The global impact of supersaturation in a coupled chemistry-climate model A. Gettelman and D. E. Kinnison National Center for Atmospheric Research, Boulder, CO, USA
Abstract. Ice supersaturation is important for understanding condensation in the
upper troposphere. Many general circulation models however do not permit
supersaturation. In this study, a coupled chemistry climate model, the
Whole Atmosphere Community Climate Model (WACCM), is modified to include
supersaturation for the ice phase. Rather than a study of a detailed
parameterization of supersaturation, the study is intended as a sensitivity
experiment, to understand the potential impact of supersaturation, and of
expected changes to stratospheric water vapor, on climate and chemistry.
High clouds decrease and water vapor in the
stratosphere increases at a similar rate to the prescribed supersaturation
(20% supersaturation increases water vapor by nearly 20%). The stratospheric
Brewer-Dobson circulation slows at high southern latitudes, consistent with
slight changes in temperature likely induced by changes to cloud radiative
forcing. The cloud changes also cause an increase in the seasonal cycle
of near tropopause temperatures, increasing them in boreal summer over
There are also impacts on chemistry, with small increases in
ozone in the
tropical lower stratosphere driven by enhanced production.
The radiative impact of changing water vapor is dominated by the reduction
in cloud forcing associated with fewer clouds (~+0.6 Wm−2) with a
small component likely from the radiative effect (greenhouse trapping)
of the extra water vapor (~+0.2 Wm−2), consistent
with previous work.
Representing supersaturation is thus important, and changes to supersaturation
resulting from changes in aerosol loading for example,
might have a modest impact on global radiative forcing, mostly through
changes to clouds. There is no evidence of a strong impact of water vapor
on tropical tropopause temperatures.
Citation: Gettelman, A. and Kinnison, D. E.: The global impact of supersaturation in a coupled chemistry-climate model, Atmos. Chem. Phys., 7, 1629-1643, doi:10.5194/acp-7-1629-2007, 2007.