The global impact of supersaturation in a coupled chemistry-climate model

[摘要] Ice supersaturation is important for understanding condensation in theupper troposphere. Many general circulation models however do not permitsupersaturation. In this study, a coupled chemistry climate model, theWhole Atmosphere Community Climate Model (WACCM), is modified to includesupersaturation for the ice phase. Rather than a study of a detailedparameterization of supersaturation, the study is intended as a sensitivityexperiment, to understand the potential impact of supersaturation, and ofexpected changes to stratospheric water vapor, on climate and chemistry.High clouds decrease and water vapor in thestratosphere increases at a similar rate to the prescribed supersaturation(20% supersaturation increases water vapor by nearly 20%). The stratosphericBrewer-Dobson circulation slows at high southern latitudes, consistent withslight changes in temperature likely induced by changes to cloud radiativeforcing. The cloud changes also cause an increase in the seasonal cycleof near tropopause temperatures, increasing them in boreal summer overboreal winter.There are also impacts on chemistry, with small increases inozone in thetropical lower stratosphere driven by enhanced production.The radiative impact of changing water vapor is dominated by the reductionin cloud forcing associated with fewer clouds (~+0.6 Wm⁻²) with asmall component likely from the radiative effect (greenhouse trapping)of the extra water vapor (~+0.2 Wm⁻²), consistentwith previous work.Representing supersaturation is thus important, and changes to supersaturationresulting from changes in aerosol loading for example,might have a modest impact on global radiative forcing, mostly throughchanges to clouds. There is no evidence of a strong impact of water vaporon tropical tropopause temperatures.

[学科分类] 大气科学