STRATOSPHERIC TROPOSPHERIC MASS-EXCHANGE DURING THE PRESIDENTS DAY STORM

Using data generated from a model simulation, the exchange of mass bet ween the stratosphere and the troposphere is estimated for the Preside nts' Day storm during a 24-h period beginning at 1500 UTC 18 February 1979. This 24-h interval coincided with a strongly developed tropopaus e depression and the onset of explosive surface cyclogenesis. The init ial part of the study consists of identifying a surface of isentropic potential vorticity (IPV) to represent the tropopause. The 3.0-IPV-uni t surface is chosen since the pressure distribution on this surface cl osely 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 baroclini c amplification of the Presidents' Day storm. Using a quasi-Lagrangian transport model, stratospheric-tropospheric mass exchange is estimate d 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 x 10(14) kg. The transport from t he troposphere to the stratosphere is 2 x 10(14) kg yielding a net tra nsport across the tropopause of 3 x 10(14) 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 refer red to as the ''dry airstream,'' arcing counterclockwise around the po lar-front jet and trough system from northwest to east. Reverse transp ort from the troposphere to the stratosphere occurs northeast of the d epression and agrees with trajectories of air parcels within the end r egion of rising ''conveyer belts. ''