The need to understand differences among general circulation model projections of CO2-induced climatic change has motivated the present study, which provides an intercomparison and interpretation of climate feedback processes in 19 atmospheric general circulation models. This intercomparison uses sea surface temperature change as a surrogate for climate change. The interpretation of cloud-climate interactions is given special attention. A roughly threefold variation in one measure of global climate sensitivity is found among the 19 models. The important conclusion is that most of this variation is attributable to differences in the models' depiction of cloud feedback, a result that emphasizes the need for improvements in the treatment of clouds in these models if they are ultimately to be used as reliable climate predictors. It is further emphasized that cloud feedback is the consequence of all interacting physical and dynamical processes in a general circulation model. The result of these processes is to produce changes in temperature, moisture distribution, and clouds which are integrated into the radiative response termed cloud feedback. INTRODUCTIONProjected increases in the concentration of atmospheric carbon dioxide and other greenhouse gases are expected to have an important impact on climate. The most comprehensive way to infer future climatic change associated with this perturbation of atmospheric composition is by means of three-dimensional general circulation models (GCMs). Schlesinger and Mitchell [1987] have, however, demonstrated that several existing GCMs simulate climate responses to increasing CO2 that differ considerably. Cess and Potter [1988], following a suggestion by Speltnan and Manabe [1984], indicate that differences in global-mean warming, The global-mean direct radiative forcing G of the surfaceatmosphere system is evaluated by holding all other climate parameters fixed. It is this quantity that induces the ensuing climate change, and physically, it represents a change in the net (solar plus infrared) radiative flux at the top of the atmosphere (TOA). For an increase in the CO2 concentration of the atmosphere, to cite one example, G is the reduction in the emitted TOA infrared flux resulting solely from the CO2 increase, and this reduction results in a heating of the surface-atmosphere system. The response process is the change in climate that is then necessary to restore the TOA radiation balance, such that that is either too warm or too cold, then it will respectively produce a climate sensitivity parameter that is too small or too large, and clearly, the intercomparison simulation had to be designed to eliminate this effect. There was also a practical constraint: the CO2 simulations require large amounts of computer time for equilibration of the rather primitive ocean models that have been used in these numerical experiments.An attractive alternative that eliminated both of the above mentioned difficulties was to adopt +_2øK sea surface temperature ( The perpetual July simulation e...

Snow feedback is expected to amplify global warming caused by increasing concentrations of atmospheric greenhouse gases. The conventional explanation is that a warmer Earth will have less snow cover, resulting in a darker planet that absorbs more solar radiation. An intercomparison of 17 general circulation models, for which perturbations of sea surface temperature were used as a surrogate climate change, suggests that this explanation is overly simplistic. The results instead indicate that additional amplification or moderation may be caused both by cloud interactions and longwave radiation. One measure of this net effect of snow feedback was found to differ markedly among the 17 climate models, ranging from weak negative feedback in some models to strong positive feedback in others.

Some climatological information from 14 atmospheric general circulation models is presented and compared in order to assess the ability of a broad group of models to simulate current climate. The quantities considered are cross sections of temperature, zonal wind, and meridional stream function together with latitudinal distributions of mean sea level pressure and precipitation rate. The nature of the deficiencies in the simulated climates that are common to all models and those which differ among models is investigated; the general improvement in the ability of models to simulate certain aspects of the climate is shown; consideration is given to the effect of increasing resolution on simulated climate; and approaches to understanding and reducing model deficiencies are discussed. The information presented here is a subset of a more voluminous compilation which is available in report form (Boer et al., 1991). This report contains essentially the same text, but results from all 14 models are presented together with additional results in the form of geographical distributions of surface variables and certain difference statistics.

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Snow feedbacks produced by 14 atmospheric general circulation models have been analyzed through idealized numerical experiments. Included in the analysis is an investigation of the surface energy budgets of the models. Negative or weak positive snow feedbacks occurred in some of the models, while others produced strong positive snow feedbacks. These feedbacks are due not only to melting snow, but also to increases in boundary temperature, changes in air temperature, changes in water vapor, and changes in cloudiness. As a result, the net response of each model is quite complex. We analyze in detail the responses of one model with a strong positive snow feedback and another with a weak negative snow feedback. Some of the models include a temperature dependence of the snow albedo, and this has significantly affected the results. sonal cycle of snow cover were discussed by Robock [1980]. Of particular interest are possible climatic feedbacks involving changes in snow cover in response to externally forced perturbations of the climate system [Robock, 1983]. According to Groisman et al. [1994], the annual snow cover in the northern hemisphere has in fact declined by about 10% over the past 20 years.The concept of climatic feedback has been discussed by many authors. A useful introduction is given by Schlesinger [ 1989]. The climate system is considered to involve a number of internal parameters, denoted by Ij, and to be subject to possibly variable external forcing, denoted here by G. We interpret G as a change in the net radiation at the top of the atmosphere, which could be due to a variety of external causes, including increasing greenhouse gas concentrations and/or changes in solar output or the Earth's orbital parameters. (The notation used here differs from Schlesinger's.)The response of the system to changes in the external forcing is determined in part by the changes of the various internal parameters. The changes of the internal parameters represent the feedbacks at work in the system. As an example, suppose that the climate state is characterized by the globally averaged surface temperature T. As discussed by Schlesinger [1989], the change of T due to G is (1) Here (AT) 0 is the temperature change that would occur in the absence of feedbacks, and fj is the feedback due to process j, which satisfies or' (2) where 3f is the net radiation at the top of the atmosphere, defined so that it is positive into the planet. It should be clear 20,757 20,758 RANDALL ET AL.: ANALYSIS OF SNOW FEEDBACKS Changes In Cloudiness Induced by Melting Snow Affect the Earth's Rad•tion Wa•er Ground Wa•er Ground rker Grou• Emits More • • •ses More [ Absorbs More Rad•t•n I I Sensible an•or ] So•r Figure 1. Schematic illustrating various snow feedbacks at work in nature.As snow melts, darker ground is exposed. This leads to more absorption of solar radiation. The warmer ground emits more longwave radiation and also gives up more sensible and latent heat. These changes can indirectly affect the cloudiness, which then further alters the flo...