The boundary condition for vertical velocity and its interdependence with surface gas exchange
Metadata
Show full item recordAuthor
Kowalski, AndrewEditorial
European Geosciences Union
Materia
Transport Stomata Surface gas exchanges Vertical velocity Ecosystems
Date
2017-07-05Referencia bibliográfica
Kowalski, A.S. The boundary condition for vertical velocity and its interdependence with surface gas exchange. Atmospheric Chemistry and Physics, 17: 8177-8187 (2017). [http://hdl.handle.net/10481/48369]
Sponsorship
Investigation into this matter was funded by Spanish national project GEISpain (CGL2014-52838-C2-1-R).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 = E/ρ. This is true even right down at the surface where the boundary condition is w|0 = E/ρ|0 (where w|0 and ρ|0 represent the vertical velocity and density of air at the surface). 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, for example) 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 airstream – 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 characterisations of ecosystem and plant functioning.