Numerical Studying On A Rainstorm Process In Ningxia

Based on the study about the climatic characteristics of the east of Northwest China, a rainstorm process, which happened during29th-30th, July2012("7.29rainstorm process" for short) is analyzed and discussed by using routine observation data, encryption automatic station hourly data in Ningxia, Yinchuan doppler radar data, Ningxia lightning observation data, FY-2E satellite TBB data and the output of high resolution in temporal and spatial from WRF model. The main results are as follows:(1) The analyses on the climatic characteristics of precipitation in the east of Northwest China show that the amount of precipitation dicreases from southeast to northwest in the region and with obvious seasonal differences. Moreover, the precipitation frequency in the study area is dominated by weak and moderate precipitation events. Although the frequency of heavy rain is low, one strong rainfall process may make a large contribution to the annual precipitation, sometimes, it can reach40%of the annual precipitation. The Mann-Kendall test reveals that the heavy rainfall frequency has a clear decrease trend after1990. Besides, the wavelet analysis on the heavy rainfall indicates that a3-year period is obvious in1960s, and the period is getting longer after that, until it becomes unconspicuous in1990.(2) Analyses on the large scale circulation feature in "7.29" rainstorm process shows that the development of rainstrorm is closely related to upper-level jet at200hPa, short-wave trough at500hPa, the shear line and eastward low vortex at700hPa.(3) Radar echo shows that continuous strong echo of convection cells occurred before02BT30th, the coverage range and strength of radar echo decreased after that. The moving and variation of convective clouds associated with the mesoscale vortex has closely relationship with precipitation intensity in the process by TBB data. Spatial distribution of lightning was consistent with the area of heavy rain, which mainly occurred between20on29th and02on30th. The lightning frequency obviously decreased from02on30th to12on30th, when stable precipitation dominated.(4)The simulation results of WRF model indicates that the formation and development of rainstrorm is related to eastward low vortex and its warm-type shear line, while the former is stable and the latter is moving northward slowly during the period of convective precipitation. And in the period of stable precipitation, the low vortex moves to the center of Ningxia, the cold-type shear line gets stronger, and the warm shear line moves to the region of Inner Mongolia. The low level jet (LLJ) came into being on22BT29th, which maintained for about7hours, and got weaker and moved out of Ningxia after04BT on30th. The LLJ has an important effect to the rainstrom.(5) The diagnostic analyses with high resolution data of WRF model show that the continuous water-vapor transfer and deep water vapor layer bring abundant water vapor for the rainstorm. The water vapor transfer is mianly from the south boundry, which is corresponded to the formation and development of the LLJ. The variation of net water vapor transport is consistent with the feature of convective precipitation and stable precipitation. Strong divergence at the right side of upper-level jet exit region, the quality ajustment of atmosphere between upper level and low level make pressure gradient and allobaric wind increase, which is crucial for the formation and maintenance of low level jet. The strong confluence of warm and cool air and the positive differential vorticity advection broke up the quasi-geostrophic balance and inspire the secondary circulation, and caused the formation of the strong ascending movement in northern Ningxia. Further more, the region of A NBE has a corresponding relationship with the rainfall region in the period of convective precipitation, and which is quite small in stable precipitation stage, and the characteristic of quasigeostrophic balance is obvious in this period.(6) Mutiscale charaters were analyzed with two dimension Discrete Consine Transform (2D-DCT) indicate that the low vortex develops and moves eastward in large scale as well as the development and variation of Pmesoscale vortex about200-300km in mesoscale at low level of troposphere, the variation of Pmesoscale vortex is corresponed with the convective cloud cluster analyzed from TBB data. The characteristics of middle level the moving and development of the trough at middle level of troposphere. The variation of precipitation intensity is related to water-vapor flux at large scale and mesoscale, and the large scale water-vapor flux takes a large contribution to water-vapor transfer in northen Ningxia. The decrease of water-vapor flux in large scale and mesoscale is one of main causes that convective precipitation turns to stable precipitation.