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| Titel |
Decoupling in the land-atmosphere for carbon exchange during severe droughts |
| VerfasserIn |
Marie Combe, Jordi Vilà-Guerau de Arellano, Huug G. Ouwersloot, Wouter Peters |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250109203
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| Publikation (Nr.) |
 EGU/EGU2015-9095.pdf |
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| Zusammenfassung |
| When analyzing the terrestrial carbon cycle, a strong focus is generally placed on its surface
drivers (e.g. leaf area index and soil moisture). However, free-tropospheric conditions
and processes occurring at the top of the atmospheric boundary layer (ABL), like
subsidence or cloud formation, can strongly impact entrainment fluxes and the surface
energy balance, which in turn affect the atmospheric CO2 mole fraction and surface
CO2 uptake. The free-tropospheric state and ABL-top processes are controlled
by large-scale meteorological conditions, and can change drastically during for
instance severe droughts. To quantify the importance of these upper-atmospheric
processes for the carbon cycle, we perform a sensitivity analysis across a range of
meteorological conditions inspired by field observations. We use a conceptual model that
represents the daytime surface fluxes of carbon, water and energy for a maize field,
coupled to the dynamics of a convective boundary layer. We find that the importance
of upper-atmosphere conditions for the atmospheric CO2 budget is strengthened
under low soil moisture conditions, exceeding the influence of surface fluxes by
a factor of four or more. Under these conditions the surface carbon, water, and
energy exchange get decoupled from the atmosphere, and the surface energy is
directed mainly towards sensible heat, which increases both the direct and entrainment
heating of the ABL. This in turn contributes to further soil moisture depletion and
thus forms a positive drought feedback. The occurrence of a decoupled state in our
conceptual model strongly depends on how we parameterize soil moisture stress.
Since the soil moisture stress parameterization impacts the rate of day-to-day soil
moisture depletion under prolonged drought conditions, it is key to modeling drought
situations and heat waves. We show how the decoupling, drought feedback and
atmospheric CO2 budget differ under various parameterizations for soil moisture stress. |
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