Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
doi:10.5194/acp-2017-172
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
23 Mar 2017
Review status
A revision of this discussion paper is under review for the journal Atmospheric Chemistry and Physics (ACP).
The boundary condition for the vertical velocity and its interdependence with surface gas exchange
Andrew S. Kowalski 1Departmento de Física Aplicada, Universidad de Granada, Granada, 18071, Spain
2Instituto Interuniversitario de Investigación del Sistema Tierra en Andalucía, Centro Andaluz de Medio Ambiente (IISTA-CEAMA), Granada, 18071, Spain
Abstract. The law of conservation of linear momentum is applied to surface gas exchanges, employing scale analysis to diagnose the vertical velocity (w) in the boundary layer. Net upward momentum in the surface layer is forced by evaporation (E) and defines non-zero vertical motion, with a magnitude defined by the ratio of E to the air density, as w = Eρ. This is true even right down at the surface where the boundary condition is w0 = Eρ0. This Stefan flow velocity implies upward transport of a non-diffusive nature that is a general feature of the troposphere but is of particular importance at the surface, where it assists molecular diffusion with upward gas migration (of H2O, e.g.) but opposes that of downward-diffusing species like CO2 during daytime. The definition of flux-gradient relationships (eddy diffusivities) requires rectification to exclude non-diffusive transport, which does not depend on scalar gradients. At the microscopic scale, the role of non-diffusive transport in the process of evaporation from inside a narrow tube – with vapour transport into an overlying, horizontal air stream – was described long ago in classical mechanics, and is routinely accounted for by chemical engineers, but has been neglected by scientists studying stomatal conductance. Correctly accounting for non-diffusive transport through stomata, which can appreciably reduce net CO2 transport and marginally boost that of water vapour, should improve characterizations of ecosystem and plant functioning.

Citation: Kowalski, A. S.: The boundary condition for the vertical velocity and its interdependence with surface gas exchange, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-172, in review, 2017.
Andrew S. Kowalski
Andrew S. Kowalski
Andrew S. Kowalski

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Short summary
An analysis based on physical conservation law demonstrates that surface-atmosphere exchanges include a non-diffusive component. This implies the need to revise flux gradient relationships including eddy diffusivities in micrometeorology, and stomatal conductances in plant physiology.
An analysis based on physical conservation law demonstrates that surface-atmosphere exchanges...
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