Upward Momentum and Non-diffusive Transport in the Surface Layer

The laws of classical mechanics are applied to surface-atmosphere gas exchanges to derive, from vertical momentum in the surface layer, the surface boundary condition for the vertical velocity as the ratio of the evapo-transpiration rate to air density (w0=E/rho). This generally upward momentum implies, both at the surface and throughout the surface layer, non-diffusive (i.e., convective) transport of all gases; the role of such mass flow has been neglected in descriptions of surface-atmosphere exchanges. The correct decomposition of net transport into diffusive and convective components is shown to have important consequences for accurately defined flux-gradient relationships and resistance models, including the characterization of physiological influence over plant gas exchanges via stomatal regulation.