Eric Smith

Department of Meteorology

An observational and modeling study was conducted to help assess how well current GCMs are predicting surface fluxes under the highly variable cloudiness and flow conditions characteristic of the real atmosphere. The observational data base for the study was obtained from a network of surface flux stations operated during the First ISLSCP Field Experiment (FIFE). The study included examination of a surface-driven secondary circulation in the boundary layer resulting from a persistent cross-site gradient in soil moisture, to demonstrate the sensitivity of boundary layer dynamics to heterogeneous surface fluxes. The performance of a biosphere model in reproducing the measured surface fluxes was evaluated with and without the use of satellite retrieval of three key canopy variables with RMS uncertainties commensurate with those of the measurements themselves. Four sensible heat flux closure schemes currently being used in GCMs were then evaluated against the FIFE observations. Results indicate that the methods by which closure models are calibrated lead to exceedingly large errors when the schemes are applied to variable boundary layer conditions.

The FIFE study-area was a 15 km by 15 km region of semi-complex terrain in central Kansas containing the Konza Prairie Natural Area. Twenty-two sites across the study-area were selected in an attempt to sample the natural inhomogeneity in terrain and phenology influencing the fluxes. The region was studied for 143 days in 1987 and 21 days in 1989. Annual, intraseasonal, synoptic, and diurnal time scales are the four predominant temporal scales over which the fluxes were observed to vary. Cloudiness was found to be the dominant control on flux magnitudes. Precipitation and its resultant effects on soil moisture distribution was found to be the dominant control on evaporative fraction or Bowen ratio.

The effects of burn treatment, grazing conditions, topography, and cloudiness on radiative, sensible heat, and moisture fluxes were examined for both growing season and senescent periods. Cloudiness was the far more dominant control on variations in available heating than phenology or topography, and thus was the dominant control on the modulation of sensible and latent heat fluxes. For sensible heat, the amplitude of the effect of cloudiness was largest during the senescent period, while for latent heat, it was largest during the growing season. The RMS uncertainties in the measured fluxes were estimated to be approximately 30 Wm-2.

During 1989, a persistent gradient of soil moisture was observed across the site throughout the 21-day study period. This was due to the irregular distribution of antecedent rainfall in the 2 months prior to the experimental period. The soil moisture gradient was independently observed through gravimetric, L-band microwave, and gamma-ray soil moisture measurements. Even though the cloudiness variability tended to remove any site heterogeneity in available heating, the gradient in soil moisture maintained a gradient in evaporative fraction and thus a cross-site difference in sensible heating of the boundary layer. A resulting ABL secondary circulation was established with daytime vertical velocities approaching one cm s-1; Smith et al. (1994).

A biosphere model was then evaluated using the FIFE data. The model is driven in a top-down scheme by standard surface meteorological variables produced in any GCM scheme at every time step; these are precipitation, temperature, relative humidity, pressure, winds, and the down-welling short- and long-wave radiative fluxes; Smith et al. (1993). In the model performance calculations, these 8 variables were obtained from actual measurements. The model allows for fractional vegetation cover, has several vertical layers in the canopy for purposes of radiative transfer, and three soil layers which are thermally and hydrologically prognosed. To be closed, the model requires three additional variables: canopy albedo, leaf area index (LAI), and stomatal resistance. These three slowly changing variables must be measured, arbitrarily specified, or retrieved from satellite measurements for the purpose of integrating the model over a time period. In contrast to simple heat transfer closure models and bucket evaporation models, a biosphere model provides more realistic treatment of transpiring vegetation and the canopy-soil interface, some vital checks and balances which prevent the occurrence of pathological fluxes, and a more detailed treatment of water transfer and phase change in the canopy and solid. The modeled and measured latent heat fluxes show good agreement with a bias of 8 Wm-2 and an RMS uncertainty of ~40 Wm-2 for the growing season.

A pair of numerical experiments were conducted in which a control run was first compared to a Òsatellite runÓ, in which the three slow canopy variables were obtained by satellite-derived parameters, retrieved from AVHRR measurements during clear-sky periods and linearly interpolated during cloudy periods, and then to a Òsynthetic runÓ in which the canopy albedo and LAI were specified by measurements but the stomatal resistance by an independent formula (Crosson et al., 1993). The control run itself was based on use of measured canopy albedo, LAI, and stomatal resistance. All three runs produced small biases; the RMS differences were approximately 35 Wm-2 for the control run, 45 Wm-2 for the satellite run, and 55 Wm-2 for the synthetic run (see Figure 17.1).

Finally, four popular closure schemes used in a number of limited area and large scale models (including 19 GCMs) for calculation of sensible heat flux were evaluated (Wai and Smith, 1994). The schemes consist of (1) the bulk aerodynamic method of Laval et al. (1981); (2) the stability adjusted parametric scheme based on bulk Richardson number of Louis (1979); the modified parametric scheme of Louis et al. (1981); and the two-level turbulent closure scheme of Mellor & Yamada (1982). All four schemes involve empirical coefficients; such constants for the first two schemes were obtained from fitting measured flux data obtained for idealized boundary layer conditions, by tuning 10-day ECMWF forecasts in the case of the Louis-81 scheme, and from laboratory data in the case of the Level-2 closure scheme.

For the 1989 FIFE data, the Louis, and Mellor and Yamada schemes perform best and worst, respectively, but these differences are overshadowed by the overall errors found for all four schemes (see Figure 17.2). All schemes significantly overestimate sensible heat fluxes, particularly under unstable conditions, with RMS uncertainties exceeding 100 Wm-2 up to 160 Wm-2 for the noon period. The uncertainties of the Louis-79 and Level-2 schemes could be reduced to levels consistent with uncertainties in the measured data by recalibration with the FIFE data itself.

It appears that a number of closure schemes currently being used in GCMs are simply too idealized to handle the diverse set of flow and cloudiness situations found in the real boundary layer, representing turbulence conditions for which the closure schemes were never calibrated. Biosphere models are generally more adaptable to non- idealized boundary layers and may be able to yield significant improvement in flux accuracy as well as increased understanding of GCM behavior. However, the design of biosphere models should be reduced to a level of complexity consistent with the scales at which GCMs are operated to avoid the problem of having to specify too many biophysical parameters in an ad hoc fashion.

References

Crosson W.L. , H.J. Cooper, and E.A. Smith, 1993: Estimation of surface heat and moisture fluxes over a prairie grassland. Part 4: Impact of satellite remote sensing of slow canopy variables on accuracy of a hybrid biosphere model. J. Geophys. Res., 98, 4979- 4999.

Smith, E. A., W. L. Crosson, H.J. Cooper, and H.-Y. Weng, 1993: Estimation of surface heat and moisture fluxes over a prairie grassland. Part 3: Design of a hybrid physical/remote sensing biosphere model. J. Geophys. Res., 98, 4951-4978.

Smith, E.A., M.M.-K. Wai, H.J. Cooper, M.T. Rubes, and A. Hsu, 1994: Linking boundary circulations and surface processes during FIFE 89. Part 1: Observational analysis. J. Atmos. Sci., 51, 1497- 1529.

Wai, M.M.-K., and E.A. Smith, 1994: Evaluation of current surface flux parameterization schemes in limited area and large scale models using FIFE and HAPEX Sahel measurements. J. Geophys. Res., submitted.