Two perspectives on the coupled carbon, water and energy exchange in the planetary boundary layer

[摘要] Understanding the interactions between the land surface and theatmosphere is key to modelling boundary-layer meteorology and cloudformation, as well as carbon cycling and crop yield. In this studywe explore these interactions in the exchange of water, heat andCO₂ in a cropland–atmosphere system at the diurnal andlocal scale. To that end, we couple an atmospheric mixed-layer model(MXL) to two land-surface schemes developed from two differentperspectives: while one land-surface scheme (A-g_s)simulates vegetation from an atmospheric point of view, the other(GECROS) simulates vegetation from a carbon-storage point ofview. We calculate surface fluxes of heat, moisture and carbon, aswell as the resulting atmospheric state and boundary-layer dynamics,over a maize field in the Netherlands, on a day for which we havea rich set of observations available. Particular emphasis is placedon understanding the role of upper-atmosphere conditions likesubsidence in comparison to the role of surface forcings like soilmoisture. We show that the atmospheric-oriented model(MXL-A-g_s) outperforms the carbon storage-oriented model(MXL-GECROS) on this diurnal scale. We find this performance is partly due tothe difference of scales at which the models were made to run. Mostimportantly, this performance strongly depends onthe sensitivity of the modelled stomatal conductance towater stress, which is implemented differently in each model. Thissensitivity also influences the magnitude of the surface fluxes ofCO₂, water and heat (surface control) and subsequentlyimpacts the boundary-layer growth and entrainment fluxes (upperatmosphere control), which alter the atmospheric state. Thesefindings suggest that observed CO₂ mole fractions in theboundary layer can reflect strong influences of both the surface andupper-atmosphere conditions, and the interpretation of CO₂mole fraction variations depends on the assumed land-surfacecoupling. We illustrate this with a sensitivity analysis where highsubsidence and soil moisture depletion, typical for periods of drought,have competing and opposite effects on the boundary-layer height h.The resulting net decrease in h induces a change of 12 ppm inthe late-afternoon CO₂ mole fraction. Also, the effect of such high subsidenceand soil moisture depletion on the surface Bowen ratio are of the samemagnitude. Thus, correctly including such two-way land-surface interactionson the diurnal scale can potentially improve our understanding andinterpretation of observed variations in atmospheric CO₂, as well asimprove crop yield forecasts by better describing the water loss and carbongain.

[学科分类] 地球化学与岩石