A THEORTICAL STUDY OF LOW-LEVEL FRONTAL STRUC-TURE IN THE BOUNDARY LAYER OVER OROGRAPHY PART Ⅰ:COLD FRONT AND UNIFORM GEOSTROPHIC FLOW

In this study a simplified two-layer frontal model is incorporated to investigate the effect of orography and boundary layer friction on the structure and circulation o f surface cold front(SCF).The inclination of SCF mainly depends on the geostrophic wind in the war m sector, frontal moving speed and the position of SCF relative to orography.Frontal slope is increased in the upward side, but decreased in the leeward side.There are two different circulation regimes in the cold sector for the steady cold front, i.e.a closed anti-clockwise one in the low -level near the SCF and a clockwise one away from the SCF.In the upward side the intensity and scope of the closed circulation in cold sector are enlarged and the turning point of frontal circulation at the frontal interface is lifted which leads to an enhancing upward motion of warm air ahead of the SCF.There is a cont rary result when the cold front moves to the leeward side.The clockwise circulation in the coldair can be obstructed wholly in the upward side by orography for the moving slowly steady cold front.Alee troug hover the leeward side is induced by the interaction of orography with the cold front.The vertical motion in frontal zone would be induced by the oro graphy dynamic lifting and the dist ribution o f convergence which related to the interaction between the boundary layer friction, frontal structure and orography.An intense upwar dmotion ahead of the SCF may produced by the boundary layer friction that can beinflated in the upward side and reduced in the leew ard side.There are multiple vertical upwarding zones in the war mside when the cold front passages in the leeward side.