The interaction of orographic disturbance with fi-out is investigated with a nonhydrostatic filllycompressible mesoscale model (ARPS). It is shown that the fi-out is dominated mainly by the orographic disturbance if the fi-out is weak. Firstly, because the stratified airstream is forced to f’low along the topographicsurface. the topographic surface almost coincides with the lowest isentrope for the barotropic flow. The potential temperature gradients are opposite on upwind slope and downwind slope. As the cold fi.ont movesacross [he mountain, its intensity decreases on the upwind side and increases on the downwind side due tothe thermal superposition. Conversely, the warm front is strengthened on the upwind slope and weakenedon the downwind slope. This is the thermal superposition effect. Secondly, the mountain-threed circulationand orographic waves, which depend on the shape and size of topography and characteristics of airflow,contribute to fi-ontogenesis and/or fi.ontolysis. This is referred as dynamical action. For the mesoscalemountain ridge of gentle slope, the dynamical action weakens the cold fi-out on the upwind slope, andstrengthens the cold ti.ont on the lee side. While for the mesoscale mountain of steep slope, the dynamical effect weakens the cold ti.ont on the upwind side and strengthens the cold fi.ont on the mountain top, thefrontal intensity is decreased when fi-out moves downslope rapidly. As fi-out moves into the convergent zonenear the mountain base, its intensity is enhanced severely. If the fi-out is intensive, there is strong interactionbetween the orographic disturbance and the fi.ont. The cold fi-out dramatically increases downslope windand lee side gravity wave activity. And these in turn act upon the fi-outal intensity and frontal structure. Forthe baroclinic basic flow, the southerly warm advection on the upwind side makes the cold fi.ont lessfrontolysis, the northerly on the lee side violently intensifies the clod fi-out.
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