Research On The Initiation And Development Mechanisms Of Convective Systems In The Vincinity Of The Mei-yu Front

Based on observations and convection-permitting simulations,three aspects of moist convection which occurs in the vicinity of the Mei-yu front are investigated in this paper.First is the diurnal variability of precipitation.Diurnal cycles of the Mei-yu frontal precipitation,including their regional differences are revealed and associated mechanisms are also examined.Second is the convection initiation(CII).The ARPS model is utilized to study three types of CII which are associated with the formation of a squall line in the southern flank of the Mei-yu front Last is the maintenance and organization of banded convection.In combination of a idealized model and a real case study,the mechanisms responsible for the development and organization of the Mei-yu rainband are discussed.The main results are as follows:(1)Diurnal cycle of the Mei-yu frontal precipitation and its controlling mechansimsThe Mei-yu frontal precipitation could be divided into south and north rainbands.Both rainbands present obvious diurnal variations and have peak rainfall in the early morning while the south band has a secondary peak in the afternoon,indicating the regional differences.Net moisture flux into a control volume enclosing each rainband plays a paramount role in modulating the diurnal cycles of Mei-yu frontal precipitation and the time-lagged correlations for rainbands are maximized with a 2-3 hour time lead in the net flux.Net moisture flux is almost purely due to low-level ageostrophic winds(AGWs)and the ageostrophic convergence also dominates the large-scale forcing.AGWs exhibit diurnal cycles with clockwise rotations and their directions and magnitudes depend on the background geostrophic winds and such behavior agrees very well with the prediction.In the afternoon when active surface heating exists,surface friction and boundary layer mixing slow down the boundary layer flow significantly,the ageostrophic winds are directed in roughly the opposite direction as the geostrophic winds,making the total wind speed sub-geostrophic,effectively drawing air out of the frontal zone,thereby causing the frontal precipitation to weaken quickly in the late afternoon and evening.Usually between midnight and early morning,the surface friction ceases to exert a drag on the boundary flow and the ageostrophic wind vector points in a similar direction as the geostrophic wind,making the total wind super-geostrop hic.It is the ageostrophic wind that creates the low-level flow convergence and hence low-level lifting.The net moisture flux gradually points to the frontal zone and reaches its maximum.Strong,sustained super-ageostrophic flows from the south also transport more moisture into the frontal zone.The two processes create precipitation maximum in the early morning.A two-dimensional analytical model which includes both diurnal thermal forcing(the Holton mechanism)and diurnally varying boundary-layer friction(the Blackadar mechanism)is able to account for the rainband diurnal cycles.In fact,the Blackadar mechanism is primarily responsible for the diurnal variations,such as the amplitude while the Holton mechanism also contributes to the phase.Even if the rainbands are also affected by the thermal circulations,like the mountain-plain and land-sea circulations,they apparently play a secondary role.(2)The initiation of a squall line within a Mei-yu frontal systemThe initiation of convective cells that evolved into a squall line(IOP8)on the southern flank of a Mei-yu front is investigated using the ARPS and ARPS-3DVAR which is used to assimilate the dense surface observations.The initiations of four convective cells,denoted as CII-A through CII-D,actually are associated with three different processes.For CII-C,initiation occurs directly over the gust front with the frontal convergence providing the main forcing.For CII-A(B),locally enhanced convergence bands associated with boundary layer horizontal convective rolls(HCRs)play a crucial role in determining the exact locations of CIIs while the secondary front from earlier frontal precipitation also contributes to additional convergence forcing.CII-D occurs south of and sufficiently away from the gust front,and is mainly forced by the HCR circulations.With surface heat flux is turned off,CIIs are much delayed.Surface heating increases convective instability of air south of the front and causes the development of HCRs;it also enhances the gust frontal convergence by mixing higher southwesterly momentum toward the surface.When condensation process is turned off,HCR and/or gust front forcing are still able to lift the low-level air to super-saturation where CII is expected.Surface and boundary layer processes therefore are still important for CIIs as in much drier environments like those associated with drylines in the United States Southern Great Plains even though the Mei-yu season is usually characterized as being wet,cloudy and short of sunshine.(3)The Mei-yu rainband maintenance and organization processesIdealized simulations and convection-permitting numerical simulations are used to investigate the maintenance and organization of long-lived convection in the formation of a Mei-yu rainband.The Mei-yu rainband is composed by multiple convective precipitation bands and therefore is qualified to have the two-dimensional banding nature.Its life-cycle usually includes three episodes,strong convection,reinvigorations and dissipations.Following the southeastward progression of the mid-level trough,cold and dry northwesterly wind advances toward the rainband region.In the meanwhile,accompanied dry air from the upper level descends and intrudes downward to the lower moist sector,thereby inducing the vertical extent of potential instability and resulting in the strengthened frontogenesis.As the consequence,convection is reinforced to support the rainband development.The simultaneous presence of the conditional instability,conditionally symmetric instability(CSI)and inertial instability appeare to be the best hypothesis to explain the organization of this rainband.In response to the releases of above instabilities,the mesoscale circulations in deep convection are actually the coexistence of the upward and slantwise motions.