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The Stratospheric-Troposphere Exchange Project (STEP) has two objectives:
A. Investigate the mechanism and rates of irreversible transfer of mass, trace gases, and aerosols from troposphere to stratosphere and within the lower stratosphere.

B. Explain the observed extreme dryness of the stratosphere

The first objective derives from the need for a better description of how natural and manmade chemicals move from their tropospheric sources to the stratospheric ozone shield. The second objective, though closely related to the first, deserves separate mention because it has been a fundamental mystery of atmospheric science for decades.

The STEP measurement platforms are the NASA U-2 and ER-2 aircraft. The complete set of STEP-measured variables can be grouped as:

State Variables:
Three-dimensional Wind, Pressure, and Temperature; Temperature Profiles (about 1 km above and below the aircraft).

Stratospheric Tracers:
Ozone; Odd Nitrogen; Cosmogenic Radionuclides (e.g., Be7, P32); Potential Vorticity (derived from the wind and temperature profile measurements).

Tropospheric Tracers:
Water Vapor; Water Total (vapor plus particles); Carbon Monoxide; Radon.

Condensation Nuclei; Aerosol Size Distribution; Cloud Particle Image and Size Distribution.

Infrared Upwelling (Narrow Field of View); Infrared and Solar Up- and Down-welling (Hemispheric); Cloud Images.

Underlined variables are measured at frequencies of 1 Hz or better, so that small-scale structures can be resolved and eddy fluxes and cross correlations can be determined.

Instruments to make the above measurements are being developed and phased in over a schedule of four science missions and two intercalibration missions. The science missions are designed to investigate different aspects of stratosphere-troposphere exchange, including cloud-free and cloud dominated mechanisms, in both midlatitudes and the tropics. The final mission, using the ER-2 based in Darwin, Australia in January-February 1987, will access the world's highest, coldest tropopauses, as well as the largest penetrating cumulonimbus anvils. Flights will be designed to test a dehydration mechanism proposed by Danielsen (1982*), as well as to acquire sufficient data to test and develop other hypotheses.

Step's first mission was flown in April-May 1984 using six instruments on the U-2. The most striking result was the discovery in the stratosphere of highly laminated structures of ozone, water vapor, and condensation nuclei. U-2 wind measurements indicate that the laminae are caused by waves that fold mixing ratio and potential vorticity surfaces. The folding process greatly increases the vertical gradients for the mixing ratios and the potentials for small-scale instabilities. The latter lead to irreversible mixing; thus a reversible; wave-generated transport is rendered irreversible by small-scale instabilities.

All funding fro STEP is provided by the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA).