This review discusses the land‐surface‐atmosphere interaction using observations from two North American field experiments (First International Satellite Land Surface Climatology Project Field Experiment (FIFE) and Boreal Ecosystem Atmosphere Study (BOREAS)) and the application of research data to the improvement of land surface and boundary layer parameterizations in the European Centre for Medium‐Range Weather Forecast (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 ground heat flux. We consider the impact on the boundary 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 summer. Results from a new ECMWF model with four predicted soil layers illustrate the interaction between the soil moisture reservoir, evaporation and precipitation on different timescales and space scales. An analysis of an ensemble of 30‐day integrations for July 1993 (the month of the Mississippi flood) showed a large sensitivity of the monthly precipitation pattern (and amount) to different initial soil moisture conditions. Short‐range forecasts with old and new land surface and boundary layer schemes showed that the new scheme produced much better precipitation forecasts for the central United States because of a more realistic thermodynamic structure, which in turn resulted from improved evaporation 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 and summarize recent experience with nudging of soil moisture at ECMWF and improvements in the surface energy budget coming from the better prediction of clouds.
Abstract. Using the Boreal Ecosystem-Atmosphere Study mesonet data, we show the annual cycle of albedo for 1994 and 1995 at 10 sites; two over grass, one over an aspen forest, and an average of seven over coniferous forests. Representative daily average albedo values in summer are 0.2 over grass, 0.15 for aspen, and 0.083 for the conifer sites. In winter the corresponding mean albedo for snow-covered grass, aspen, and conifer sites with snow under the canopy are 0.75, 0.21, and 0.13. The jack pine sites have a higher mean winter albedo of 0.15 than the predominantly spruce sites for which mean albedo is only 0.11. Forest albedo increases at all sites in winter (with snow on the ground under the canopy) as the ratio of diffuse to total solar flux increases. The albedo of the conifer sites in winter rarely reaches 0.3. Table 1 lists their locations, elevation, the type of vegetation, and the station numbers we have assigned to them. Two sites in the south and southwest were over grass; one was over a stand of old aspen in the Prince Albert National Park; and three were over jack pine sites: two at BOREAS flux tower sites in the southern and northern study areas (SSA and NSA, respectively) and one at Lynn Lake, the most northerly site. The remaining four were over mixed spruce and poplar stands , from an analysis of polar orbiting satellite data, showed that the northern boreal forests stand out on a global map as regions of low maximum albedo in winter; they quote a mean value of 0.36 for the maximum winter albedo of the boreal forest. In a study using an energy balance climate model, Otterman et al. [1984] showed that the low winter albedo of the high-latitude forests increase the surface temperature at 65øN by 5 K. There has been considerable work on the development of detailed snow cover models for climate simulations [e.g., Loth et al., 1993], as the issue of the impact of snow cover on winter 28,901
This paper analyzes the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment site-average datasets for near-surface meteorology, soil moisture, and temperature; the surface fluxes of radiation, sensible, and latent heat; and the ground heat flux, for the period May 1987-November 1989. The diurnal and seasonal variation of surface albedo for this grassland site are discussed. The coupling of precipitation, soil moisture, evaporation, pressure height to the lifting condensation level, and equivalent potential temperature ( E ) on seasonal and diurnal timescales is also discussed. The 1988 data confirm the authors' result, shown earlier from the 1987 data that over moist soils increased evapotranspiration lowers afternoon lifting condensation level and increases afternoon E , suggesting a mechanism for a local positive feedback between soil moisture and precipitation on horizontal scales greater than 200 km. The seasonal cycle of ground heat flux and soil temperature is examined and the authors show that the coupling in the warm months between E and soil temperature on seasonal scales is similar over land to the coupling found over warm oceans despite very different controls on the surface fluxes. The boundary layer equilibrium over the ocean is contrasted with the diurnal cycle over land, which is soil moisture dependent.
We analyze the diurnal cycle of the 2‐m thermodynamic data averaged over the First International Land Surface Climatology Project (ISLSCP) Field Experiment site near Manhattan, Kansas, during 1987, using supporting soil moisture data, surface flux data, rainfall, and cloud information. Conserved variable plots are our primary analysis method. We present a summer mean, stratified into dry and wet days, and the monthly seasonal cycle. Further stratifications indicate the control of soil moisture on the surface evapotranspiration, vegetative conductance, and mean diurnal cycle for the boundary layer. We extract composite data sets for the daytime diurnal cycle over grassland in midsummer as a function of soil moisture and show that these are consistent with a mixed layer model for a rapidly entraining boundary layer.
[1] An integrated data set with simultaneous observations at the surface, from tethered balloons within the boundary layer and from rawinsonde ascents, was collected during the wet season experiment of the Large-Scale Biosphere-Atmosphere (LBA) Experiment in Amazonia during January and February of 1999 in support of the ground validation for the Tropical Rainfall Measuring Mission (TRMM). We analyze the surface diurnal cycles of temperature, humidity, lifting condensation level, equivalent potential temperature, andbx64the surface fluxes of sensible and latent heat, ground heat flux and net radiation, for easterly and westerly wind regimes in the lower troposphere. During the easterly wind regimes, the diurnal evolution of mixing ratio shows that the flux of water vapor through cloud base exceeds the large surface evaporation. There is a trend toward a wetter and cooler subcloud layer as the rainy season progresses. Daytime surface Bowen ratio for this pasture site is about 0.4, and falls slightly as the rainy season progresses. Typically in the afternoon, evaporatively driven downdrafts from convective rainbands transform the boundary layer. The fall of equivalent potential temperature in the boundary layer is similar for both regimes, but the boundary layer cooling by convective events during the westerly regimes is reduced, because the subcloud layer is shallower on average. Tethersonde ascents through the edges of gust fronts show that subcloud air is first cooled and moistened by rainfall evaporation before the arrival of downdraft air at the surface. These measurements provide a detailed observational basis for the validation and improvement of parameterizations for shallow and deep convection in numerical forecast models.
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