Triggering And Maintaining Mechanism Research Of A Heavy Rainstorm In Northwestern-Parts Of Fujian In Pre-flood Season

By using the NCEP/NCAR reanalysis data, the ground and upper-air observation data and WRF numerical simulation results, the synoptic cause,trigger and maintain mechanisms of a northwestern Fujian rainstorm process was studied in this paper. The torrential rain occurred in the context of warm and moist air from stable Western Pacific Subtropic High and dry cold air from small trough in westerlies, the low-level southwesterly jet offers a lot of water vapor and make the atmosphere unstable, shear line vortex activities triggered the heavy rain. TBB field show that in early stage of the precipitation convection is weak, precipitation comes mostly from stratiform clouds, and subsequently TBB is rapidly reduced and the convective cloud developed, then precipitation is strengthened. The high-altitude jet stream induced high-level divergence convergence coupled with low level shear line to produce a strong upward motion, warm and humid air lifting condensation into rain, by analysing difference of divergence between300hPa and850hPa, we found that it has a good correspondence with heavy rain.The moisture flux divergence is also a good indication of the rain. The unstable energy analysis shows that, before the rainstorm, there is a pseudo-equivalent temperature high energy tongue in northwestern Fujian, after rainstorms, high energy tongue weakened because of the release of unstable energy. K index and rainfall intensity relationship is better, in the heyday of the precipitation the K index reaches38～40℃, which cause strong convective activity.By comparing the simulation results and observation data, we discovered that the WRF model successfully simulated the northwest of Fujian rainstorm process, accurately reproduce the500hPa circulation situation, high-altitude subtropical jet, low level southwesterly jet. Further analysis of the mechanism of the heavy rain simulation was done using the WRF output of high-resolution, found that the low-level mesoscale shear line triggered heavy rainfall. The Q vector is significant in rainstorm diagnostic.Q vector convergence corresponds to the upward motion, but in the area of heavy rain, the convergence and divergence exercise intensity and their space scales are small, indicating that the rise and sinking motion strongly alternately,the rainstorm have significant convective and mesoscale features. Moist potential vorticity has threatened significance of heavy rainfall, before the rainstorm, MPVl in northwest Fujian was generally negative, meant large number of unstable energy savings, after rainstorm occurs, MPV1value declined, heavy rain occurred in the combination of convective instability and baroclinic instability region of MPV1and MPV2positive and negative transition zone.