A THEORTICAL STUDY OF LOW-LEVEL FRONTAL STRUCTURE IN THE BOUNDARY LAYER OVER OROGRAPHY, PART Ⅱ: WARM FRONT AND UNIFORM GEOSTROPHIC FLOW

A simplified two-layer shallow frontal model which including the effect of orography and boundary layer friction is incorporated to study theoretically the vertical structure and circulation of low-level warm front in the boundary layer flow over orography. The inclination of surface warm front (SWF) chiefly depends on the frontal moving speed and the geostrophic flow in the warm sector that is decreased with the frontal speed increasing. The feature is opposite to that of cold fronts. The effect of orography on the slope of warm front is relative smaller than that on the cold front. Comparing with the features of flat case shows that the closed circulation near the SWF in the cold sector for stationary warm front is enlarged when the SWF locates the upstream of orography and contracted on the upslope. However, it will be expanded again when the SWF moving to the leeside. The clockwise circulation away from the SWF in the cold side may be blocked in the leeside whose blocking will be increased when the orography height increasing. There are three different vertical zones in the warm front.(a) A small horizontal scale and strong vertical motion in the vicinity of SWF due to the convergence induced by boundary layer friction that decreased with the increasing of frontal moving speed. Moreover the vertical motion related to the SWF will be increased in the upside, and decreased in the leeside that is associated with the orography dynamic effect. (b) A uniform upwarding motion along the frontal interface in the warm sector that is related to the lifting effect of frontal inclination. (c) A wide vertical motion zone is away from the SWF in the cold side whose center is placed the top of boundary layer and will be blocked in the leeside when the SWF located the upslope.