ABSTRACT - DRUM - University of Maryland

Troposphere Exchange Project (STEP/Tropical) showed the same result as in WVEE,
observing tropospheric air penetrating and mixing into the stratospheric air.
It seems clear that penetrative overshooting clouds do exist. However, the
question of their frequency, i. e. whether this kind of overshooting occurs often enough
to generate significant dehydration, is still unanswered. Recent work suggests that
tropical convection is generally capped at altitudes 2-4 km below the tropical coldpoint
tropopause, so there is only infrequent penetrative overshooting that reaches to
the tropical tropopause level [Highwood and Hoskins, 1998; Folkins et al., 1999; Keith,
2000; Gettelman et al., 2001].
Sherwood and Dessler [2000, 2001] extended this convective dehydration
hypothesis, and suggested a dehydration model by convective detrainment mixing into
the TTL. In this TTL hypothesis, air is dehydrated rapidly as suggested by Danielsen
[1982] but detrains at variable levels throughout the depth of the TTL. The bottom of
the TTL is close to the typical level of neutral buoyancy (LNB) of tropical convection
[Selkirk, 1993]. Air detraining near the bottom of the TTL will have typical values of
water vapor mixing ratio (10-15 ppmv), which is relatively moist compared to the
stratosphere. Air detraining at higher altitudes, however, must have originated in more
energetic and warmer updrafts and/or overshot significantly. This air can be
dehydrated to moisture levels far below average for the TTL.
This newly dehydrated air, provided by overshooting clouds, spreads
horizontally from the convective region, then mixes with moister air that detrained
earlier at lower altitudes and has slowly risen to this altitude. Air on any given θ
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