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The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.

The Agulhas Current is the major western boundary current of the Southern Hemisphere. South of Africa it retroflects back into the southwest Indian Ocean, transporting relatively warm water into the midlatitudes. Large sensible and latent heat transfers from the Agulhas Current and its retroflection to the atmosphere occur throughout the year, but particularly during winter. This study suggests that the NCEP and ECMWF models tend to underestimate these fluxes because they are unable to adequately represent the air–sea fluxes over the warmest waters in the core of the current. This core is only 80–100 km wide and it is suggested that the SST data used by these models do not have fine enough spatial resolution to properly represent the Agulhas Current and its mesoscale variability.

Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropical oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin–Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s−1. The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convection. It is shown that these revisions to the transfer coefficients at very low wind speeds (<5 m s−1) have a dramatic positive impact on almost all aspects of the model's simulation of the tropics. These include much improved seasonal rainfall distributions (with the virtual elimination of a tendency to generate a double ITCZ in both winter and summer), a much improved Indian monsoon circulation, and substantially reduced tropical systematic errors. The model previously had an easterly bias in the zonal-mean upper tropical tropospheric flow with a corresponding cold bias in the deep tropics; it is shown that the flux revision substantially reduces this. Furthermore, the revision to the fluxes greatly enhances the model's ability to represent interannual and intraseasonal variability (see also the companion paper by Palmer et al.).

A new version of the ECMWF land surface parameterization scheme is described. It has four prognostic layers in the soil for temperature and soil moisture, with a free drainage and a zero heat flux condition at the bottom as a boundary condition. The scheme has been extensively tested in stand-alone mode with the help of long observational time series from three different experiments with different climatological regimes: the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment in the United States, Cabauw in the Netherlands, and the Amazonian Rainforest Meteorological Experiment in Brazil. The emphasis is on seasonal timescales because it was felt that the main deficiencies in the old ECMWF land surface scheme were related to its capability of storing precipitation in spring and making it available for evaporation later in the year. It is argued that the stand-alone testing is particularly important, because it allows one to isolate problems in the land surface scheme without having to deal with complicated interactions in the full three-dimensional model.

This paper discusses the sensitivity of short- and medium-range precipitation forecasts for the central United States to land surface parametrization and soil moisture anomalies. Two forecast systems with different land surface and boundary layer schemes were running in parallel during the extreme rainfall events of July 1993. One forecast system produces much better precipitation forecasts due to a more realistic thermodynamic structure resulting from improved evaporation in an area that is about 1 day upstream from the area of heaviest rain. The paper also discusses two ensembles of 30-day integrations for July 1993. In the first ensemble, soil moisture is initialized at field capacity (100% availability); in the second ensemble it is at 25% of soil moisture availability. It is shown that the moist integrations produce a much more realistic precipitation pattern than the dry integrations. These results suggest that there may be some predictive skill in the monthly range related to the time-scale of the soil moisture reservoir. The mechanism responsible for the precipitation differences is concluded to be the result of differences in surface heating in the area 1 day upstream, impacting the atmospheric thermodynamic structure. Increased evaporation and reduced heating in moist soil conditions upstream result in the absence of significant boundary layer capping inversion and hence little inhibition of deep precipitating convection.

In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.

This paper describes and interprets the 1987 data from Cabauw, the Netherlands, which can be used to test land surface schemes in stand-alone mode. The data are available from the authors for model development and research. It consists of half-hour averages of forcing data (wind, temperature, specific humidity at 20-m height, downward solar and thermal radiation, and precipitation) and validation data (net radiation, sensible heat flux, latent heat flux, ground heat flux, and soil temperature). To obtain a continuous time series of the forcing parameters and the surface energy fluxes, it was necessary to use a model to fill in the missing observations. The quality of the observations and the reliability of model data are assessed by exploiting the redundancy in the observations and by comparing the model output with the data when both are available. The monthly averages of sensible heat flux are believed to be accurate to within ±5 W m−2and the monthly means of net radiation and latent heat flux to within ±10 W m−2. An analysis of the evaporation data shows that evaporation from the interception reservoir is very common and that the canopy resistance can be modeled in terms of solar radiation, soil moisture, and atmospheric moisture deficit.

In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m−2in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (±10 W m−2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of 30 W m−2and 25 W m−2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m−2for sensible heat flux and 10 W m−2for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.

Data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment for the summer season of 1987 are used to assess the land-surface interaction of the ECMWF reanalysis. In comparison with an earlier study, using the 1992 ECMWF operational model, the land-surface interaction is greatly improved. The bias in the incoming solar radiation has been removed, although there seems to be a small low bias in the incoming longwave, which is significant at night. The four-layer soil moisture model depicts the seasonal cycle well, and the root zone is recharged satisfactorily after major rain events. Consequently, the evaporative fraction (EF) over the season is now generally quite good. There is, however, a low bias in EF in June and high bias in October, which is probably due to the absence of a seasonal cycle in the model vegetation. The evaporative fraction also appears too high in the model just after rainfall. It also appears that the model lacks a realistic seasonal control on the soil heat flux. The surface diurnal thermodynamic cycle has two noticeable errors. The temperature minimum at sunrise is too low, because the surface uncouples too much at night under the stable boundary layer, and the incoming longwave radiation is biased low. There is also an unrealistic diurnal cycle of mixing ratio, q, with too strong a midmorning peak, and too large a fall during the day to a late afternoon minimum that is biased low. These errors in the diurnal cycle of q may feed back on the diurnal cycle of precipitation. The morning peak is partly related to the too-strong inversion at sunrise, which slows the deepening of the boundary layer. The late afternoon minimum of mixing ratio (below that of the model analysis) leads to a positive nudging of soil moisture in the analysis cycle. The model summer mixing ratio has a small high bias of 0.5 g kg super(-) super(1) .

Two simplifying assumptions adopted in the current ECMWF surface scheme are explored: a uniform skin temperature for all grid-box fractions with variable latent heat release and a fixed value of an effective heat conductivity defining the soil heat flux density. This paper proposes relatively simple modifications of the ECMWF scheme with a better physical basis, without large input or computer infrastructure requirements. A uniform skin temperature overestimates evaporation from relatively wet surface fractions when the other surface components are dry and warm. This is shown to be the case for an evaporating soil after rain and vegetation evaporation in a sparse Mediterranean vineyard canopy. Allowing different temperatures for each surface fraction significantly reduces the overestimations and introduces only little additional computation. The default effective conductivity value (7 W m−2 K−1) employed by the current ECMWF scheme is shown to be too low for the sparse vineyard canopy. By raising the conductivity to 17 W m−2 K−1 for the bare-soil part of the surface, the daytime simulated soil heat flux was improved considerably.