Numerical Study On The Formation Mechanism Associated With A Tropical Cyclone Remote Precipitation

A heavy rainfall event occurred in shandong Province on17-19July2008, caused mainly by typhoon Kalmaegi and cold air. Fy2D TBB and NCEP/NCAR reanalysis data, routine observational data and simulated data from WRF model are used to analyse the far distance typhoon and heavy rain process. The characteristics of the vapor conditions, the dynamic and thermal fields, the dry cold air activities and the structures and mechanism ofamesoscale low vortex that caused the heavy rain process are analyzed. The sensitivity tests have been done by removing the typhoon and removing the terrain to study the role of the typhoon and topography in this heavy precipitation. Main conclusions are as follows:1. The mid-latitude trough, typhoon, subtropical anticyclone and upper-level jet stream provide a favorable background field for this precipitation. The mesoscale low vortex and mesoscale convective system are the main makers of the heavy rainfall event. The precipitation has obvious mesoscale features. The precipitation area removes from the southwest of Shandong Province to the northeast with the movement of mesoscale low vortex.2. The results show that the development of the mesoscale vortex can be clearly seen from simulation data. A new vortex center formed and replaced the old vortex center. It is not the simple easterly movement of the mesoscale vortex along shear line. The mesoscale vortex formed after the occurrence of the heavy precipitation.Obvious temperature rising, deep saturated layer and updraft can be found over the mesoscale vortex before the formation of the mesoscale vortex. Downdraft dominated the center of the vortex, convection area move to the east of the center of the mescale vortex. During the weaken stage of the mesoscale vortex, downdraft became strong.Analysis of vorticity equation suggests that divergence term and tilting term in the low-troposphere, horizontal advection term and convection term in the mid-troposphere are the main contributors of the positive vorticity.Tilting term is very important to the formation of mesoscale vortex and the persistence of heavy rainfall.3. There exists obvious dry cold intrusion process during the precipitation, the dry cold air originated from the mid-high latitudes and the mid-high level. The dry air intrusion plays a dual role to lower the temperature and humidity. The dry intrusion is tilt to the west with height. The disturbance of positive potential vorticity on the upper tropospher downward transporte with the dry air intrusion and stimulate the development of the low-level mesoscal vortex. The rain area is always located in the500hPa dry area forefront. Large potential vorticity in500hPa have instructions of the development and movement of the precipitation.4. The wet symmetry instability and weak convection instability exists during the rainfall process, and on the vertical distribution is discontinuous. The unstable stratification is mainly distributed in the middle troposphere and over200hPa when the rain occurred in the southwest of Shandong Province. The unstable stratification is mainly distributed under200hPa when the rain occurred in the center of Shandong Province.Tilt upward motion from the south of rain area upward transport warm and moist air from the South to the area over the rain, and to promote the release of unstable energy.5. The water vapor is mainly come from the Bay of Bengal and the west of the Pacific.The water vapor from the Bay of Bengal to the Shandong region is influenced by the typhoon circulation. After removing the typhoon, the rainfall of the process significantly reduced. The typhoon has different effect on rainfall reduction in southwest and central of Shandong Province. During the precipitation in southwest of Shandong Province, the mesoscale low vortex and the vapor convergence intensity is weaken, the rainfall is also weakened obviously when remove the typhoon. During the precipitation in the central of Shandong area, the mesoscale low vortex and the vapor convergence intensity is enhanced instead when removing the typhoon, but the time of raining is greatly shortened leads to the total rainfall decrease. After remove the terrain, the rainfall of the process also decrease, but the reduced amplitude is not large. The influence of typhoon on the precipitation process is more than the terrain’s.