Numerical Simulation And Sensitivity Test Of Plateau Shear Line

Firstly,using the non-hydrostatic mesoscale WRF?Weather Research Forecast?model,and the National Centers for Environment Prediction Final Analysis?NCEP-FNL?data,the conventional observation data,the temperature of black body?TBB?data from the Fengyun-2F?FY-2F?satellite,the Climate Prediction Center Morphing Technique?CMORPH?precipitation data,the plateau transverse shear line process occurring from 29 June 2014 to 1st July was simulated,and the structural characteristics in the course of evolution were analyzed.Then the numerical experiments of the impact of intensity of the upper-level jet stream on the plateau shear line were carried out,and combined with?equation the factors of vertical upward motion of plateau shear line are analyzed.Finally,based on the experiments of the influence of Tibetan Plateau terrain height on plateau shear line,we discussed preliminarily the influence of the dynamic affections of Plateau topography on the structural characteristics of plateau shear line.The main conclusions can be summarized as follows:?1?During the shear line process,there are significant differences in the structural characteristics of different stages.It usually corresponds to the cloud sector ofTBB<-20?nearby the shear line;with the developing of the shear line,the TBB decreases,and there are many convective activities centers of TBB<-60?, corresponding to the main period of precipitation;TBB increases again during the weakening stage of the shear line and the precipitation goes to end.?2?The plateau shear line has a thermodynamic structure of“warm south but cold north”,the vertical distribution characteristic of high level stability and low layer instability appears during the development and maintenance stage of the shear line;the plateau shear line is the water vapor congregated zone,the change of water vapor flux divergence has a certain effect on the development of plateau shear line.Above the vertical direction of the nascent,developing and maintenance stage,there are the positive vorticity and converge centers which both present the dynamic system of lower troposphere positive vorticity coupled with the high potential vorticity.The cyclonic shear is favorable for maintaining the positive vorticity of plateau shear line;lower level convergence and upper level divergence structure on divergence field is conducive to the development of vertical ascending motion of the shear line;and it's a characteristic signal of plateau shear line weakened that the converge zone begins to subside then disappear in advance of the positive vorticity does.?3?The horizontal distribution characteristics of the whole level apparent heat source ?Q₁?and apparent water vapor sink?Q₁?can reflect the distribution characteristics of precipitation on the plateau shear line,and the latent heat of condensation released by the precipitation heating the atmosphere,thus forming a heat source and strong ascending motion in the active region of the shear line,which is conducive to the production of precipitation.During the plateau shear line process,the vertical distributions of Q₁ and Q₂ are both single peak structures,and their distributions are coincident basically,indicating that the precipitation process of the plateau shear line is based on continuous precipitation,however,the peaks of Q₁ and Q₂ have obvious differences in different stages of the plateau shear line.The vertical advection term is the main contribution of the Q₁ and Q₂ on the plateau shear line.?4?The intensity of the upper-level jet stream has an important influence on low layer wind field,the increase of jet stream will increase the wind shear on the plateau shear line,and the shear line will become longer,at the same time,the enhancement of the upper-level jet intensity is also beneficial to the convergence of the water vapor on the plateau shear line.The upper jet can act on the positive vorticity column and convergence ascending motion on the plateau shear line by influencing the vertical configuration of divergence field that lower-level convergence and upper-level divergence.?5?The diagnostic analysis of the?equation shows that the Laplace term of temperature advection plays a dominant role in the vertical upward motion on the plateau shear line,and the warm advection of the lower layer is favorable to the ascending motion on the shear line.The influence of the variation of upper-level jet stream intensity on the difference term of the vorticity advection is greater than that of the Laplace term of temperature advection,and the enhancement of the upper-level jet intensity will enlarge the positive contribution of difference term of the vorticity advection and the term of temperature advection,which is more favorable for the ascending motion and the maintenance of the plateau shear line.?6?Tibetan Plateau terrain has an important influence on the formation and maintaining of the plateau shear line.Firstly,with the development of the terrain height reduced, the shear line will be strengthened in the early stage,but it is not conducive to the subsequent development and maintenance of the shear line.Secondly,the positive vorticity centers,low level convergence,and upward motion of those kind of dynamic characteristics of the shear line will also be weakened.Finally,the convergence of the wind field on the plateau shear line and the decrease of water vapor content weaken the convergence of water vapor on the shear line,thus inhibiting the development and maintenance of the plateau shear line.