed. The main results and conclusions are summarized as follows.1. The movement of West Pacific subtropical high (WPSH) westward and northward, and retreat to southward and eastward can cause heavy rainstorms in Ningxia. Interaction of multiple scale weather systems, such as low vortex, shear, meso-scale surface convergence line relate to rainstorm, these are important systems which can induce rainstorm in Ningxia. Especially, the low-level vortex and shear are more common systems. The persistence of meso-scale convergence line on the ground is critical dynamic lifting conditions, which intensity are close relate to precipitation intensity, and consistent with spatial distributions of heavy rainfall, the center located on precipitation center slightly. Strong rainfall region locates at the right side of the700hPa shear line, the left side of low level jet or southern wind convergence region.2. There is a deep wet layer and a super high humidity region in the strong rainstorms, which is much higher than normal rainstorm. Specific humidity can reach to13g/kg and9g/kg at700hPa and500hPa respectively. Water vapor flux divergence can reach up to-2×10-7g.cm-2.hPa-1.s-1at low level between700hPa and850hPa. Not only heavy rainfall has transportation of water vapor in horizontally, but also has a strongly vertical transport of water vapor. The maintenance of water vapor transport provides sufficient vapor during formation of heavy rain; meanwhile that increases releasing of latent heat and strengthens the unstable stratification of atmosphere, which has a major impact on the intensity and persistence of precipitation.3. In duration of formation of the two rainstorms in Ningxia, there exists significantly sudden moisture convergence and divergence in the Northern Plains region, while there is no sudden changing of moisture in southern mountain region. The reason may be explained by the different sea level heights between the north and south Ningxia and the terrain of Helan Mountain. The vapor rapidly converges into the relatively lower level in the Yinchuan plain after climbing the mountains, which lead to a sudden convergence of water vapor. This provides a useful signal of forecasting in heavy rainfalls in the northern regions of Ningxia.4. The dynamic mechanism is revealed that trigger of the two heavy rainfalls is, negative vorticity divergence at high levels and positive vorticity convergence at low levels, and persistently strong upward motion. The overlap region of Low-level convergence and high-level divergence is coincident well with the heavy rainfall center.5. During the mature stage of MβCS and the occurrence of a strong wedge cusp in strong TBB gradient zone, the TBB strong gradient region locates near the cold cloud center region or at the greatest curvature zone of TBB contour, which correspond to the strong rainfall region, which also is a good signal for forecasting heavy rainfall region. The wind fields retrieved from high spatial and temporal resolution Doppler radar can effectively indicates the characteristics of the dynamical field and the evolution of heavy rainfall, especially the major dynamical factors that cause heavy rain such as occurrences of low-level jet and shear line convergence and their evolutions.