ABSTRACT - DRUM - University of Maryland

atmospheric waves [Potter and Holton, 1995; Boehm and Verlinde, 2000], to name just
two. One theory of stratospheric dehydration postulates that formation and subsequent
sedimentation of particles could occur during these episodic exposures to very cold
temperatures, leading to dehydration [Holton and Gettelman, 2001]. The other theory
postulates that air detrains from convection already dehydrated to stratospheric
abundances [Sherwood and Dessler 2000; Sherwood and Dessler 2001]. In this case,
no further dehydration is needed.
The convective dehydration theory [Sherwood and Dessler 2000; Sherwood and
Dessler, 2001] requires overshooting deep convection to reach the tropopause and
higher in order to expose the air to temperatures low enough to dry the air to
stratospheric abundances [Johnston and Solomon, 1979]. Measurements show that the
equivalent potential temperature (θ e ) in the boundary layer of convective regions peaks
at 345 K and is generally less than ~355-360 K [Folkins et al., 2000]. Thus,
convection reaching potential temperatures higher than ~355-360 K has likely overshot
its level of neutral buoyancy (LNB). We call this ‘overshooting’ deep convection. If
this air subsequently mixes with higher potential temperature air during the time it is
above its LNB, then the resulting mixture will settle at altitudes above the parcel’s
initial LNB. In the absence of mixing, the convecting air will eventually reach
equilibrium at its LNB.
Evidence that clouds penetrate to near the tropopause is based largely on field
missions that provide results for a single time and place [Cornford and Spavins, 1973;
Danielsen, 1993; Kritz et al., 1993]. Observations of clouds during these missions do
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