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STEP Methods & Schedule
To address the objectives stated in Section 1, STEP includes a four-year period of phased instrument development and flight missions, followed by a period of data analysis, publication, and archival. The measurement platforms are the NASA U-2 and ER-2 aircraft, chosen for their ability to sample the atmosphere up to altitudes of about 70,000 ft (20 km) and to follow scientist-direct sampling paths that focus on transient targets of interest (e.g. a jet stream overlying a tropopause fold, a cumulonimbus anvil).
The STEP measurement set is designed to permit the identification of sampled air's recent origin (stratosphere vs. tropospheric, tropical vs. midlatitude) as well as to test other aspects of hypothesized exchange mechanisms. The variables measured can be conveniently grouped as follows:
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.
Particles:
Condensation Nuclei; Aerosol Size Distribution; Cloud Particle Image and Size Distribution.
Radiation:
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, to permit calculation of eddy fluxes and cross correlations. The latter aid in identifying not only the altitude, but also the latitude of a sampled parcel's source.
Figure 2 shows the planned final configuration of instruments on the ER-2 and identifies instrument Principal Investigators. Appendix C gives more information on each instrument, including measurement method, sensitivity and frequency.
The instruments shown in Figure 2 are being developed and phased in over a mission schedule designed to address different aspects of stratosphere-troposphere exchange. Table 1 summarizes the objectives, aircraft, staging site, and payload of each mission.