FRONTAL INTERACTION WITH THE APPALACHIAN MOUNTAINS .1. A CLIMATOLOGY

A comparison between the climatological structure of retarded and unre tarded fronts aligned parallel to the Appalachian Mountains is investi gated. With the average height of the Appalachians being 1 km, retarde d and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numeric al studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 19 90, over 55% of all cold fronts were retarded by the mountains. Statis tical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Comp osite fields of sea level pressure; 850-, 500-, and 200-mb heights; qu asigeostrophic potential vorticity and its advection; and potential he ight (U/N) were computed for all retarded and unretarded fronts. Unret arded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, an d more amplified how at all levels. There was no significant differenc e between the potential height fields of the two types of fronts. In a ddition the average potential height, for both groups of fronts, easil y met the criteria for retardation. Rather than depending upon the Fro ude number of the flow, it is hypothesized that the strength of the sy noptic-scale circulations in the middle and upper troposphere primaril y determines whether or not a front will be retarded by the Appalachia n Mountains.