Interaction between the Geobotanic State and Climate: A Suggested Approach and a Test with a Zonal Model

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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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