Numerical simulation study of interactional effects of the low-level vertical wind shear with the cold pool on a squall line evolution in North China
A preliminary analysis of low-level dynamical and thermo-dynamical effects on a squall line case occurred in North China on 23 July 2009 is implemented, based on 4DVar assimilation to 6 CINRAD radar observations and numerical simulation with a three-dimensional cloud model under a rapid update cycling mode. The results indicate that the squall line is under the conditions of low-level moderate shear environments and the interaction of the low-level vertical wind shear with the cold pool is the key mechanism for structure and evolution of the squall line. There is strong low-level vertical wind shear but weak cold pool during the initial period of squall line development with a ratio of C, the speed of the cold pool propagation, to △U, the component of the low-level shear perpendicular to the squall line, less than 1 (I. E. , C/△U<1). The squall line echo observed by the radars leans downshear during the initial period. The buoyant environment with high CAPE and low LFC has a significant effect on development and intensification of the squall line and overcomes the disadvantageous impact of imbalance between the low-level shear and the cold pool to the squall line evolution during the initial period. The cold pool is distinctly enhanced because cold air descends to and deposits near the surface, which results from the convective precipitation induced by squall line storms so as to cause the line-normal low-level shear and the cold pool to gradually attain an approximate balance and optimal state (I. E. , C/△U≈1) which leads low-level air to generating the strongest vertical updraft during the enhanced and mature phase of the squall line. The squall line has the strongest intensity and the line echo observed by the radars is much more upright lifting during the period. With time and continued precipitation, the disadvantageous conditions that the strength of the storm line system-generated cold pool evidently overmatches the strength of the low-level shear (I. E. , C/△U>1) causes the squall line to gradually dissipate. The storm line echo observed by radars distinctly broadens and tilts upshear with very low height of strong echo top during the decay period of the squall line. Both the qualitative analysis and quantitative calculation from the simulated results show the influence explanation of the low-level shear and cold pool interaction on the squall line development is accordant to the RKW theory for addressing the squall line structure, evolution, and intensity by the relative balance between the cold pool intensity and the low-level shear magnitude that advanced by Rotunno and Weisman et al. In addition, simulated results indicate low-level 0-3 km shear is most important to squall line development whereas 0-6 km shear has also positive effects on squall line evolution, especially while the squall line is at its best survival state.
Low-level vertical wind shear, Cold pool, Squall line, Numerical simulation
国家科技期刊平台
NSTL
万方数据
维普
