The Canadian Land Surface Scheme (CLASS): Its history and future

The Canadian Land Surface Scheme "CLASS" was developed in the late 1980s for the Canadian GCM, in response to the perceived need for a "second-generation" land surface model which would adequately treat the effects of vegetation, snow and soil on exchanges of heat and moisture with the atmosphere. CLASS has been tested both in coupled mode with the GCM and in various experiments that have been carried out in conjunction with PILPS, the international Project for Intercomparison of Land-Surface Parameterization Schemes. In the context of those runs, CLASS has been shown to perform quite well. However; it is recognized that a more rigorous framework of testing against field data is required before the model can be used with confidence for studies involving the scaling up of surface fluxes, or the modelling of severely heterogeneous landscapes. The series of papers which follows (to which this one provides an introductory overview) describes the testing of CLASS against a wide variety of micrometeorological datasets, and the model development that has proceeded alongside this work, under the auspices of the Canadian Climate Research Network, from 1994 to 1997. The highlights of this research include the development of new algorithms for bare soil evaporation; forest transpiration; heat and moisture transfers in organic soils; and regional-scale river routing.