The tropical experiment of the Stratosphere‐Troposphere Exchange Project (STEP): Science objectives, operations, and summary findings

Abstract

The Stratosphere‐Troposphere Exchange Project Tropical Experiment (STEP Tropical) investigated stratosphere‐troposphere exchange and dehydration processes in that region and season with the coldest average tropopause temperatures, the tropical western Pacific and northern Australia during the winter monsoon (January‐February) of 1987. This is also a period of extensive convective activity and rainfall. In addition to this primary goal, STEP Tropical (1) extended stratospheric aircraft chemical tracer surveys to the southern hemisphere subtropics and mid‐latitudes and (2) sampled the anticyclone that dominates the circulation over Australia in the lower stratosphere during the winter monsoon. Advanced fast‐response instruments on NASA's ER‐2 aircraft measured meteorological variables, stratospheric and tropospheric tracers, particles, and radiative fluxes. ER‐2 sorties included six ferry flight legs across the Pacific and eleven flights in the Australian region. The 1986–1987 monsoon was atypical in that (1) its date of onset, January 14, was 3 weeks later than normal and (2) it was unusually intense and sustained once it did arrive. As a consequence, almost all the flights were conducted under monsoon conditions, with only limited sampling of premonsoon and break‐monsoon continental convection. Illustrations show flight paths for each sortie on satellite images and on 100 hPa synoptic flow charts, as well as the timing of flights with respect to overall cloudiness in the Australian region. STEP Tropical results, reported in the accompanying set of papers, include (1) observational documentation of a convective scale cold trap that dries air of recent tropospheric origin to prevailing stratospheric minimum water vapor mixing ratios (3 ppmv or less); (2) indications that this drying mechanism can be effective not only in the anvils of the tallest clouds (which occur during “break‐monsoon” conditions) but also in the anvils of tropical cyclones and monsoon mesoscale convective systems; (3) demonstration that convectively generated gravity waves account for a significant part of the momentum forcing of the semiannual and quasi‐biennial oscillations in the stratosphere; and (4) documentation of an upper tropospheric source of NOy and a significant upward flux of NOy at the tropical tropopause. For monsoon convection to be an important contributor to the net transfer of tropospheric air to the stratosphere, a mechanism is required to move air from the cloud tops to significantly greater heights. STEP Tropical results suggest that gravity‐wave‐induced small‐scale turbulence or radiative heating of anvils provides this mechanism. Observational proof of the theoretically required high heating rates remains to be obtained.