Effects of coastal orography on landfalling cold fronts. Part II: Effects of surface friction

The role of surface friction in modifying cord fronts as they make landfall in regions of steep coastal orography is examined by means of idealized si mulations. Both the effects of the surface-roughness change at the coast in the absence of orography and the effects of orography are considered. Flow over a large and abrupt change in surface roughness generates an inertia-g ravity wave above the boundary layer with characteristics similar to that a ssociated with how over a plateau. Deceleration of the cross-coast flow occ urs over land, as well as for a short distance upstream, and causes retarda tion of frontal motion. A prescribed northerly postfrontal jet weakens rapi dly after landfall. Maximum vertical motions are several centimeters per se cond; however, only small rainfall enhancement is expected since the updraf t is very narrow and produces only small vertical displacements. Friction modifies the flaw over the orography by increasing the upstream fl ow deceleration and reducing the magnitude of the barrier jet. The reductio n of the barrier-jet strength (when compared to inviscid simulations) by su rface friction becomes more pronounced as the mountain forcing of the jet i ncreases. With surface friction, frontal-motion retardation by the orograph y is strong and upstream frontogenesis is enhanced. The frontal updraft is strongest at the coast and remains strong far a short distance inland along the lower portion of the windward slope. The coastal enhancement of the fr ontal updraft results from the combined effects of the orography and the su rface-roughness change, but large parcel displacements are due mainly to th e orographic forcing. Along-coast winds in the coastal zone during frontal passage are approximately determined by a superposition of the southerly ba rrier jet and the frontal jets.