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

Abstract

Rapid cyclogenesis commences between 0300 and 0600 UTC 19 February and proceeds through the remaining 18h 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 the advection of PV increases in the middle to upper troposphere over the East Coast. Model trajectories and eulerian analyses indicate that three airstreams converge into the cyclogenetic region during the rapid development phase. 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 θ due to the release of latent heat near the coastal region. Absolute vorticity associated with the simulated storm is realized primarily through vortex stretching associated with the convergence of the airstreams below the 700mb level. Finally, the divergence of these airstreams within the 700 to 500mb 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. -from Authors