Characteristics Of Meso-?-scale Vortices In The Warm Season Over The Yangtze And Huai River Basin

Mesovortices(MVs)are meso-y-scale vortices,which are in general formed on the leading convective line of mesoscale convective systems(MCSs)or convective cells.MV can often produce severe weather like heavy rainfall,large hail,etc.In the warm season of Yangtze and Huai River Basin(YHRB),convective systems especially MCSs occur more frequently than usual.In this work,three-year(2013-2015)Doppler radar observations from April to July at 17 stations and reanalysis data are used to study the statistical characteristics and environment features of MCSs and MVs in the YHRB.The main results are as follows:From April to July of 2013-2015,there are 95 MCSs detected in the YHRB,most of which are formed in the eastern Hubei and western Anhui province.The occurrence frequency of MCSs is of apparent monthly and diurnal variations.More than 60%of the MCSs are found in June and July,especially in July.MCSs most often occur from late afternoon to midnight with a secondary peak in the early morning.In June and July,more warm,moist air are transported to the YHRB from South China Sea and Indian Ocean under the influence of Asia summer monsoon.The convective available potential energy(CAPE)is notably increased in June and July,while the low-level vertical shear is stronger in April and May.As a result,there are more MCSs in June and July than in April and May.In particular,MCSs occur more frequently in Meiyu period of the YHRB,i.e.,from mid-to-late June to early-to-mid July.MCSs in this season have longer lifetime and bigger size than others.MCSs are divided into two types according to their shapes.One is linear MCSs,while the other is nonlinear MCSs.Linear MCSs have more apparent monthly variation than nonlinear ones.The former also has longer lifetime than the latter because of more plentiful warm and moist air being transported to the YHRB,leading to larger CAPE and low-level vertical wind shear that are favorable for the development of MCSs.From April to July of 2013-2015,in total 3790 MVs are identified,half of which occur over the central YHRB.The number of MVs increases from April to July as in the case of MCSs.But the diurnal variation of MVs is weaker than that of MCSs.According to their lifetimes,MVs are divided into three types,i.e.,short-lived(18-30 min),moderately-lived(30-60 min)and long-li-ved MVs(>60min).All the three kinds of MVs have similar monthly variations to that of the total MVs.However,long-lived MVs have more apparent diurnal variations than the other two types.They occur more often in mid-afternoon.Long-lived MVs also have stronger intensities and larger sizes than the other two,MVs are divided into to types,i.e.,MCS-type and cell-type MVs.The former is generated within MCSs,while the latter is generated by convective cells.The MCS-type MVs are primarily formed during the developing and mature stages of the parent MCS.They are larger,stronger and longer-lived than cell-type ones.MCS-type MVs in general occur in an environment of stronger inertial instability in the upper troposphere and synoptic forcing in the middle troposphere,both of which are conductive to the development of organized MCSs.More importantly,there is a southwesterly low-level jet(LLJ)formed at 850 hPa which can more efficiently transport warm air with plentiful moisture into the YHRB.The LLJ can also increase the low-level vertical wind shear,which favors the development of strong and long-lived MVs.