Observational Analysis And Modeling Study Of A Local Heavy Rainstorm Close To The Coastal Zones

A local heavy rainstorm occurred in Jiaonan on September 21,2012. It rained in a limited area with a great precipitation in a short period of time. Because of the lack of signs from the normal synoptic situation, the weather forecast had been failed. In order to explore the mechanism of the rainstorm, automatic weather observation data, doppler radar data, wind profile radar data and atmospheric reanalysis data are used to do observational analysis. And then, WRF (Weather Research and Forecast) mesoscale model and its Cycle-3DVAR (Three-Dimensional Variational) assimilation system are utilized to carry on modeling study. Additionally, a series of sensitivity experiments are performed to verify the role of Jiaonan coastal waters and local terrain in the rainstorm. The results show that (1) The upper level synoptic situation is not conducive to cause strong convection. The pseudo-equivalent temperature shows distribution characteristics of High θsc-Low θsc-High θsc from high to low, and with an unstable atmospheric stratification, which are potential conditions of the heavy rainstorm. (2) There existed a low level jet along the coast, and it is just located in the west side of the subtropical anticyclone. The heavy rainstorm occurs at 100 km range at the left side of the low level jet. The low level jet begins to form about 12 hours before the heavy rainstorm, and it provides an important pass for the transporting of water vapor, momentum and heat. (3) The subtropical anticyclone and the land-sea thermal difference are important factors for the formation of the low level jet. The coupling of high and low level jet and the high level momentum transmission provide the conditions for the maintenance and development of the low level jet. The latent heat release during the rainstorm may have a positive effect in strengthening the low level jet. (4) WRF simulation can better reproduce the heavy rainstorm event, which generates under the interaction of shallow trough and cold tongue at 500 hPa, low level convergence line and the subtropical anticyclone. The low level jet is the major channel for transporting water vapor. The convergence and updraft near the coastal zones triggered the rainstorm. (5) Jiaonan coastal waters, which have more than 80% contribution to the precipitation, are the main source of water vapor of the rainstorm. The land-sea distribution has a very important impact on the low level jet. Jiaonan local terrain has a significant impact on the position and strength of local convergence, thereby affecting the precipitation with an increase by about 20%.