THE BOREAL FORESTS AND CLIMATE

The sensitivity of modelled northern hemisphere climate to modification of the snow-covered surface albedo is investigated using the United Kingdom Meteorological Office (UKMO) general circulation model (GCM). The UKMO GCM is a global, primitive-equation model with 11 layers in the atmosphere. Surface processes in the model are highly parametrized, with bulk aerodynamic formulation of the surface fluxes and a 'bucket' soil-moisture accounting method. The experiment represents in a highly simplified fashion the role of the boreal forests in reducing the surface albedo under snow-covered conditions. A parametrization of snow-covered land was developed which allows the maximum albedo attainable with a snow cover to be prescribed as a function of vegetation type. In the standard version of the model the maximum snow-covered surface albedo attainable is 0.60, which exceeds observed values for the forested areas of the northern hemisphere. The model was integrated twice, with different albedos representing forested and deforested conditions. The sensitivity of the heat and hydrologic budgets for the northern hemisphere and deforested areas is discussed. A detailed analysis of the deforested regions reveals systematic reductions in temperature of up to 2.8 K. Precipitation shows a systematic decrease in the affected regions. The largest decreases occur generally in the months with largest evaporation changes. For the case of no masking by forest vegetation (equivalent to boreal deforestation) the model produces a significant change in the pattern of snow-melt. The removal of forest affects both the magnitude and the timing of spring snow-melt, and consequently also the runoff. There are delays in snow-melt-induced runoff peaks by a month and increases in the magnitudes by on average 32%. The role of large-scale advection is investigated by comparison with the results from a single-column-model experiment. A realistic representation of the snow-covered surface albedo is evidently a requirement for simulations of the northern hemisphere climate.