STEP Early Results

Step flew its first mission in Spring 1984. This was an exploratory extratropical mission, flown with a limited complement of existing instruments on the U-2. The objective was to determine the processes that transport air irreversibly between the troposphere and stratosphere, and within the lower stratosphere, during large-scale cyclogenesis. Four U-2 flights were made, each designed to document tropospheric extrusions into the stratosphere, above the jet stream core. The U-2 flights were coordinated with CV-990 and Electra flights (sponsored by NASA's Global Tropospheric Experiment), which documented stratospheric extrusions into the troposphere, below the jet core.

U-2 instruments measured horizontal wind, temperature, pressure, water vapor, ozone, condensation nuclei, and cosmogenic radionuclides in the stratosphere. Instruments were provided, and data analyzed, by teams from NASA Ames, NOAA, University of Minnesota, and State University of New York. All three aircraft (the STEP U-2 and the GTE CV-990 and Electra) were directed to regions where tropopause folds were predicted. The predictions, developed especially for these experiments, proved extremely accurate, and all aircraft sampled the desired atmospheric structures.

The most striking result of the U-2 flights was the discovery in the stratosphere of highly laminated structures of ozone, water vapor, and condensation nuclei, with layers of maximum and minimum concentrations stacked atop one another, each about 1 km thick. These structures were located above the jet-stream core, which was in turn above an underlying tropopause fold. These stratospheric laminae raise two questions: (1) What causes them? and (2) What do they imply for irreversible transport?

The U-2 wind measurements shed light on both questions. Superimposed on the large-scale, mean winds are wave-induced velocities that rotate with height, turning through 360 degrees every 2 km. Differential advections of the mixing ratios of ozone, water vapor, and condensation nuclei and of the potential vorticity by these wave-induced velocities fold the mixing ratio and potential vorticity surfaces, creating a laminar structure of alternating maxima and minima. Also, the folding process greatly increases the vertical gradients of 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. The transfer is not from troposphere to stratosphere, but rather from the subtropical to the polar side of the jet.

Another striking result of the Spring 1984 U-2 flights was the positive correlations between water vapor and ozone observed in the dry stratosphere (i.e. above the hygropause, or water vapor minimum, at about 15 km in these experiments). These positive correlations, observed at large, medium, and small scales, are evidence of a stratospheric source of water vapor, presumably methane oxidation. Below the hygropause, the expected negative correlations between water vapor and ozone were observed. The above results were presented in a special session of the spring 1985 AGU Meeting (Chan et. al., 1985; Danielsen, 1985a, b; Kelly, 1985; Kritz, 1985; Russell et. al. 1985; Starr et. al., 1985; Wilson et. al., 1985). Abstracts are reproduced in Appendix D.

Step flew its first mission in Spring 1984. This was an exploratory extratropical mission, flown with a limited complement of existing instruments on the U-2. The objective was to determine the processes that transport air irreversibly between the troposphere and stratosphere, and within the lower stratosphere, during large-scale cyclogenesis. Four U-2 flights were made, each designed to document tropospheric extrusions into the stratosphere, above the jet stream core. The U-2 flights were coordinated with CV-990 and Electra flights (sponsored by NASA's Global Tropospheric Experiment), which documented stratospheric extrusions into the troposphere, below the jet core.

U-2 instruments measured horizontal wind, temperature, pressure, water vapor, ozone, condensation nuclei, and cosmogenic radionuclides in the stratosphere. Instruments were provided, and data analyzed, by teams from NASA Ames, NOAA, University of Minnesota, and State University of New York. All three aircraft (the STEP U-2 and the GTE CV-990 and Electra) were directed to regions where tropopause folds were predicted. The predictions, developed especially for these experiments, proved extremely accurate, and all aircraft sampled the desired atmospheric structures.

The most striking result of the U-2 flights was the discovery in the stratosphere of highly laminated structures of ozone, water vapor, and condensation nuclei, with layers of maximum and minimum concentrations stacked atop one another, each about 1 km thick. These structures were located above the jet-stream core, which was in turn above an underlying tropopause fold. These stratospheric laminae raise two questions: (1) What causes them? and (2) What do they imply for irreversible transport?

The U-2 wind measurements shed light on both questions. Superimposed on the large-scale, mean winds are wave-induced velocities that rotate with height, turning through 360 degrees every 2 km. Differential advections of the mixing ratios of ozone, water vapor, and condensation nuclei and of the potential vorticity by these wave-induced velocities fold the mixing ratio and potential vorticity surfaces, creating a laminar structure of alternating maxima and minima. Also, the folding process greatly increases the vertical gradients of 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. The transfer is not from troposphere to stratosphere, but rather from the subtropical to the polar side of the jet.

Another striking result of the Spring 1984 U-2 flights was the positive correlations between water vapor and ozone observed in the dry stratosphere (i.e. above the hygropause, or water vapor minimum, at about 15 km in these experiments). These positive correlations, observed at large, medium, and small scales, are evidence of a stratospheric source of water vapor, presumably methane oxidation. Below the hygropause, the expected negative correlations between water vapor and ozone were observed. The above results were presented in a special session of the spring 1985 AGU Meeting (Chan et. al., 1985; Danielsen, 1985a, b; Kelly, 1985; Kritz, 1985; Russell et. al. 1985; Starr et. al., 1985; Wilson et. al., 1985). Abstracts are reproduced in Appendix D.

Step flew its first mission in Spring 1984. This was an exploratory extratropical mission, flown with a limited complement of existing instruments on the U-2. The objective was to determine the processes that transport air irreversibly between the troposphere and stratosphere, and within the lower stratosphere, during large-scale cyclogenesis. Four U-2 flights were made, each designed to document tropospheric extrusions into the stratosphere, above the jet stream core. The U-2 flights were coordinated with CV-990 and Electra flights (sponsored by NASA's Global Tropospheric Experiment), which documented stratospheric extrusions into the troposphere, below the jet core.

U-2 instruments measured horizontal wind, temperature, pressure, water vapor, ozone, condensation nuclei, and cosmogenic radionuclides in the stratosphere. Instruments were provided, and data analyzed, by teams from NASA Ames, NOAA, University of Minnesota, and State University of New York. All three aircraft (the STEP U-2 and the GTE CV-990 and Electra) were directed to regions where tropopause folds were predicted. The predictions, developed especially for these experiments, proved extremely accurate, and all aircraft sampled the desired atmospheric structures.

The most striking result of the U-2 flights was the discovery in the stratosphere of highly laminated structures of ozone, water vapor, and condensation nuclei, with layers of maximum and minimum concentrations stacked atop one another, each about 1 km thick. These structures were located above the jet-stream core, which was in turn above an underlying tropopause fold. These stratospheric laminae raise two questions: (1) What causes them? and (2) What do they imply for irreversible transport?

The U-2 wind measurements shed light on both questions. Superimposed on the large-scale, mean winds are wave-induced velocities that rotate with height, turning through 360 degrees every 2 km. Differential advections of the mixing ratios of ozone, water vapor, and condensation nuclei and of the potential vorticity by these wave-induced velocities fold the mixing ratio and potential vorticity surfaces, creating a laminar structure of alternating maxima and minima. Also, the folding process greatly increases the vertical gradients of 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. The transfer is not from troposphere to stratosphere, but rather from the subtropical to the polar side of the jet.

Another striking result of the Spring 1984 U-2 flights was the positive correlations between water vapor and ozone observed in the dry stratosphere (i.e. above the hygropause, or water vapor minimum, at about 15 km in these experiments). These positive correlations, observed at large, medium, and small scales, are evidence of a stratospheric source of water vapor, presumably methane oxidation. Below the hygropause, the expected negative correlations between water vapor and ozone were observed. The above results were presented in a special session of the spring 1985 AGU Meeting (Chan et. al., 1985; Danielsen, 1985a, b; Kelly, 1985; Kritz, 1985; Russell et. al. 1985; Starr et. al., 1985; Wilson et. al., 1985). Abstracts are reproduced in Appendix D.

IV Funding Sources

All funding for STEP is provided by the National Aeronautics and Space Administration (NASA) or by the National Oceanic and Atmospheric Administration (NOAA). NASA provides all funding for (1) research and analysis (e.g. instrument development and operation, purchases, computer support, platform aircraft flights, and contractor/grantee salaries, overhead, and travel) and (2) NASA civil service salaries, overhead, and computer support for NOAA personnel.