Cyclone–Anticyclone Couplets over North America. Part II: Analysis of a Major Cyclone Event over the Eastern United States

Abstract

The evolution of a major cyclone over Eastern North America in March 1971 is described. A noteworthy feature of the development, in addition to the extreme baroclinity, was the substantial thermal advection upward from the middle troposphere to the lower stratosphere in conjunction with a strongly sloping tropopause. Strong cold air advection into the trough aloft was associated with subsidence and vortex tube stretching above 500 mb. The vorticity so generated was advected downstream to support exceptional surface development. A deep layer of air with potential vorticity values characteristic of stratospheric air was advected downward to 600 mb immediately upstream of the deepening surface cyclone. The region of positive potential vorticity advection coincided with the area of computed quasi-geostrophic height falls, and lay along the leading edge of the massive cold advection zone aloft. A strong sloping tropopause led to a sloping level of nondivergence across the trough axis with substantial cyclonic vorticity produced by vortex tube stretching as descent changed to ascent along an isobaric surface crossing the trough axis from west to east. This vorticity was advected downstream over the surface cyclone. The relatively high level of nondivergence (300 mb) immediately upstream of the surface cyclone ensured convergence and cyclone spinup through a deep layer. Immediately downstream of the surface cyclone the level of nondivergence lowered to below 500 mb as convergence and vorticity generation was concentrated in the lower level moist air from which heavy precipitation was falling. Stratospheric values of potential vorticity are generated in the 900?700 mb layer in the region of heavy precipitation beneath the level of maximum diabatic heating.