Vertical momentum in the boundary layer and its consequences for transport of greenhouse gases

Like mass and energy, momentum is an extensive quantity and so the momentum of a system is equal to the sum of the momenta of its components. Considering boundary-layer air in the surface-normal ("vertical") direction, its components have momentum in different directions. That of carbon dioxide is downward due to photosynthetic uptake by ecosystems, while those of oxygen and water vapor are upward due to photosynthetic and evaporative sources, respectively. And so on. Importantly, evaporation is several orders of magnitude greater than any other form of surface exchange, and therefore the vertical momentum of air can be precisely approximated by that of water vapor. This allows estimation of the vertical velocity as simply the ratio of the evaporative flux density to the air density, and has important consequences for many aspects of boundary-layer meteorology.

We analyzed atmospheric data from numerous flux towers over a range of climatological and ecological contexts to characterize the vertical velocity and consequent magnitudes of non-diffusive greenhouse gas transport. Our results invalidate long-standing and intuitive attribution of diffusion of some greenhouse gas directly to its sources and sinks, which has neglected the key role of water vapor in diluting the components of dry air.

This work was supported by the project PID2020-117825GB-C21 and PID2020-117825GB-C22 (INTEGRATYON3) funded by MCIN/AEI/10.13039/501100011033, and by projects BRNM-60-UGR20 (OLEAGEIs) and P18-RT-3629 (ICAERSA) including European Union ERDF funds.