The NASA spring 1984 Stratosphere‐Troposphere Exchange experiment: Science objectives and operations

Abstract

We present an overview of the first experiment of the Stratosphere‐Troposphere Exchange Project (STEP), conducted in spring 1984 over the southwestern United States. The objective was to identify modes of transport during large‐scale cyclogenesis, both (1) within the stratosphere, from the anticyclonic to the cyclonic side of the jet, and (2) from the troposphere to the stratosphere. The primary platform was the NASA U‐2 high‐altitude research aircraft, supported by the regular twice‐daily radiosonde network. Measurements included temperature, pressure, horizontal wind, ozone, cosmogenic radionuclides, water vapor, and condensation nuclei. U‐2 flights were made on 4 days. On 3 days the flight path was in a vertical plane orthogonal to the jet stream flow, extending from the anticyclonic side of the jet, across it, and into the cyclonic vortex. The other flight was directed at processes associated with the upper level cirrus outflow from a tropical vortex. Aircraft direction was performed using analysis of NMC prognostications, commercial diagnostics and predictions, radiosonde data, and satellite infrared imagery. The cross‐jet flight most intensively analyzed (April 20–21, 1984) was coordinated with simultaneous flights in the troposphere and lower stratosphere made by the NASA CV‐990 and Electra as a part of the Global Tropospheric Experiment. This paper briefly reviews the U‐2 instrument payload, shows the flight paths of April 20–21 and May 6 in relation to the respective meteorological situations, and reviews the procedures used to predict large‐scale cyclogenesis and associated tropopause folding and to direct the aircraft. The following papers report the measurement data and interpret the results in terms of reversible and irreversible cross‐jet transport mechanisms.