A model-based diagnostic study of the rapid development phase of the Presidents' Day Cyclone

Jeffrey S. Whitaker

Dept. of Meteorology, The Florida State University, Tallahassee, FL

Louis W. Uccellini

Laboratory for Atmospheres, NASA/Goddard Space Flight Center, Greenbelt, MD

Keith F. Brill

General Sciences Corporation, Laurel MD

Abstract

A model simulation of the rapid development phase of the Presidents' Day cyclone of 19 February 1979 is analyzed in an effor to complement and extend a diagnostic analysis based only on 12--h radiosonde date over the contiguous United States, with a large data-void area over the Atlantic Ocean (Uccellini et al, 1985). As indicated by the SLP and 850 mb absolute vorticity tendencies, rapid cyclogenesis commences between 0300 and 0600 UTC 19 February and proceeds through the remaining 18 h of the simulation. This rapid development phase occurs as stratospheric air (marked by high values of potential vorticity (PV)) approaches and subsequently overlies a separate, lower-tropospheric PV maximum confined to the East Coast, or during the period when advection of PV increases in the middle and upper troposphere over the East Coast. The onset of rapid deepening is marked by 1) the transition in the mass divergence profiles over the surface low from a diffuse pattern with two or three divergence maxima to a two-layer structure, with maximum divergence located near 500 mb and the level of nondivergence located near 700 m; 2) the intensification of precipitation just north of the surface low pressure system; and 3) an abrupt increase in the low-level vorticity.

Model trajectories and Eulerian analyses indicate that three airstreams converge into the cyclogenetic region during the rapid development phase. One of these airstreams descends within a tropopause fold on the west side of an upper-level trough over the north-central United States on 18 February and approaches the cyclone from the west-southwest as the rapid development commences. A second airstream originates in a region of lower tropospheric subsidence within the cold anticyclone north of the storm, follows an anticyclonically curved path at low levels over the ocean, and then ascends as it enters the storm from the east. A third airstream approaches the storm from the south at low levels and also ascends as it enters the storm circulation. All of the airstreams pass through the low-level PV maximum as they approach the storm system, with the PV increase following a parcel related to the vertical distribution of diabatic heating due to the release of latent heat near the coastal region.

A vorticity analysis shows that absolute vorticity associated with the simulated storm is realized primarily through vortex stretching associated with the convergence of airstreams below the 700 mb level. Although the maximum vorticity is initially confined below the 700 mb level, the convergence of the various airstreams is shown to be directly related to dynamic and physical processes that extend throughout the entire troposphere. Finally, the divergence of these airstreams within the 700 to 500 mb layer increases the magnitude of the mass divergence just north and east of the storm center and thus enhances the rapid deepening of the surface low as measured by the decreasing sea level pressure.