Leonhard Pfister and Henry B. Selkirk

The purpose of this task is to develop an understanding of mesos- cale transport processes in the lower stratosphere and at the tropo- pause. Among the specific scientific problems of interest are: (1) gravity and inertia-gravity waves excited by convection; (2) inertia- gravity waves in midlatitude jets; (3) turbulent and convective tran- sport mechanisms near the tropical tropopause; (4) stratospheric water budget as affected by the cold tropical tropopause; (5) radiative and microphysical processes near the tropical tropopause; (6) small and mesoscale motions in the arctic and antarctic polar vortex; and (7) horizontal transport mechanisms within the lower stratosphere and between stratosphere and troposphere. The approach combines the analysis of aircraft measurements from several major field projects (STEP -- the Stratosphere-Troposphere Exchange Project; AASE 1 and 2 -- the first and second Airborne Arctic Stratosphere Expeditions; SPADE -- the Stratospheric Photochemistry, Aerosols, and Dynamics Experiment; and ASHOE/MAESA -- the Airborne Southern Hemisphere Ozone Experiment/Measurements for the Assessment of the Effects of Stratos- pheric Aircraft), mechanistic modeling using linear gravity wave models, and air parcel trajectory calculations.

Among the results of this work are: (1) the first estimates of the net irreversible effect of convectively generated gravity waves on the tropical upper stratosphere; (2) evaluation of upward fluxes of horizontal momentum due to gravity waves in the midlatitude and arctic stratosphere; (3) extensive measurements of turbulence in the tropi- cal, midlatitude, and arctic stratosphere; (4) estimation of the stra- tospheric dehydration potential of the Australian monsoon circulation; and (5) demonstrating that a leaky barrier to horizontal lower stra- tospheric transport exists in the subtropics, and that this barrier is related to structures in the local wind field. Future problems of interest include; (1) examination of midlatitude convection in the southern hemisphere as a source for stratospheric gravity waves; (2) establishing the mechanisms for generating observed turbulence in the tropical, midlatitude, and polar stratosphere; (3) evaluating the con- tributions of convectively generated gravity waves to the momentum budget of the tropical stratosphere; (4) evaluating the annual cycle of water injection into the tropical stratosphere; and (5) assessing the ability of global models to represent stratosphere-troposphere exchange by comparing the results of model-based trajectory analyses with aircraft measurements.

Ames-Moffett contact: Dr. Leonhard Pfister

or tel: (415) 604-3183

Headquarters program office: Y