A Numerical Simulation Study Of A Topographic-induced Heavy Rain Case At Northern Tianshan

According to the special topographical condition of Tianshan Mountains, precipitation process has complex spatial distribution and temporal trend in this area. In order to study the precipitation mechanism affected by the complex topography of Tianshan Mountains, a heavy rainfall process happened during June29th to July4th,2011, is studied using Weather Research&Forecast (WRF) model, the data from ordinary observations and automatic weather stations, NCEP/NCARR reanalysis1dataset, and NCEP FNL dataset. Detailed analysis is conducted on dynamical structure, transport of water vapor and microphysical mechanism of a local severe convective process over North Slope of mid-section Tianshan Mountains and Tuoli. The results indicate that:Dynamical and water vapor conditions which are conducive to heavy rain process over the mid-section of Tianshan Mountains are:western jet which converged along Ili Valley climbed over North Tianshan Mountains, flow on the right side lifted by North Sloped after turning right due to the anti-cyclone movement tendency for the right side of jet; another flow from south, blowing by Yanqi Valley, is lifted by South Slope of East-section of Tianshan Mountains. These two airflows converged over the North Slope of mid-section of East Tianshan Mountains, which is conducive to local convection development sue to its substantial water vapor providing.This heavy rainfall process is generally dominated by deep mid-Asian trough over western Xinjiang Province. Large-scale, dominating southwest airflow ahead of the trough drives high-level to mid-section of Tianshan Mountains; large-scale high-level ice clouds interact with low-level convective clouds, which forms advantageous conditions for precpitation development and generation of regional heavy downpour at North Slope of Tianshan Mountains.Advantagous dynamical and water vapor conditions to short-term heavy rainfall process are:water vapor driving by northwestern flow from Norskehavet turned to northeastern direction under the effect of horn-sized topography, thus forming north-section flow; After crossing Dzungarian Gate, northwestern flow turned to southeastern flow affecting by southern flow, and then merged with northwestern flow to its north, thus forming south-section flow. Two flows converged over Tuoli site after climbing up, which provided dynamical and water vapor conditions to this precipitation process.Multi-cell storms are the dominated short-term (2hours) heavy rainfall system which hits Tuoli observation site, this storm includes6convection cells.5th and6th cells make the majority of precipiation that is observed by Tuoli site. This storm lasts as long as2hours and50minutes.5th cell accelerated precipitation by development of icy-phase precipitation in super-cooling layer. It is conductive to solid hydrometeor formation, which melted after falling down to warm layer;6th cell accelerated precipitation by super-cooled water, which is lifted to higher level, merged with ice clouds and helped development of hydrometeor. This two factors together involved to short-term heavy precipitation.