Evolution of Dynamic and Thermal Structure and Instability Condition Analysis of the Extreme Precipitation System in Beijing-Tianjin-Hebei on July 19 2016

The evolution of dynamic and thermal structure and instability condition of an extreme precipitation system in Beijing-Tianjin-Hebei are analyzed using the NCEP/NCAR Global Forecast System (GFS) data combined with national automatic stations observations of precipitation provided by the Meteorological Information Center of China Meteorological Administration, the CMORPH satellite precipitation data and the fusion of precipitation data from the FY2 precipitation and radar quantitative estimation of precipitation. This study reveals the configuration of weather systems in different air pressure zones over the Beijing-Tianjin-Hebei region. Vertical motion, water vapor condition and unstable stratification evolution during the precipitation process are explored. The results are as follows. (1) The circulation at 500 hPa presented an east-high-west-low pattern, which was coordinated with a low-level vortex at 700 hPa and jet streams in low and high levels. The subtropical high blocked the eastward movement of the low-level vortex in North China, making it stagnant in the Beijing-Tianjin-Hebei region. (2) The development and eastward-moving of the low level vortex was important for the occurrence of the rainstorm. (3) The potential divergence analysis was applied to explore changes in convective instability. Results indicate that in the rear of the precipitation area, lower-level potential instability was mainly determined by vertical wind shear, which reflected the joint effects of vertical wind shear and moist baroclinicity. Lower-level potential divergence was negative in weak precipitation area and behind the precipitation area, which was conducive to regional potential instability. Potential divergence was positive in strong precipitation area and in front of precipitation area, inhibiting the development of potential instability. The change in potential divergence affected precipitation region through affecting the atmospheric stability. The high value region of potential divergence corresponded to high value region of precipitation, especially the 700 hPa potential divergence was a good indicator for precipitation region, which could be estimated by the change of potential divergence at 700 hPa.