The Presidents' Day Cyclone of 18–19 February 1979: Influence of Upstream Trough Amplification and Associated Tropopause Folding on Rapid Cyclogenesis

Abstract

A diagnostic analysis of an amplifying polar jet-trough system and associated tropopause fold which preceded the 19 February 1979 Presidents' Day cyclone is presented. The analysis is based on conventional radiosonde data, infrared and visible satellite imagery, 6.7 μm water vapor measurements from the Temperature-Humidity Infrared Radiometer (THIR), and ozone measurements from the Total Ozone Mapping Spectrometer (TOMS). The case study indicates that dynamically forced meso-α scale vertical circulations played an important role in the extrusion of stratospheric air along the axis of a polar jet and the subsequent development of the storm system. Specific findings include: 1) Tropopause folding accompanying an amplifying polar jet–trough system occurred along the axis of the intensifying polar jet in response to subsidence forced by geostrophic deformation patterns associated with the jet streak. 2) The folding process extruded dry stratosphere air marked by high values of potential vorticity down toward the 700 mb level, 1500 km upstream of the East Coast, 12 to 24 h prior to the explosive development phase of the cyclone. This result differs from previous case studies which have emphasized the concurrent development of a folded tropopause and cyclogenesis. 3) During the 12 h preceding rapid cyclogenesis, the stratospheric air descended toward the 800 mb level and moved toward the East Coast to a position just upstream of the area in which rapid cyclogenesis occurred. Even though potential vorticity was not strictly conserved, the absolute vorticity increased in the lower to middle troposphere in association with adiabatic mass convergence, vertical stretching and the related decrease of static stability of the air mass originating in the stratosphere. 4) As was inferred from the infrared and visible satellite imagery and the ozone measurements, the stratospheric air mass was nearly colocated with the storm center as explosive deepening and vortex development occurred, suggesting that the explosive development of the cyclone was likely influenced by the stratospheric air mass as it descended toward a deep oceanic planetary boundary layer immediately off the East Coast.