Stratospheric–Tropospheric Mass Exchange during the Presidents' Day Storm

Abstract

Using data generated from a model simulation, the exchange of mass between the stratosphere and the troposphere is estimated for the Presidents' Day storm during a 24-h period beginning at 1500 UTC 18 February 1979. This 24-h interval coincided with a strongly developed tropopause depression and the onset of explosive surface cyclogenesis. The initial part of the study consists of identifying a surface of isentropic potential vorticity (IPV) to represent the tropopause. The 3.0-IPV-unit surface is chosen since the pressure distribution on this surface closely matches the tropopause pressures reported by radiosonde stations. The IPV surface portrays the depression of the tropopause associated with the polar-front jet and trough system accompanying the baroclinic amplification of the Presidents' Day storm. Using a quasi-Lagrangian transport model, stratospheric–tropospheric mass exchange is estimated for the region including and immediately adjacent to the tropopause depression. The estimated mass transport from the stratosphere to the troposphere for the 24-h period is 5 × 10¹⁴ kg. The transport from the troposphere to the stratosphere is 2 × 10¹⁴ kg yielding a net transport across the tropopause of 3 × 10¹⁴ kg from the stratosphere to the troposphere. These results are confirmed by a second, independent model simulation. The mass transport from stratosphere to troposphere across the 3.0-IPV surface coincides with descending air, often referred to as the “dry airstream,” arcing counterclockwise around the polar-front jet and trough system from northwest to east. Reverse transport from the troposphere to the stratosphere occurs northeast of the depression and agrees with trajectories of air parcels within the end region of rising “conveyer belts."