Sa. Braun et al., Effects of coastal orography on landfalling cold fronts. Part II: Effects of surface friction, J ATMOS SCI, 56(19), 1999, pp. 3366-3384
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.