Abstract The Hydrologic Atmospheric Pilot EXperiment-Modélisation du Bilan Hydrique (HAPEX-MOBILHY) experiment which investigated contrast of canopies (a coniferous forest and corn fields), offers a good example of vegetation-atmosphere coupling variability at mesoscale. This study gives a new interpretation of some results already obtained by a mesoscale meteorological model with a 10 km horizontal resolution, and which was used to simulate a real tridimensional situation. According to the concept of vegetation-atmosphere coupling factor, Ω, introduced by Jarvis and McNaughton (1986), an expression based upon the vegetation parameterization currently used in the mesoscale model, is derived. The new definition of the coupling factor includes a sensitivity to the vegetation cover, radiative emissivity and stem/leaf area index ratio. A numerical simulation was performed for the case of 16 June 1986 of the HAPEX-MOBILHY experiment, in which both aerodynamic and canopy conductances are interactively simulated according to the radiative and atmospheric forcings. The results show very contrasted behaviour between the forest and the agricultural crops. Over the forest, where Ω < 0.1, the transpiration rate is mainly controlled by leaf stomatal activity, while over the crops where 0.2 < Ω < 0.3, the reduced net radiative energy supply limits the transpiration rate. Further implications for surface flux variability and relationships involving leaf-air temperature differences are also discussed.