Open AccessFast cloud adjustment to increasing CO 2 in a superparameterized climate model
Author(s) -
Wyant Matthew C.,
Bretherton Christopher S.,
Blossey Peter N.,
Khairoutdinov Marat
Publication year - 2012
Publication title -
journal of advances in modeling earth systems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.03
H-Index - 58
ISSN - 1942-2466
DOI - 10.1029/2011ms000092
Subject(s) - atmospheric sciences , cloud albedo , environmental science , cloud fraction , longwave , cloud cover , troposphere , downwelling , entrainment (biomusicology) , albedo (alchemy) , climatology , subtropics , radiative transfer , cloud forcing , boundary layer , climate model , cloud computing , geology , upwelling , climate change , physics , thermodynamics , art , oceanography , computer science , acoustics , biology , operating system , quantum mechanics , rhythm , art history , fishery , performance art
Two‐year simulation experiments with a superparameterized climate model, SP‐CAM, are performed to understand the fast tropical (30S‐30N) cloud response to an instantaneous quadrupling of CO 2 concentration with SST held fixed at present‐day values. The greenhouse effect of the CO 2 perturbation quickly warms the tropical land surfaces by an average of 0.5 K. This shifts rising motion, surface precipitation, and cloud cover at all levels from the ocean to the land, with only small net tropical‐mean cloud changes. There is a widespread average reduction of about 80 m in the depth of the trade inversion capping the marine boundary layer (MBL) over the cooler subtropical oceans. One apparent contributing factor is CO 2 ‐enhanced downwelling longwave radiation, which reduces boundary‐layer radiative cooling, a primary driver of turbulent entrainment through the trade inversion. A second contributor is a slight CO 2 ‐induced heating of the free troposphere above the MBL, which strengthens the trade inversion and also inhibits entrainment. There is a corresponding downward displacement of MBL clouds with a very slight decrease in mean cloud cover and albedo. Two‐dimensional cloud‐resolving model (CRM) simulations of this MBL response are run to steady state using composite SP‐CAM simulated thermodynamic and wind profiles from a representative cool subtropical ocean regime, for the control and 4xCO 2 cases. Simulations with a CRM grid resolution equal to that of SP‐CAM are compared with much finer resolution simulations. The coarse‐resolution simulations maintain a cloud fraction and albedo comparable to SP‐CAM, but the fine‐resolution simulations have a much smaller cloud fraction. Nevertheless, both CRM configurations simulate a reduction in inversion height comparable to SP‐CAM. The changes in low cloud cover and albedo in the CRM simulations are small, but both simulations predict a slight reduction in low cloud albedo as in SP‐CAM.