Sensible Heat Flux-Radiometric Surface Temperature Relationship Over Sparse Vegetation: Parameterizing B-1
2000
Lhomme, J. P. | Chehbouni, A. | Monteny, B.
In the bulk formulation of vegetation-atmosphere transfer, theparameter B⁻¹ is needed for evaluating the sensible heatflux from radiometric surface temperature. An excess resistance,expressed as a function of B⁻¹ (rᵣB⁻¹/u*, whereu* is the friction velocity), shouldbe added to the aerodynamic resistance calculated between thelevel of apparent sink of momentum and the reference height.Over sparse vegetation, B⁻¹ (and consequently the excessresistance) can be very large and variable. A one-dimensionaltwo-layer model of the canopy-atmosphere interaction is used toinvestigate the behaviour of this fitting parameter and to derive anoperational parameterization in terms of structural and viewingcharacteristics. Besides canopy structural characteristics andradiometer viewing angle, input variables include weather data,stomatal and substrate resistances. B⁻¹ varies with almostall the input variables; however, the leaf area index (LAI) andthe view angle of the radiometer appear as the most significantfactors of variation. Using a set of weather data and componentresistances randomly generated between fixed limits, `average'curves representing B⁻¹ as a function of LAI for differentview angles are inferred from the model and polynomial expressionsare fitted to the simulated curves. This set of parameterizations isobtained from ranges of input data wide enough to be representativeof a large variety of experimental conditions. It is successfully testedagainst other parameterizations, using both simulated data andmeasurements made over contrasted surfaces in Niger, France andCalifornia. As the formulations proposed depend on the range ofvalues prescribed in the simulation process for each input data, theyare modifiable and adjustable to any experimental conditions.
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