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| Titel |
Two perspectives on the coupled carbon, water and energy exchange in the planetary boundary layer |
| VerfasserIn |
M. Combe, J. Vilà-Guerau de Arellano, H. G. Ouwersloot, C. M. J. Jacobs, W. Peters |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 1 ; Nr. 12, no. 1 (2015-01-08), S.103-123 |
| Datensatznummer |
250117761
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| Publikation (Nr.) |
 copernicus.org/bg-12-103-2015.pdf |
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| Zusammenfassung |
| Understanding the interactions between the land surface and the
atmosphere is key to modelling boundary-layer meteorology and cloud
formation, as well as carbon cycling and crop yield. In this study
we explore these interactions in the exchange of water, heat and
CO2 in a cropland–atmosphere system at the diurnal and
local scale. To that end, we couple an atmospheric mixed-layer model
(MXL) to two land-surface schemes developed from two different
perspectives: while one land-surface scheme (A-gs)
simulates vegetation from an atmospheric point of view, the other
(GECROS) simulates vegetation from a carbon-storage point of
view. We calculate surface fluxes of heat, moisture and carbon, as
well as the resulting atmospheric state and boundary-layer dynamics,
over a maize field in the Netherlands, on a day for which we have
a rich set of observations available. Particular emphasis is placed
on understanding the role of upper-atmosphere conditions like
subsidence in comparison to the role of surface forcings like soil
moisture. We show that the atmospheric-oriented model
(MXL-A-gs) outperforms the carbon storage-oriented model
(MXL-GECROS) on this diurnal scale. We find this performance is partly due to
the difference of scales at which the models were made to run. Most
importantly, this performance strongly depends on
the sensitivity of the modelled stomatal conductance to
water stress, which is implemented differently in each model. This
sensitivity also influences the magnitude of the surface fluxes of
CO2, water and heat (surface control) and subsequently
impacts the boundary-layer growth and entrainment fluxes (upper
atmosphere control), which alter the atmospheric state. These
findings suggest that observed CO2 mole fractions in the
boundary layer can reflect strong influences of both the surface and
upper-atmosphere conditions, and the interpretation of CO2
mole fraction variations depends on the assumed land-surface
coupling. We illustrate this with a sensitivity analysis where high
subsidence 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 in
the late-afternoon CO2 mole fraction. Also, the effect of such high subsidence
and soil moisture depletion on the surface Bowen ratio are of the same
magnitude. Thus, correctly including such two-way land-surface interactions
on the diurnal scale can potentially improve our understanding and
interpretation of observed variations in atmospheric CO2, as well as
improve crop yield forecasts by better describing the water loss and carbon
gain. |
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