Global land‐surface albedo modelling

Abstract

AbstractThe climatic impact of changes in terrestrial albedo has been studied using numerous climate models, ranging from simple, one‐dimensional energy balance climate models to the most complex, three‐dimensional general circulation models of the atmosphere. In the majority of these studies, the land‐surface albedos have been prescribed both spatially and temporally from albedo observations. The effects of variations in albedo, due to such factors as the angular distribution of incident radiation and plant phenology (especially leaf emergence, senescence, and drop), cannot be deduced from these observation‐based albedos.To overcome the limitations of using observed land‐surface albedos in climate models, a model of radiation transfer in plant canopies was used to predict vegetation albedo. This model incorporates the optical properties of the vegetation and soil surface as well as the micro and macrostructure of the canopy. Because of the model's reliance on these physical properties of the land‐surface cover, it is able to account explicitly for albedo variations caused by factors both internal and external to the vegetation canopy.Using the physiognomic classification of natural vegetation developed by Küchler, representative canopy architectures 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 compared with existing observations of plant canopy albedo, the degree of correspondence is, in many cases, quite good. Moreover, the dependence of albedo on irradiance distribution predicted by the model agrees well with established theory.Global maps of land‐surface albedo are produced for both clear‐sky and overcast conditions in January, April, July, and October using a simple solar radiation model to determine the incident radiation field. These maps are compared with previously compiled 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.