ABSTRACT - DRUM - University of Maryland

4.3 Overshooting deep convection and dehydration in the
lower stratospheric humidity
The findings in this dissertation have important implications for explaining the
mechanism of tropical lower stratospheric humidity regulation. Since Brewer
postulated the Brewer-Dobson circulation, the question of how air is dehydrated as it
enters the stratosphere is one of the classic problems of atmospheric science. Processes
in the TTL are important because they control the humidity of air entering into the
tropical lower stratosphere.
There are several theories that postulate how air enters the lower stratosphere
and how this air is dehydrated to the level of lower stratospheric humidity. Air
detraining from convection in the TTL subsequently experiences slow ascent as part of
the mean overturning circulation and rises toward the stratosphere. As the air slowly
ascends, it is periodically exposed to temperatures considerably below that of the
zonally averaged tropopause. One theory postulates the formation and subsequent
sedimentation of particles could occur during these episodic exposures to very cold
temperatures, leading to dehydration. The other theory postulates that air detrains from
overshooting deep convection already dehydrated to stratospheric abundance. The
convective dehydration theory requires overshooting deep convection to reach the
tropopause and higher in order to expose the air to temperatures low enough to
dehydrate the air to stratospheric humidity levels.
This dissertation reveals important points about the convective dehydration
theory. First, this study demonstrates that the TTL is cooled by convection, at a rate of
69