Analysis Of A Regional Heavy Rain Process Caused By Southwest Low Vortex In Sichuan Basin

Southwest low vortex is a meso-scale system, its strong development and movement can bring heavyrainfall etc severe weather to most regions of our country. In this paper, by using automatic weather stationrainfall data in Sichuan, NECP1°×1°reanalysis data, radar mosaic products of SWAN system andTRMM satellite data, combining synoptic meteorology and physical diagnostic methods, we study thesouthwest low vortex’s large scale circulation background, its development and movement mechanism, itsdynamic and thermodynamic structure characteristics, its radar echo characteristics, its precipitation andprecipitation clouds of macro-and micro-structure characteristics. The main conclusions are as follows:(1) The large scale circulation background of this low vortex heavy rain occurrence are typical500hPa“saddle field” circulation and the high-low altitude configuration of “north trough and south vortex”,typhoon “Conson” and low-level jet have important influence on the evolution of the vortex and thevapor transport. The heavy rainfall concentration period of the two rainfall centers-Leshan andBazhong/Dazhou are consistent with the activity period of vortex in these areas. The vorticityadvection transportation near the vortex areas at700hPa plays an important role in the development ofvortex. When the positive vorticity advection centers strengthen, the southwest vortex developsstrongly, on the contrary, the vortex does not develop. The vortex will only move where coldadvection in the back and warm advection in the front both appear at850hPa, the movement directionare the center zero line direction of clod and warm advection intersection, the stronger the temperatureadvection transport, the faster the vortex move, on the contrary, the slower.(2) The dynamic and thermodynamic structure characteristics are: the positive vorticity areas appearbelow500hPa height, positive vorticity centers are at700hPa height, the negative vorticity areasappear in the middle and upper troposphere; the convergence zone are below600hPa, correspondingbetter with the positive vorticity district, strong convergence center appear in the lower troposphere,below850hPa, the divergence areas are above600hPa. The positive vorticity center in lowertroposphere layer appearing period has a good correlation with the period of heavy rainfall appeared.The convergence center in lower troposphere and the divergence center in higher troposphere basicallyboth appear a little earlier than the heavy precipitation center come, this is a good indication. Theheavy rainfall period has a good correspondence to the strongly rising period, the vertical velocitycenter are at600-500hPa, the strongest can develop to300hPa nearby. Stratification are unstablebelow850hPa, the time interval between the maximum convective unstable stratification appear and the heavy rainfall come show the occurrence of high intensity precipitation required a certain time ofunstable energy accumulation. In addition, the vapor flux convergence center at850hPa has a goodcorrespondence to the strong precipitation center.(3) COTREC wind radar mosaic products in SWAN system capture the cyclonic circulation(or shear)feature repeatedly in the vortex regions, so this cyclonic circulation features can be well reflected thedisturbance characteristics of vortex. The radar echo characteristics show it mainly stratus-convectivemixed rainfall pattern, the development and movement of strong echo center has a good correlationwith the development and movement of vortex. The max vertical liquid water content values are nothigh, water vapor is mainly form the low-level warm and moist airflow transportation. Moreover, themaximum vertical liquid water content, the maximum combination reflection index and the highestecho top are basically had the same evolution trend, high value center of vertical liquid water contentcorrespond with strong combination reflection index and high echo top. The development andmovement of vortex can promote clouds echo to merger, the surge of maximum vertical liquid watercontent make clouds rapidly accumulate energy, so that the radar echo intensity and scope can expandfurther.(4) The characteristics of vortex precipitation clouds in TRMM are: in the horizontal direction theprecipitation clouds mainly locate in right front of vortex, it is obvious mixed clouds structure in theinfrared brightness temperature map, the lower the cloud top temperature are, the stronger theconvective activity is. Infrared brightness temperature map is much larger than the microwavebrightness temperature map, the low value areas in the microwave brightness temperature correspondwell to the rainfall areas, the lower the microwave brightness temperature value are, the stronger theprecipitation intensity is. In the vertical direction, it is always the highest content of ice particles,followed by precipitation particle content, the third is cloud water particle content and cloud iceparticle content is at least. Ice particles can throughout the low level to the high level, mainlyconcentrate below10.0km, the maximum appear in the condensation layer, precipitation particles andcloud water particles mainly distribute below5.0km, cloud ice particles are mainly above8.0km, thestronger the upward motion are, the greater cloud ice particles content in the higher troposphere are.(5) The characteristics of vortex precipitation in TRMM are: in the horizontal direction, vortexprecipitation regions locate in the north and northeast of vortex, precipitation clouds develop fromisolated cloud block to a precipitation rain band in the northeast-southwest direction, the stratiformrain surrounded convective rain in the rain band, the stratiform sample are up to90%in the whole rainband, the range of rainfall intensity is mainly concentrated in the0-5mm·h-1, smaller rain(<10mm·h-1)are more than60%of total precipitation, convective precipitation mainly concentrate in the range of10-20mm·h-1, with more than30%contribution to the total precipitation, with the development of convective precipitation, the contribution to the total rainfall of larger precipitation intensity(>30mm·h-1) in the convective precipitation spectrum increase. In the vertical direction, the maximum raintop height can reach17km in the mature stage of vortex, the instantaneous intensity of heavyprecipitation center is over50mm·h-1, appear in the5km, it is obvious stratiform rain surroundconvective rain structure, the rain top basically maintain near10km height in the development andreduced stage of southwest, and the bright band structure of stratiform rain is clear near5km Whenvortex develop again, the precipitation intensity in the whole rain band strengthen again, but the raintop height and precipitation intensity are far less than vortex mature stage, and it is obvious spiral rainband structure near heavy precipitation center in the vortex region.(6) TRMM data obtained the average precipitation profile feature of vortex are: convective precipitationprofiles can reflect the development and evolution of vortex precipitation, in the mature stage,precipitation increased significantly in the upper troposphere(10-18km), many precipitation particlesare uplifted to the high-level to form the solid precipitation particles. The maximum rainfall valueappear below6km, it explains coagulation growth process in clouds play a major role in thisprecipitation process. Convective and stratiform rain have increasing also reducing with altitude raisedbelow5km, it may has relation with the micro-physical processes in that height and the complexterrain of Sichuan region. The latent heat profile feature of vortex is: the maximum latent heat releasealways appear near7km height in four stages. The latent heat release strongest in each layer in themature stage of vortex, it increases rapidly to a peak value below7km, close to8000K h-1, thendecreases rapidly in the range of7-9km, the latent heat release has a slight rebound in the range of9-12km with altitude increasing, and increase rapidly again above14km, these change may be relatedto the phase change process in the cloud(top). The total latent heat release value in each layer at theother stage have little difference, the total value in vortex mitigation phase is slightly lower than theother two phases.