Divergency-Vorticity And Bore Dynamics Study In Warm Season Precipitations

The dynamic and thermodynamic mechanisms of precipitation systems in warm season are complex,rainfalls from these systems have characteristic of long lasting,high intensity and of large scale.Associated precipitation cause significant impact and loss to people.So,improvements of understandings on convective systems' initiation and maintenance mechanism have great significances on improving weather forecast accuracy and enhancing the capabilities of disaster prevention and reduction.Based on the observations that meso-scale system has the characteristic of vorticity-divergency coexistence(i.e.the vorticity and diversity share the same magnitude).The paper raise the divergency-vorticity structure(hereafter,DV structure,the structure where the vorticity and divergency of cyclone in lower level of convections share the same magnitude)model firstly.Based on this,DV structure identifying method,its intensity parameter M()and M tendency equation are also raised.Results show that parameter M is controlled by divergency variation and vorticity variation.Based on the establishment above,a 5-days Meiyu-front precipitation process from30 June to 05 July,and landfalling typhoon “Nepartak” precipitation process from 07 to 10 July in 2016 are studied.Analysis of Mei-yu precipitation process shows that precipitation is trigged by low vortex moving east forward along north side of Mei-yu front and constitutes of cluster/banded meso-scale precipitations.In cluster systems,DV structure mainly distribute in the region of stratus and has a high correspondence with precipitation area.In banded systems,DV structures distribute along Mei-yu front,north side of low vortex and also have a good correspondence with precipitation area.Value of M in precipitation area is close to-1 while smaller outside.Compared to storm relative helicity and low-level divergency,the distribution of DV structure has a better indication of precipitation area.Evolution of M value has a good negative correlation with that of precipitation intensity,thus will have a good role of prediction.Analysis of an individual CI case in Mei-yu front indicates the process of geostrophic adjustment and evolution reflected by M value.Analysis of the evolution of Typhoon “Nepartak”shows that the distribution of DV structures are consistent with that of spiral rain-band and eyewall area and has a indication of potential precipitation.Variations of M(e.g.,average DV number)show a better correlation with precipitation intensity than lowlevel divergency and mid-level vorticity.The studies of initiation and maintenance mechanism of nocturnal convections are based on two observed nocturnal meso-scale system associated with atmospheric bores occurred on Great plains of United States from 10-12 Jun,2016 and Yangtze-Huai plains of China from 05-06 Jun,2016 separately.Analysis of the case over Great plains shows that the atmospheric bore occurred when the associated systems transform from surface-based convective line to elevated convection.The atmospheric bore has a speed around 15ms-1,lifestyle around 4 hours and a pronounced lifting effects of air parcels in low-level inversion layer.Drop in surface pressure and minor warming can be seen as bore passed by but different from past studies,wind direction didn't show obvious change.Environmental CAPE and CIN are concentrated in mixing boundary layer.The lifting effect of the bore decreases the LFC,lifts CAPE and consumes CIN,the air parcels of high CAPE is lifted above LFC,thus CI occurred,system maintained.There exists a wave duct between 1700-4000 m AGL which help to maintain the bore propagating for a while.Diagnosis of vertical acceleration shows that the lifting effects of atmospheric bore mainly from inertial acceleration where fluid extension dominates.Ideal simulation indicates that the bore is initiated by the surface shallow cold pool moving in the environment with low-level inversion layer.The mechanism could be explained by two-layer fluid theory.It is call “elevated bore” for its propagation at low level and has a more pronounced lifting effect.Further investigation indicates that the modification of DV structure identification method is more suitable to apply in these elevated nocturnal systems.Analysis of the case occurred on Yangtze-Huai plain in the evening from 5-6` Jun2016 show that the bore occurred ahead of the convective line in the front of the associated bow echo system.The bore has a max length around 100 km,speed around12.9ms-1,a bit faster than system and lasted for 90 min.As the bore passed by,surface pressure increase,temperature showed no changes and wind speed sharply change with direction 180°shifts.Sounding show an inversion layer existed between 1500-2200 m AGL.Simulation showed that the bore evolved from the detached head of convective cold pool and has some characteristic of gravity current,thus belongs to Type C atmospheric bore.The bore has a pronounced lifting effects on environmental CAPE and has a great role on system structure modification and maintenance.