Frontal interaction with the Appalachian mountains. Part I: A climatology

A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded 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 numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure; 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection; and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.