The Stratosphere-Troposphere Exchange Project (STEP) has two objectives:

A. Investigate the Mechanisms and Rates of Irreversible Transfer Mass, Tracer Gases, and Aerosols from Troposphere to Stratosphere and within the lower Stratosphere.

The motivation for this objective is protection of the stratospheric ozone layer. Figure 1 illustrates schematically some of the transport processes and chemicals that affect stratospheric ozone. Predicting the stratospheric ozone changes caused by emissions of natural and manmade chemicals (e.g. nitrogen oxides, chlorofluorocarbons) requires an accurate description of when, where, and at what rates the chemicals enter the stratosphere and move therein to the ozone layer. STEP will study transfers in both the midlatitudes and the tropics, including both cloud-dominated and cloud-free processes (e.g. those associated with jet streams, waves, and tropopause folds).

B. Explain the Observed Extreme Dryness of the Stratosphere.

This objective, though closely related to the first, deserves separate mention because it has been a fundamental mystery of atmospheric science for a long time. Specifically, there are quantitative problems with the Brewe-Dobson hypothesis, put forth in the late 1940's. That hypothesis states that the dryness of the stratosphere in the tropics, where it is freeze dried by the extremely cold tropopause temperatures there. The problem is that typical tropical tropopause temperatures (around -80 C) are not cold enough to freeze-dry air to the commonly observed water vapor mixing ratio of 3 part per million by volume (ppmv). The problem could be solved by air entering preferentially in tropical regions with unusually cold tropopauses (e.g. the Indonesia-Micronesia area, where observed temperatures are often -84 C and below), or alternatively by air entering preferentially in clouds, provided the cloud tops are systematically colder than the surrounding air and other conditions apply.

Danielsen (1982*) hypothesized a "dehydration engine" in the anvils of penetrating cumulonimbus clouds that would satisfy the necessary conditions. An important aspect of Danielsen's mechanism is a radiatively driven connective overturning in the anvil, symbolized by the circular arrows in the anvil cloud of Figure 1. One of STEP's goals is to test the Danielsen dehydration mechanism, as well as to gather sufficient data to test or discover other mechanisms. Specifically, STEP aims to determine where and how cold are the temperature minima, whether cirrus crystals are present at the minima, and the origin and transfer rates of air moving through the minima.

*References are listed alphabetically in Section V (STEP Mission Overview).