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 aerod ynamic resistance calculated between the level of apparent sink of momentum and the reference height. Over sparse vegetation, B-1 (and consequently th e 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 parameter ization in terms of structural and viewing characteristics. Besides canopy structural characteristics and radiometer viewing angle, input variables in clude weather data, stomatal and substrate resistances. B-1 varies with alm ost all the input variables; however, the leaf area index (LAI) and the vie w angle of the radiometer appear as the most significant factors of variati on. Using a set of weather data and component resistances randomly generate d between fixed limits, 'average' curves representing B-1 as a function of LAI for different view angles are inferred from the model and polynomial ex pressions 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 again st other parameterizations, using both simulated data and measurements made over contrasted surfaces in Niger, France and California. As the formulati ons proposed depend on the range of values prescribed in the simulation pro cess for each input data, they are modifiable and adjustable to any experim ental conditions.