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8. Review of general circulation models as a basis for predicting the effects of vegetation change on climate
General circulation models
Response to variation in land surface properties
GCM Simulations of tropical rainfall
Recommendations for future research
Symbols and abbreviations
P. R. Rowntree
General circulation models (GCMs) are briefly described together with the parametrizations employed. Aspects especially relevant to changes in vegetation are emphasized. Factors such as surface albedo, ground hydrological processes, and aerodynamic surface roughness, which are affected by these changes, are considered in relation to their contribution to the sensitivity of the models. Results are summarized for experiments on both global and regional scales, simulating changes of surface albedo and surface water availability.
A removal of vegetation tends to increase albedo and reduce roughness and affects surface moisture availability by changing interception, runoff, and the depth of soil accessible to the roots. Increases in surface albedo decrease evaporation and also tend to reduce atmospheric moisture convergence and precipitation. Decreases in surface moisture availability reduce evaporation, and this generally leads to a decrease of rainfall, which helps to maintain the surface moisture anomaly. A decrease in surface roughness also affects the partitioning of upward energy flux between the sensible and latent (evaporation) forms, with evaporation increasing as roughness decreases for large stomata! resistances and decreasing for small.
This paper reviews the atmospheric general circulation model as a tool for study of the climatic response to changes in vegetation in the tropics. It concentrates on (a) the models' treatments of land surface processes, which are crucial to estimations of the climatic response to variations in vegetation, and (b) the sensitivities of the models as revealed by experiments with perturbations of these properties, mainly the surface solar albedo and moisture availability. The quality of the model simulations in the tropics is briefly reviewed, with the emphasis on rainfall over continents, for which a good simulation is most important in sensitivity experiments.
It should be noted that for studies of global climatic change it is not appropriate to assume that sea surface temperatures are unaffected. However, because of the general inferiority to date of simulations of the atmosphere with coupled ocean-atmosphere models, such perturbation experiments have been made mostly with prescribed sea surface temperatures. The review of GCM parametrizations of land surface processes is necessarily brief compared with that of Carson (1982). Similarly, reference to Dickinson (1980) is recommended for a specific discussion of the likely effects of tropical deforestation on climate.
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