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

The role of surface friction in modifying cold fronts as they make landfall in regions of steep coastal orography is examined by means of idealized simulations. 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-gravity wave above the boundary layer with characteristics similar to that associated with flow over a plateau. Deceleration of the cross-coast flow occurs over land, as well as for a short distance upstream, and causes retardation of frontal motion. A prescribed northerly postfrontal jet weakens rapidly after landfall. Maximum vertical motions are several centimeters per second; however, only small rainfall enhancement is expected since the updraft is very narrow and produces only small vertical displacements. Friction modifies the flow over the orography by increasing the upstream flow deceleration and reducing the magnitude of the barrier jet. The reduction of the barrier-jet strength (when compared to inviscid simulations) by surface friction becomes more pronounced as the mountain forcing of the jet increases. With surface friction, frontal-motion retardation by the orography is strong and upstream frontogenesis is enhanced. The frontal updraft is strongest at the coast and remains strong for a short distance inland along the lower portion of the windward slope. The coastal enhancement of the frontal updraft results from the combined effects of the orography and the surface-roughness change, but large parcel displacements are due mainly to the orographie forcing. Alongcoast winds in the coastal zone during frontal passage are approximately determined by a superposition of the southerly barrier jet and the frontal jets.