Sensible heat flux-radiometric surface temperature relationship over sparse vegetation:: Parameterizing B-1

In the bulk formulation of vegetation-atmosphere transfer, the parameter B-1 is needed for evaluating the sensible heat flux from radiometric surface temperature. An excess resistance, expressed as a function of B-1 (r(r) = B-1/u(*), where u(*) is the friction velocity), should be added to the aerodynamic resistance calculated between the level of apparent sink of momentum and the reference height. Over sparse vegetation, B-1 (and consequently the excess resistance) can be very large and variable. A one-dimensional two-layer model of the canopy-atmosphere interaction is used to investigate the behaviour of this fitting parameter and to derive an operational parameterization in terms of structural and viewing characteristics. Besides canopy structural characteristics and radiometer viewing angle, input variables include weather data, stomatal and substrate resistances. B-1 varies with almost all the input variables; however, the leaf area index (LAI) and the view angle of the radiometer appear as the most significant factors of variation. Using a set of weather data and component resistances randomly generated between fixed limits, 'average' curves representing B-1 as a function of LAI for different view angles are inferred from the model and polynomial expressions are fitted to the simulated curves. This set of parameterizations is obtained from ranges of input data wide enough to be representative of a large variety of experimental conditions. It is successfully tested against other parameterizations, using both simulated data and measurements made over contrasted surfaces in Niger, France and California. As the formulations proposed depend on the range of values prescribed in the simulation process for each input data, they are modifiable and adjustable to any experimental conditions.