On The Development Of A Torrential-rain-producing Convective System Along A Meiyu Front Over East China

During the early morning hours (0000-1400BST) of8July2007, an MCS developednear the southern edge of the Meiyu front, causing torrential rainfall over the Huai rivervalley. By analyzing ERA-interim data and a variety of high resolution observational datasets,we examine the synoptic background and mesoscale characteristics of this MCS. Then themechanisms governing the convective initiation/developmentand decay of the MCS, and airvertical velocity (w)in the MCS’s mature stage are investigated using the finest grid spacingof1.11km simulation experiments which is validated by comparing with the observations.The major conclusions are as follows.(1) During the MCS’s life cycle,a quasi-stationary Meiyu front was located at~34oN,withsharp humidity contrast and weak temperature difference across the front. Torrentialrainfall happened at the southern side of Meiyu front where the precipitable water（PW）exceeds60mm and massive high energy air is transported to the torrential rainfall area alongthe north-west edge of the subtropical high over the west Pacific.The temperature advectionat mid-level and the divergence at upper level aren’t significantsuggestingweak quasigeostrophic forcing.(2)There are two scales of convective organization modes during the torrential rainfallevent. About6-8WSW-ENE oriented meso-βrainbands move side-by-side southeastwardwith the speed of~12-15m s-1, leading to the formation of a400km×100km quasiEW-oriented convective band.This pocessis called rainband training. The meso-γconvectivecells that make up the meso-βrain-bands propagate southwestward and move eastward,forming the echo-training process. Repeated convective initiation/development andmovement of cells/rainbands along the same path result in the torrential rainfall.(3)The surface observations suggest thatthe WNW-ESE oriented rainfall band is locatedabout30-100km to the south of Meiyu front where the convectively generated cold pool andsurface outflow take place. The outflow confronts with the southwest environmental flowforming a high temperature gradient band, i.e., fake cold front.(4)The WRF model simulation is valided by comparing the accumulated precipitation,radar reflectivity and surface meso-scale characteristics with the observations. Both theobservation and simulation show the backbuilding develop model of the meso-βrain-bands, i.e., new convection is continuously triggered above a near surface cold dome left behind by aprevious MCS and then move easteward.Analysis of the simulationsuggests that: the warmand moist ai（rhigh e）is transported northeastward by the intensified nocturnal low-level jet(LLJ) and lifted by the cold dome around the Wangjiaba. When it arrives at the west portionof the heavy rainfall area, the energetic air is elevated toits level of free convection (LFC). Asnew convectionis triggeredcontinuously and move eastward driven by the mid-tropospherewesterlyflow, the meso-β-scale rainbands develop through the backbuilding mode.(5) Due to the weaking of the environmental LLJ after1000BST, CAPE cannot berestored to support the MCS’s maintenance causing rapid dissipation of the MCS. On theother hand, the convectively generated surface outflow could also play an important role inmodulating the MCS’s dissipation.(6) The MCS evolution in the sensitive experiment with the topography of mountainDabie removed is nearly identical to the CONTROL simulation, suggesting negligibleimpacts of mountain Dabie on the torrential rainfall event of interest.(7) Analysis of air vertical velocity (w) suggests that,(a) in the DC regions, the airvertical motion is mainly ascent with the maximum w at the mid-troposphere (~6km); At thelow altitudes (below1.5km), environmental high-eair ascends and the convectivelygenerated low-eair descends. In the RST, air ascends weakly at the mid-to-high altitudes(above5km) and descends weakly below4-5km altitudes.(b) The perturbation air densitybuoyancy term (B1), vertical pressure gradient force term (PGA) and hydrometer loadingterm (B2) play important roles in the predictive equation of w. In the DC regions below the~1.5km altitude, both the thermal (B1) and dynamical (PGA) terms help forming newconvection at the leading edge; At2-10km altitudes, latent heating induced by changing ofwater phases supports the strong up-motion of air in the DC regions; Near the cloud tops, B1is negative and PGA is positive likely because of long-wave radiative cooling and adiabaticcooling in association with weak air upward motion. Compared to the DC regions, the RSTregions see smaller values of B1, PGA and B2. Below~5km, B1and B2are negative whilePGA is positive in the RST regions, suggesting that rain evaporate cooling is the majorcontributing factor for the weak air descent; At5-10km altitudes in the RST regions,detrainment of positively buoyant air from the DC regions to the RST regions and thesubsequent depositional heating result in positive B1which supports the upward air motionthere.