Study Of Mesoscale Disturbance Vortex In The Planetary Boundary Layer Of The Mei-yu Front In Lower Reaches Of The Yangtze River

Rainstorm has great impact to China for its locating in the world's famous monsoon region. Every early summer, it comes into mei-yu season and will bring plenty of rainfalls. There are several mesoscale convective systems (MCS) or mesoscale vortexes generated on the shear line during the mei-yu rainstorm.In order to further understand the origin and development of the mesoscale disturbance vortex in planetary boundary layer (PBL) in lower reaches of Yangtze River, discover the trigger mechanism of the rainstorm and provide some scientific references for the refined short-range forecast, the advanced research WRF model is employed to simulate this kind of rainstorm processes in recent years. Results reveal that there's a low level vortex or shear line over the reaches of Yangtze River before rainstorm happens. Generally, the rainfall, which always accompanies with low level jet, will fall to south of shear line. While there are short wave troughs and westerly zone at500hPa, there exists up-level jet with the wind value more than40/s at200hPa.Physics and mesoscale convergence line/zone in PBL are very important for atmosphere. There will produce a local convergence line under suitable terrain. It will generate a new gust front when the existing gust front and the local convergence line meet, which can intensify the upward movement and break out of strong convection.The analysis also indicates that there're generally mesoscale vortexes that produced in low level during mei-yu rainstorm. However, some vortexes with the horizontal scale of about100km or smaller initial from PBL and most of them develop by shear or confluence. In order to distinguish from generalized vortexes, these vortexes, that the horizontal scale is about100km, generates in PBL initially and is responsible for the heavy rainfall, are called PBL mesoscale disturbance vortex (PMDV).The mesoscale numerical model WRF with WRF-Var3D assimilation and high resolution observation data can simulate the formation and development of mei-yu rainstorm and mesoscale vortexes well.There are kinds of types for formation of PMDV. Some are single, while some are pairs with interaction each other, and some even are cluster. Whatever, there are some common features for all of them. These vortexes are not at the same level, but they initial all in PBL and then develop upwards. The diameters of the vortexes are about50-100km, with warm wet flow from south and plenty of unstable energy to south of shear line before vortexes happen. There will turn up frontogenesis at the region of confluence or shear. What's more, the vortex center will be superposition with the center of frontogenesis and they will develop with each other. The main dynamic conditions of vortex formation is that the confluence and shear of wind in PBL, especially the enhancement and aggression of north wind.There's close relationship between PMDV and MCS when the PMDV happens. MCS existed before PMDV generates, will promote the formation of PMDV for its surface outflow will bring cold wind into PMDV. Of course, the PMDV will enhance the convective, convergence and rain in back. So it can be considered that the PMDV forms when the north jet to north of the shear line in the PBL encounters with the southwest flow in the PBL of the warm section south of the shear line. The mesoscale vortex is accompanied with a strong wind velocity gradient. At the stage of development of the vortex, there's high equivalent potential temperature center in PBL, which extends upwards and form a high energy tongue. However, there will appear a cold center in middle of troposphere and it will cause static instability. The convective instability energy can be released with the strengthenment of the convergence and lower-level flow. So it can bring short-term storm and increase the precipitation of the region near vortex.