THE LAND SURFACE-ATMOSPHERE INTERACTION - A REVIEW BASED ON OBSERVATIONAL AND GLOBAL MODELING PERSPECTIVES

This review discusses the land-surface-atmosphere interaction using ob servations from two North American field experiments (First Internatio nal Satellite Land Surface Climatology Project Field Experiment (FIFE) and Boreal Ecosystem Atmosphere Study (BOREAS)) and the application o f research data to the improvement of land surface and boundary layer parameterizations in the European Centre for Medium-Range Weather Fore cast (ECMWF) global forecast model. Using field data, we discuss some of the diurnal and seasonal feedback loops controlling the net surface radiation and its partition into the surface sensible and latent heat fluxes and the found heat flux. We consider the impact on the boundar y layer evolution and show the changes in the diurnal cycle with soil moisture in midsummer. We contrast the surface energy budget over the tropical oceans with that over both dry and wet land surfaces in summe r. Results from a new ECMWF model with four predicted soil layers illu strate the interaction between the soil moisture reservoir, evaporatio n and precipitation on different timescales and space scales. An analy sis of an ensemble of 30-day integrations for July 1993 (the month of the Mississippi flood) showed a large sensitivity of the monthly preci pitation pattern (and amount) to different initial soil moisture condi tions. Short-range forecasts with old and new land surface and boundar y layer schemes showed that the new scheme produced much better precip itation forecasts for the central United States because of a more real istic thermodynamic structure, which in turn resulted from improved ev aporation in an area that is about 1-day upstream. The results suggest that some predictability exists in the extended range as a result of the memory of the soil moisture reservoir. We also discuss briefly the problem of soil moisture initialization in a global forecast model an d summarize recent experience with nudging of soil moisture at ECMWF a nd improvements in the surface energy budget coming from the better pr ediction of clouds.