GLOBAL LAND-SURFACE ALBEDO MODELING

The climatic impact of changes in terrestrial albedo has been studied using numerous climate models, ranging from simple, one-dimensional en ergy balance climate models to the most complex, three-dimensional gen eral circulation models of the atmosphere. In the majority of these st udies, the land-surface albedos have been prescribed both spatially an d temporally from albedo observations. The effects of variations in al bedo, due to such factors as the angular distribution of incident radi ation and plant phenology (especially leaf emergence, senescence, and drop), cannot be deduced from these observation-based albedos. To over come the limitations of using observed land-surface albedos in climate models, a model of radiation transfer in plant canopies was used to p redict vegetation albedo. This model incorporates the optical properti es of the vegetation and soil surface as well as the micro and macrost ructure of the canopy. Because of the model's reliance on these physic al properties of the land-surface cover, it is able to account explici tly for albedo variations caused by factors both internal and external to the vegetation canopy. Using the physiognomic classification of na tural vegetation developed by Kuchler, representative canopy architect ures for each of 31 vegetation types were developed for each month of the year. Monthly albedos were then simulated for each vegetation type under a range of irradiance distributions. When these results are com pared with existing observations of plant canopy albedo, the degree of correspondence is, in many cases, quite good. Moreover, the dependenc e of albedo on irradiance distribution predicted by the model agrees w ell with established theory. Global maps of land-surface albedo are pr oduced for both clear-sky and overcast conditions in January, April, J uly, and October using a simple solar radiation model to determine the incident radiation field. These maps are compared with previously com piled maps of land-surface albedo. Large differences between the model - and observation-based global albedo maps occur for each of the four months. However, the model reproduces many of the large-scale features and seasonal trends evident in albedo observations.