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

The Canadian Land Surface Scheme "CLASS" was developed in the late 1980s fo r the Canadian GCM, in response to the perceived need for a "second-generat ion" land surface model which would adequately treat the effects of vegetat ion, snow and soil on exchanges of heat and moisture with the atmosphere. C LASS has been tested both in coupled mode with the GCM and in various exper iments that have been carried out in conjunction with PILPS, the internatio nal Project for Intercomparison of Land-Surface Parameterization Schemes. I n the context of those runs, CLASS has been shown to perform quite well. Ho wever; it is recognized that a more rigorous framework of testing against f ield data is required before the model can be used with confidence for stud ies involving the scaling up of surface fluxes, or the modelling of severel y heterogeneous landscapes. The series of papers which follows (to which th is one provides an introductory overview) describes the testing of CLASS ag ainst a wide variety of micrometeorological datasets, and the model develop ment that has proceeded alongside this work, under the auspices of the Cana dian Climate Research Network, from 1994 to 1997. The highlights of this re search include the development of new algorithms for bare soil evaporation; forest transpiration; heat and moisture transfers in organic soils; and re gional-scale river routing.