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George Gutman

Abstract

A zonally-averaged steady-state hemispheric mean-annual climate model is used for conducting a series of experiments on land surface alterations: desertification, deforestation and irrigation. In each experiment a fixed perturbation of surface albedo and water availability is imposed in a single latitude belt (but a different perturbation is specified in each experiment). The desertification and deforestation experiments simulate modifications to the geobotanic state due to destruction of vegetation by overgrazing and excessive cultivation of the land in the semiarid and tropical zones, respectively. The irrigation experiment simulates the climatic impact of massive irrigation of the desert belt.

Results indicate that the effect of changes in evapotranspiration rather than in surface albedo is predominant in regulating the surface temperature. It is shown that the impact of biofeedback is strongest in the area adjacent to the perturbation zone. It is also concluded that the prescribed perturbations of the geobotanic state are not sufficient to modify climate to an extent that these perturbations would persist.

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George Gutman

Abstract

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George Gutman

Abstract

It is suggested that the observed periodicity of cloud-free, visible and near-infrared data from the Advanced Very High Resolution Radiometer (AVHRR) onboard NOAA-9 for a particular target can be efficiently used for deriving the monthly mean clear-sky planetary albedo. The broadband albedo is approximated by a linear combination of visible and near-infrared albedos. The new method of 9-day compositing distributes the clear-sky observations over the sun-target-sensor geometry combinations and weights them by the occurrence of each combination during a particular month. It is shown that the monthly mean clear-sky planetary albedo can be estimated solely from the visible and near-infrared data in a model-independent manner. The surface albedo is then obtained by applying a simple atmospheric correction to the derived clear-sky planetary albedo.

A comparison of the results of the present method and those obtained by using the “minimum albedo” method indicates that the latter may lead to an underestimation of monthly values because of the angular variation in clear-sky observed albedos. The derived surface albedos are compared with those reported in climatological studies based on ground observations.

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George Gutman, George Ohring, and Joachim H. Joseph

Abstract

A method is suggested for introducing long-term interaction between the geobotanic state and climate (a biogeophysical feedback mechanism) into climate models. It is based upon making the geobotanic state, characterized by the snow-free surface albedo and the water availability parameter, dependent upon the ratio of annual radiation balance to annual precipitation (the so-called radiative index of dryness).

This approach is illustrated using a zonally averaged annual steady-state climate model which is based on the hemispheric climate model of Ohring and Adler. Zonal data statistics are employed to obtain simple relationships consistent with the zonality of the system. The heating parameterization of the original model is modified so that precipitation and cloud amount are computed using vertical velocity at 500 mb, which is calculated from the thermodynamic equation.

Experiments with the model indicate that the simulated climate and geobotanic zones are in good agreement with observations. Sensitivity studies suggest that biogeophysical feedback has a negligible effect on the model's response to solar constant variations but may be important in the evaluation of the long-term impact of surface albedo changes.

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