一次飑线内MVs和后向入流对地面大风形成影响的模拟研究

A simulation study on the influence of MVs and rear inflow in a squall line on the formation of strong wind near ground

通过实况资料以及WRF模式对广西地区的一次飑线过程进行数值模拟,根据模拟结果对飑线内中尺度涡旋MVs(Mesoscale Vortices,MVs)和后向入流与地面大风的成因关系进行了分析。通过涡度收支和涡线分析得出,弓状回波中存在东西涡旋对,其生成主要是散度项造成,中间相对弱的反气旋涡旋是涡线拱起产生的水平涡度向垂直涡度转换引起。通过计算正负涡旋对引起的旋转风从而量化了涡旋对对地面大风的贡献。结果发现,本次过程中涡旋对引起的旋转风在地面大风中占有较大比重,约40%~50%。当去除涡旋对引起的旋转风时,地面大风的强度减弱,位置偏移。由三维流线可以看出后向入流的下沉是产生地面大风的另一个影响因子。通过对浮力加速度和动力加速度进行诊断发现,后向入流的下沉主要是由于负的浮力加速度引起,水平方向的密度不均匀是负浮力加速度产生的主要因子,但在风速突然加强时,动力加速度也有明显的影响。

Through the WRF model and observational data,a numerical simulation was performed for a squall line process which had occurred in Guangxi Zhuang Autonomous Region of China.Based on the simulation results,the relationship of the MVs(mesoscale vortices)and the rear inflow in the squall with the surface high winds is analyzed.The Vorticity Budget and analysis of vortex line both show that there are east-west vortex couplets present in the bow echo,which are mainly caused by the divergence term.In addition,the relatively weak anticyclone vortex in the center is generated due to the conversation of horizontal vorticity to vertical vorticity,caused by the arch of the vortex line.Next,the contribution of the vortex couplet to the high winds on the ground is quantified by calculating cyclostrophic wind caused by the vortex couplet.The results show that the cyclostrophic winds account for 40%—50%of the ground winds in this squall line process.When the cyclostrophic winds are removed,then the intensity of the surface high winds will be weakened and the position will shift.It can be seen from the three-dimensional streamline that the subsidence of the rear inflow is another factor that produces the surface high winds.After completing the diagnosis of the buoyancy acceleration and dynamic acceleration,it is found that the sink of rear inflow jet is caused by the buoyancy acceleration.In addition,the uneven density in the horizontal direction is the main factor in generating the negative buoyancy acceleration,but the dynamic acceleration also has an obvious effect when the wind speed is suddenly strengthened.