ATMOSPHERE VEGETATION INTERACTION IN LOCAL ADVECTION CONDITIONS - EFFECT OF LOWER BOUNDARY-CONDITIONS

Models describing the atmospheric surface layer after a sudden change in surface conditions appear to be very sensitive to the imposed lower boundary conditions. This paper considers three possible formulations for the description of the lower boundary conditions of the second-order closure model of Rao et al. (1974). The three conditions considered in this paper are: constant surface relative humidity (Rao's original one, Model I), constant surface resistance (r(s)) in the Penman-Monteith approach (Model II) and a variable surface resistance, depending on atmospheric temperature and humidity as proposed by Noilhan and Planton (1989) (Model III). Model III introduces a feedback mechanism in the vegetation-atmosphere interaction as the surface resistance also determines atmospheric temperature and humidity. We found that Model I yields unrealistic, sometimes even unphysical, results. The results of Models II and III are almost identical in circumstances when the temperature and humidity effects on the surface resistance approximately cancel. According to the chosen parameterization this happens at temperatures below 25-degrees-C. At higher temperatures both effects amplify each other. Then the results of Models II and III diverge. A quantitative analysis is presented using the Penman-Monteith formula. Analysis of the sensitivity of the results to a small perturbation in the form of a simultaneous change in surface roughness indicates that a smooth-to-rough transition, as a rule, will enhance the surface heat fluxes. The roughness effect is less pronounced when the difference in the humidity of the two surfaces is small. A preliminary comparison with measurements indicates that there is not yet sufficient data to indicate if Model III will give systematically better results than Model II.