Effects of cloud parameterization on the simulation of climate changes in the GISS GCM

Climate changes obtained from five doubled CO2 experiments with different p arameterizations of large-scale clouds and moist convection are studied by use of the Goddard Institute for Space Studies (GISS) GCM at 4 degrees lat x 5 degrees long resolution, The baseline for the experiments is GISS Model II, which uses a diagnostic cloud scheme with fixed optical properties and a convection scheme with fixed cumulus mass fluxes and no downdrafts. The global and annual mean surface air temperature change (Delta T-s) of 4.2 de grees C obtained by Hansen et al. using the Model II physics at 8 degrees l at x 10 degrees long resolution is reduced to 3.55 degrees C at the finer r esolution. This is due to a significant reduction of tropical cirrus clouds in the warmer climate when a finer resolution is used, despite the fact th at the relative humidity increases there with a doubling of CO2. When the n ew moist convection parameterization of Del Genio and Yao and prognostic la rge-scale cloud parameterization of Del Genio et al. are used, Delta T-s is reduced to 3.09 degrees C from 3.55 degrees C. This is the net result of t he inclusion of the feedback of cloud optical thickness and phase change of cloud water, and the presence of areally extensive cumulus anvil clouds. W ithout the optical thickness feedback, Delta T-s is further reduced to 2.74 "C, suggesting that this feedback is positive overall. Without anvil clouds , Delta T-s is increased from 3.09 degrees to 3.7 degrees C, suggesting tha t anvil clouds of large optical thickness reduce the climate sensitivity. T he net effect of using the new large-scale cloud parameterization without i ncluding the detrainment of convective cloud water is a slight increase of Delta T-s from 3.56 degrees to 3.7 degrees C. The net effect of using the n ew moist convection parameterization without anvil clouds is insignificant (from 3.55 degrees to 3.56 degrees C). However, this is a result of a combi nation of many competing differences in other climate parameters. Despite t he global cloud cover decrease simulated in most of the experiments, middle - and high-latitude continental cloudiness generally increases with warming , consistent with the sense of observed twentieth-century cloudiness trends ; an indirect aerosol effect may therefore not be the sole explanation of t hese observations. An analysis of climate sensitivity and changes in cloud radiative forcing ( CRF) indicates that the cloud feedback is positive overall in all experimen ts except the one using the new moist convection and large-scale cloud para meterization with prescribed cloud optical thickness, for which the cloud f eedback is nearly neutral. Differences in Delta CRF among the different exp eriments cannot reliably be anticipated by the analogous differences in cur rent climate CRE The meridional distribution of Delta CRF suggests that the cloud feedback is positive mostly in the low and midlatitudes, but in the high latitudes, the cloud feedback; is mostly negative and the amplificatio n of Delta T-s is due to other processes, such as snow/ice-albedo feedback and changes in the lapse rate. The authors' results suggest that when a suf ficiently large variety of cloud feedback mechanisms are allowed for, signi ficant cancellations between positive and negative feedbacks result, causin g overall climate sensitivity to be less sensitive to uncertainties in poor ly understood cloud physics. In particular, the positive low cloud optical thickness correlations with temperature observed in satellite data argue fo r a minimum climate sensitivity higher than the 1.5 degrees C that is usual ly assumed.