Characteristics Of Warm Season Mesoscale Vortices Over The Middle And Lower Reaches Of Yangtze River

Mesoscale vortex which appears in the middle and low levels,is a cyclonic circulation with its size between several dozen kilometers and several hundred kilometers.It is one of the major weather systems causing summer rainstorms in the middle and lower reaches of Yangtze River,which always shows close relationships with the convective activities.To make a better understanding of mesoscale vortices,the CFSR(Climate Forecast System Reanalysis)data and an artificial neural network automatic vortex-searching method are used here to get the warm season mesoscale vortices in the middle and lower reaches of Yangtze River during 1996-2015.The basic characteristics of the 309 mesoscale vortices we obtained are analyzed,such as the interannual and inter-monthly variation of the frequency,generated time,longevity,horizontal scale,vertical thickness,moving path,vortex nature and so on.Totally,five wet vortices are observed forming within organized convection in the middle and lower reaches of Yangtze River in the warm season of 2015.The structural characteristics of these 5 vortices are studied via composite analysis.The vortex with the longest life span is also analyzed and its evolution is shown in detail.In addition,the mechanism of itsdevelopment is discussed.All results are shown as follows:The frequencies of mesoscale vortex in May,June and July are similar,while inAugust,the frequency shows a much smaller value than the other months.Most of vortices are local vortices,which mostly generate at 02:00 am(Beijing time,similarly hereinafter).The second peak of the generated time is 14:00 pm.The longevity of most vortices is within 30h.The radius of most vortices is about 100km.The vertical thickness is mostly less than 250hPa.And the most significant height of closed circulation is 850 hPa.Thus,mesoscale vortex in the middle and lower reaches of Yangtze River is a shallow system at low level.The moving path of the vortex is mainly northward,eastward,southward and quasi-static.Most of vortices are northward path.The mesoscale vortices are divided into three categories,which are wet vortex,dry vortex and mesoscale convective vortex(MCV)(MCV can be regarded as a special type of wet vortex),respectively.The number of wet vortices is far more than that of the dry vortices,indicating that the formation of most mesoscale vortices is inseparable from water vapor.Also,the phase change and latent heat release play a dominant role in the formation of vortices.The horizontal scale and the life span of dry vortex are small and its formation is mainly caused by the dynamic action.The composite analysis shows that the vortices are in the structures which are cold at low levels and warm at upper levels showing differences with the extratropical cyclones of cold cores and the tropical cyclones of warm cores.Instead,they have weak warm cores.The strong convergence zone of the vortex,the strong divergence area at upper layer and the strong ascending motion area are not in the center of the vortex,but on the east side of the vortex.This indicates that the development of new convection activities may be in the eastern side of the vortex,which may show a relation with high and low jet coupling.The analysis of vorticity budget shows that the configuration with the convergence at low level and the divergence at upper level plays a very important role in the growth of the vorticity.At the lower troposphere,the stretching term and the tilting term are the source of positive vorticity,while the vertical transporting term and horizontal advection term suppress the increase of local vorticity.Due to the initial convergenceat low layer and ground disturbance,the convective precipitation is triggered.The latent heat of the precipitation is conducive to the formation of the vortex.Besides transporting water vapor,the low-level jet also delivers a positive vorticity to the mesoscale vortex zone.The positive vorticity at the middle layer is the reason of the decompression at ground.As a whole,the generation of vortices is the result of multiple factors acting together.After the vortex produced,it can influence precipitation.Vortex and precipitation form a positive feedback effect.The configuration of lower-level convergence and upper-level divergence and the release of latent heat make the vortex develop and maintain.