Analysis Of Characteristics Of Rainstorm In Tengger Desert And Numerical Simulation Research

By using data from mutiple sources, the climatic background of precipitation in Tengger Desert was briefly introduced first. Then, the causes and characterisitics of two major rainstorm processes taken place in Jul,2012 were compared and analyzed from aspects of circulation background, water vapor, dynamic and thermal conditions. Afterwards, mesoscale filtering was performed according to Barnes’s filter operator. Combined with the scale-separated vorticity equation, the interaction of large scale and mesoscale fields and its impetus on the mesoscale systems were thus analyzed as well as the relationship between Mesoscale thermal circulation and rainstorm was discussed. At last, sensitivity tests of initial field wave filtering were performed in virtue of WRF model, it preliminarily studied the relative contribution of large scale and mesoscale fields to rain and the sensitivity of rain simulation depending on them. Main conclusions summarized as follows:(1)Characteristics analysis of precipitation for years suggests that the precipitation amount and yearly average days of rainstorm in Tengger Desert decrease from southeast to northwest. The most concentrated months for rainstorms are from June to September. Rainstorms have becoming more frequent since 1990s and the interannual distribution of precipitation has also been homogenized as well as the stability has been increased during the past half century.(2)An analysis of two typical rainstorm processes revealed that they were intermittent frontal precipitation by the first type of slow cold front and mixed one of frontal and convective precipitation by the second type of fast cold front respectively, which differed in circulation situation, water vapor, mesoscale convective system, dynamic and thermal conditions. However, they were all caused by the joining of swelling subtropical high and Westerly disturbance. Water vapor was input into region of two deserts mainly from north, east and south boundaries, among which the north and the east boundary got persistent input with greater net income, while the south and the west boundary received water vapor only before and after rainstorm with less net income. The intensity change of the apparent heat source and moisture sink corresponded well with rainstorm process and reached its peak when rainstorm became most intensive. The altitude of heating center of them was different, apparent heat source was dipole type:heating center appeared at both high and low level in troposphere while heating was weak at midlevel; apparent moisture sink’s appeared only at low level. Among all components of these two sorts of heat, vertical transportation was the main bearer that deciding the close relationship between ascending movement and themselves, while local term neutralized advection term. All terms of apparent heat source were able to indicate the movement of high and low level jets and front indirectly.(3)There exist differences in the role that large scale and mesoscale field played during rainstorm and the relative contribution of them to rainfall. Large scale field provided environment for rainstorm, including long distance moisture transport and the potential instability generated by the superimposition of high and low level jets. While the spatial and temporal distribution and evolution of mesoscale field decided the drop zone, intensity and beginning time of rainstorm. An analysis of scale-coupled vorticity equation suggests that the influence on mesoscale vortex of intercoupling of large scale and mesoscale vorticity field was far more notable than that of changes within themselves, among which the divergence term was main bearer while distortion and advection term only affect when vortex grew most strong. The influence of systems of two scales on precipitation through numerical simulation showed that after initial field being filtered, rain belts of all test groups were relatively north compared to approval test, the distribution of positive centers of daily precipitation difference were always determined by southwest wind field difference. Regardless of frontal or convective precipitation, all initial fields in every nested area contributed to precipitation and enhanced simulation accuracy, while the contribution of large scale initial field to precipitation was much more than that of mesoscale initial field. As precipitation grew more convective, the accuracy of rainfall simulation had the tendency of relying on the mesoscale initial field in inner nested area.(4)Numercical simulation by WRF manifested that specific mesoscale circulation that alternating between day and night dominated Tengger Desert area in summer. From the aspect of boundary layer convection and stratification stability, although dynamic conditions were more advantageous than the day before, the mere buoyancy rise that generated by heat convection was not enough for rainstorm, let alone the sinking airflow beyond 500hPa. As far as moisture was concerned, mesoscale or miniscale transportation of water vapor aroused by mesoscale thermal circulation was far from enough for triggering or sustaining a rainstorm. Moreover, the edge of oasis was liable to form an inverse wet layer which blocked water vapor transportation. As a result, synoptic scale forcing was a must for rainstorm.