Study Of The Orographic Effects Of The Dabie Mountain On The Development And Structure Of A Squall Line

A squall line process that occurred in Jianghuai region on July 11 and July 12,2014, was investigated by the WRF model based on NCEP/NCAR reanalysis data and conventional meteorological observation data, combining with topographical sensitivity experiments and comparison analysis. The case study revealed the structure characteristics of the squall line and the orographic effect of the Dabie Mountain on its development. The results are as follows:(1) Both the simulation and the observation showed that the squall line developed in the western part of the main body of Dabie Mountain and decreased in the east.And it developed when the squall line is moved to the east side of the Dabie Mountain, There are obvious back-building phenomenon in the convection line of the squall line below the moutain, and showed the characteristics of a mature squall line finally. Squall line generated and strengthened in the piedmont of the Dabie mountain and quickly reduced behind the mountain, the main reason is because the convective instability and upward movement in front of the mountain is much greater.The eastern edge of the Dabie mountain plays a positive role on the growth of the squall line: when the squall line moved to the eastern edge of Dabie mountain in the afternoon, eastern valley breeze strengthened and forced the generation of convection monomer in the front of the squall line, with an 6 m/s maximum increment of the lower wind speed according to the experiment, making the cold air at lower level warm moist air encountered and strengthened the growth of the convective monomer, and merged with the back of the convection cell, presenting the forwarding new type squall line. When there is no valley, there did not exist a new monomer any more. The reason for the newborns in the Dabie Mountains is that the relatively cool outflows in the convection line flow outward and the warm and humid air currents in the southwest side, thus striggering newborn convective, and the development of the convective lines is maintained.(2)The terrain sensitivity experiment results showed that: in the control experiment, there were not only strong back wave disturbance at the leeward side,but also abundant humid air at the same time, producing strong convection in the leeward side, and thus prompting the development of the squall line. When the whole terrain of Dabie mountain is heightened, although the lee wave disturbance was enhanced, but because of the blocking effect of the mountains, the water vapor transport in the eastside was restrained, so the development of squall line convection became weaker; After smoothing the whole terrain, lee wave disturbance is abated,the convergence of southwest and northern airflow strengthened, with an easternoverall bias of the convection development.(3) Dabie mountain has a positive effect on 700hPa frontogenesis function, from the contribution of each experiment, divergence of F1 made up of the biggest contribution in the process of the frontogenesis. When changed the terrain, the 700 hpa frontgenesis effect obviously has an southeast direction deviation, the squall lineinside will be restricted and changed by a lot of strong convective activities.