C. Ohandley et Lf. Bosart, THE IMPACT OF THE APPALACHIAN MOUNTAINS ON CYCLONIC WEATHER SYSTEMS .1. A CLIMATOLOGY, Monthly weather review, 124(7), 1996, pp. 1353-1373
A climatological study of cold fronts and cyclones crossing the Appala
chian Mountains from the west through northwest has been performed. A
sample size of 50 fronts and 40 cyclones was derived from the seven wi
nter seasons (December through March) between 1979 and 1986. The analy
sis revealed that initially linear cold fronts are slowed and deformed
by the mountains, with the mean frontal speed decreasing 40% and the
fronts developing a pronounced ''kink'' just east of the southern Appa
lachians. Furthermore, over the mountains the cross-front thermal and
pressure gradients are enhanced by about 25% and 63%, respectively. On
ce past the mountains, fronts are weaker and often difficult to locate
. In particular, the cyclonic wind shift is very ill defined, and the
cross-front pressure gradient is reduced to 38% of the premountain val
ue. Finally, small-scale (500-km diameter) ''lee lows'' form on more t
han half of the fronts. Though typically benign, some of these disturb
ances may play a role in offshore cyclogenesis. Most cyclones undergo
redevelopment across the Appalachians. During redevelopment, the prima
ry cyclone is first deflected northward, then dissipates over West Vir
ginia or Pennsylvania, while a secondary center forms approximately 35
0 km away in the lee. Two types of redevelopment are identified: the f
irst occurs nearly due south of the primary when the associated cold f
ront crosses the mountains, while the second takes place farther east
near the coast when cold-air damming is present. The exact location of
redevelopment is highly variable and apparently is a complex function
of the distribution of upper-level ''forcing'' and low-level thermal
structure. On average the net effect of the Appalachians consists of a
small and temporary but locally important course shift. A comparison
of the results with theoretical work suggests that the neglect of diab
atic processes and friction, and the use of relatively flat, symmetric
topography and simple background flows, are probably responsible for
most of the disagreement between observations and theory.