Mechanism Analysis of a Rainstorm Occurred in the Eastern Part of Northwest China

By using the data of sounding and surface intensive AWS,the satellite cloud images and the reanalysis data of NCEP 1° × 1°,the forming mechanism of a rainstorm occurred on 23 July 2010 in the eastern part of Northwest China are analyzed. The results showthat the eastern part of Northwest China is at the left side of exit zone of the upper level north jet and convergence region of the lowvortex and shear-line in the lowlevel,so the coupling of the upper and lowlevel system is easy to form strong ascend movement. The lowlevel southerly air current formed by interaction of typhoon ‘Chanthu'and the subtropical high continuously transports water vapor to the eastern region of Northwest china. During the rainstorm,precipitation rate and rainfall intensity is consistent and the precipitation rate is over 50 mm ·h-1in the strongest period of rainstorm. Under the background of the saddle pattern flowfield,the cold and warm air converge in shear lines of the lowvortex. The great total deformation zones coincide with the dense belt of equivalent potential temperature isoline,it is conducive to enhance the wet baroclinicity of the lowvortex. During the rapid development of the vortex,the vertical vorticity growth mostly come from the twisting term on 500 h Pa. The twisting term maximum ahead of vertical vorticity maximum appears,which has the very good instruction significance for rainstorm forecast. The smaller the negative center value of non-equilibrium force of atmospheric motion on 850 h Pa is,the stronger the raininess will be. The negative center value of non-equilibrium force of atmospheric motion in the extreme precipitation region is below-10. 0 × 10-9s-2.