Analysis Of Spatial-Temporal Structures And Nonlinear Subcritical Symmetric Instability In A Heavy Rainstorm In North Jiangsu

Based on conventional observational datasets,infrared satellite pictures,and NCEP reanalysis data,a heavy rain process occurred in Yangtze-Huaihe river valley during 28^thJune to 5^thJuly in the year of 2006,is synoptically analyzed and dynamically diagnosed.The mesoscale numerical model WRF(V2.2)is used to simulate the the heavy rainfall process during 00:00UTC 30 June - 12:00UTC 01 July in the region of Baoying located north of Jiangsu province,then with the high-reserlution basic physical parameters from the model output,the perpendicular shear of wind field,calculated horizontal helicity,the meridional-vertical circulation as well as calculated vorticity and divergence fields are diagnosed and analyzed.By using mesoscale band-pass filtering method,mesoscale disturbance wind field is extracted from the model output,which is used to analyze the nonlinear subcritical symmetric instability.The results are as follows:(1)During the heavy rain process from 28^thJune to 5^th July in the year of 2006 in Yangtze-Huaihe river valley,the middle-high latitude area experienced the adjusting of the two ridges- one trough circulation pattern for two times,corresponding to the cold air moving southwardly twice.The strenghening of westly in low latitudes convey water vapor to Indo-China Peninsula,the the southly flow around subtropical high transport the water vapor to the east and south of China, meeting the cold air from north between 30°-35°N,causing the persistent heavy rain process.(2)After the direction turing of vertically averaged wind shear from WNW to WSW,the maximum of south wind component indicates precipitation peak which emerges after 5-4 hours as well as absolute horizontal helicity.(3)The first emergence of strong updrafts through the total column of the atmosphere indicates the rain torrential 3 hours later.(4)Theπ/2 phase difference between divergence and vorticity at 200hPa is 9h before rainstorm,corresponding to intensely non-geostrophic gravity waves,propagates downward to 800hPa within 3hours before rainstorm.(5)The negative extream-value of moist potential vorticity in the low layer appears 3h before the precipitation peak,the magnitude of MPV1 seems approximated to that of MPV2,so that moist baroclinity and vertical wind shear in the low-level are quite strong.MPV2 is generally determined by the lower atmosphere,while the scale of MPV1 is approximated to that of the rain cluster.(6) Nonliear subcritical symmetric instability appeared in 2-1h before rainstorm and simulates the heavy rainfall,which has the indicative meaning to the rainstorm.(7) The linear symmetric instability mainly occurs at 925hPa,while nonlinear symmetric instability mainly occurs at 700hPa.(8)Heavy precipitation mainly occurs at the place between the south side of the maximum disturbance wind field at high levels, and the north side of the maximum disturbance wind field at the low levels.(9) Mesoscale disturbance wind field is strengthed at the level of 750-500hPa 6h before the rainstorm,corresponding to the nonlinear subcritical symmetric instability energy accumulating,and the unstable energy is released around 3h before the rainstorm then triggers the rainstorm.