The role of the land surface in the climate system

The role of the land surface in the climate system is illustrated, with a focus on recent experience at the European Centre for Medium-Range Weather Forecasts (ECMWF). Global energy and water budgets are discussed and compared with their counterparts over the ocean, highlighting physical mechanisms responsible for their differences. Time scales associated with the global hydrological budget are presented. Using field data and model results, soil moisture is shown to be responsible for modulating the surface-atmosphere interaction at a continental scale, on time scales ranging from the diurnal to the seasonal. After a brief review of the impact of land surface on weather, three ECMWF case studies are presented where a more realistic representation of land surface was crucial for the performance of the forecast system. They correspond, respectively, to the role of soil moisture in determining the position and intensity of the precipitation maximum in an extreme event of mid-latitudes summer, the role of albedo of the snow in the presence of forests in spring and the effect of soil water freezing as a thermal regulator of the surface in cold climates. Finally, the evolution of the systematic errors in the ECMWF forecasts of near surface temperature and humidity is presented over the last ten years; a clear signature of changes to the representation of land surface processes (and other physical processes affecting the energy and water fluxes at the surface) can be found on that record.

Contents

1. Introduction
2. Surface energy and water budget
3. Time scales and the role of soil moisture
4. Impact of land surface on weather: A brief literature survey
5. Examples from ECMWF recent experience
6. Conclusions
7. References