A Diagnostic Analysis of the Presidents' Day Storm Of February 1979

Abstract

A diagnostic analysis of the Presidents' Day storm of February 1979 is used to extend and quantify earlier findings of Bosart. The initial growth of cyclonic vorticity in the lower troposphere is driven primarily by convergence along the Carolina coastal front. Rapid cyclone spinup accompanies the approach of a potent midtropospheric short wave trough from the Ohio Valley. An analysis of the vorticity structure discloses that cyclone spinup is enhanced by the~vertical advection of vorticity in the strongly cyclonic in situ vorticity environment along the coast as the upper level trough overspreads the region. Similarly, a downward extension of high semige-strophic potential vorticity air accompanies a tropopause fold to the west of thecyclone and prior to the rapid cyclogenesis stage, confirming a result of Uccellini and others. Semigeostrophic potential vorticity is poorly conserved, however, and there is evidence for a separate region of semigeostrophic potential vorticity growth below 800 mb near the coast. Boundary layer warming and moistening approaches 400 and 1200 W m2, respectively, in the coastal waters of the precyclogenetic environment. Coastal frontogenesis responds to differential warming and moistening in conjunction with sensible heating over the ocean and the blocking of shallow cold air by the southern Appalachian Mountains. The observed wind field is frontogenetical during coastal frontogenesis whereas the large-scale geostrophic flow contributes to frontolysis. A comparison of kinematic, quasi-geostrophic and semigeostrophic vertical motions reveals that the kinematic method best captures the structure in the lower troposphere, indicative of the importance of boundary layer convergence. The semigeostrophic vertical motions are superior to the quasi..geostrophic vertical motions in depicting a sloping updraft and a closer separation of the ascent-descent dipole. Solution of the semigeostrophic vertical circulation equation from Hoskins and Draghici lends some support to the finding of Uccellini and others that the flow around the subtropical jet streak was unbalanced. Finally, kinetic energy export from the storm environment is balanced by a positive residual and generation by both the divergent and nondivergent components of the wind in a manner suggestive of an anticyclone region.