Impact of BOREAS on the ECMWF forecast model

Abstract

This paper reviews the impact of the Boreal Ecosystem-Atmosphere Study (BOREAS) on the development of a new land-surface parameterization in the European Centre for Medium-Range Weather Forecasts (ECMWF) global forecast system and discusses briefly the improvements that resulted in the model climate at high latitudes. We show how a long time series at a representative site was used to validate the model parameterizations of observed physical processes on both diurnal and seasonal timescales. As a direct result of BOREAS, the representation of several surface processes was greatly improved over the boreal forest. The introduction of separate tiles for tall and short vegetation meant that the boreal forest could be represented with some realism. In winter the albedo with snow under the trees was greatly reduced, and the introduction of a prognostic snow model with its own energy balance under the canopy meant that evaporation of snow in winter and spring was similarly reduced. An improved separate handling of liquid and frozen soil water meant that evaporation of frozen soil water is shut off, and surface runoff occurs when snow melts on frozen ground. Trees in the model now have larger more realistic unstressed vegetative resistance, as well as a stress factor for vapor pressure deficit, in addition to one for low light levels; both of which reduce summer transpiration to the lower levels observed over the boreal forest. The model does not yet have a global soil distribution, which means that it does not represent the organic soils that are characteristic of the black spruce sites, which have a larger water storage. Copyright 2001 by the American Geophysical Union.