CLASS - A CANADIAN LAND-SURFACE SCHEME FOR GCMS .2. VEGETATION MODEL AND COUPLED RUNS

In the companion to the present paper, the soil model associated with CLASS (Canadian Land Surface Scheme) was outlined. In this paper, the accompanying vegetation model is described. This model includes physic ally based treatment of energy and moisture fluxes from the canopy as well as radiation and precipitation cascades through it, and incorpora tes explicit thermal separation of the vegetation from the underlying ground. Seasonal variations of canopy parameters are accounted for. Th e morphological characteristics of the 'composite canopy' associated w ith each grid square are calculated as weighted averages over the vege tation types present. Each grid square is divided into a maximum of fo ur separate subareas: bare soil, snow-covered, vegetation-covered, and snow-and-vegetation covered. Test runs were done in coupled mode with the Canadian Climate Centre GCM, to evaluate the performance of CLASS compared with that of the simpler land surface scheme previously used . Two versions of CLASS were run: one with ponded surface water saved between time steps, and one with it discarded, For the seasons of June -July-August and December-January-February, diagnostic calculations sh owed that the old scheme underestimated the globally averaged land sur face screen temperature by as much as 3.0-degrees-C, and overestimated the globally averaged precipitation rate over land by up to 1.0 mm da y-1. CLASS, on the other hand, produced screen temperature anomalies, varying in sign, of 0.2-0.3-degrees-C, and positive precipitation anom alies of 0.6-0.7 mm day-1. The relatively poor performance of the old model was attributed to its neglect of vegetation stomatal resistance, its assumption that the contents of the soil moisture 'bucket' had to be completely frozen before the surface temperature could fall below 0-degrees-C, and its use of the force-restore method for soil temperat ures, which systematically neglects long-term thermal forcing from the soil substrate. The assumption made in most GCMs that excess surface water immediately becomes overland runoff is shown to result in substa ntial overestimates of surface screen temperatures in continental inte riors.