Edwin F. Danielsen

In situ evidence of rapid, vertical, irreversible transport of lower tropospheric air into the lower tropical stratosphere by convective cloud turrets and by larger-scale upwelling in tropical cyclones

The STEP tropical objectives were successfully met during the flight experiments conducted from Darwin, Australia, January 16 to February 16, 1987. Necessary and sufficient measurements were made in, above, and downwind from very cold cirrus clouds, produced by three convective cloud types, to demonstrate irreversible mass transports into and dehydration in the lower tropical stratosphere. The three types are defined and described in terms of the physical processes that produce them and illustrated by examples derived from in situ and remote measurements. Intense solar heating is shown to produce, in addition to the usual vertical, sea breeze circulations normal to the coastline, an unusual pair of continental spanning, horizontal circulations. An upper tropospheric-lower stratospheric anti cyclonic circulation, inclined upward toward the tropics, contributes to the dehydration of dissipating cirrus anvils and intensifies the upper level, tropical easterlies. The lower tropospheric cyclonic circulation with tropical westerlies and extratropical easterlies is in direct conflict with the normal tropical easterlies and extratropical westerlies. Impulsive switches between these two opposing lower-level wind systems create conditions favorable for each of these cloud types and explain the summer seasons aperiodic variability.

Irreversible transport in the stratosphere by internal waves of short vertical wavelength

The U-2 aircraft was instrumented and flown in the stratosphere during the Stratosphere-Troposphere Exchange Projects experiments of April 1984 to provide a set of simultaneous measurements by fast responding sensors that would aid in the identification of the modes of cross-jet transport. The measurements confirm the preexperimental deductions that transport is dominated by waves, not by large-scale circulations. Monotonic gradients of trace constituents normal to the jet axis, with upper stratospheric tracers increasing poleward and tropospheric tracers increasing equatorward, are augmented by large-scale confluence as the jet intensifies during cyclogenesis. These gradients are rotated, intensified, and significantly increased in area as their mixing ratio surfaces are folded by the differential transport of a very low frequency, transverse wave. The quasi-horizontal transport produces a laminar structure with stable layers rich in upper stratospheric tracers alternating vertically with less stable layers rich in tropospheric tracers. The transport proceeds toward irreversibility as higher frequency, shear-gravity waves extend the folding to smaller horizontal scales. It becomes irreversible when these short waves actually fold the isentropic surfaces and small-scale mixing develops. The progression to higher wave numbers is a discrete, not a continuous, cascade with major gaps in the observed horizontal wavelengths. The wave modes are identified by matching the observed amplitudes and phases against those obtained by linear perturbation theory. Prior to mixing, the wave-generated perturbations maintain the correlations produced by advecting the larger-scale mean gradients; thus the high resolution measurements support the linear turbulence closure assumption.

Air mass origins and troposphere-to-stratosphere exchange associated with mid-latitude cyclogenesis and tropopause folding inferred from 7Be measurements

The 1984 extratropical mission of NASAs Stratosphere-Troposphere Exchange Project (STEP) studied cross-jet transports in regions of cyclogenesis and tropopause folding. Correlations of 7Be, ozone, water vapor, and potential vorticity measured on a NASA U-2 research aircraft flying in high shear regions above the jet core are indicative of mixing between the cyclonic and the anticyclonic sides of the jet and are consistent with the hypothesis that small-scale entrainments of upper tropospheric air into the lower stratosphere during cyclogenesis are important in maintaining the vertical gradients of 7Be, ozone, water vapor and other trace constituents in the lower few kilometers of the mid-latitude stratosphere. Correlations between 7Be and ozone suggest a lower tropical stratospheric origin for the ozone-poor lamina observed above the jet core.

The NASA spring 1984 stratosphere-troposphere exchange experiment - Science objectives and operations

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.