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| Titel |
Enhanced air-sea CO2 flux due to water-side convection |
| VerfasserIn |
A. Rutgersson, A. Smedman |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250019630
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| Zusammenfassung |
| In the global carbon cycle, the total exchange of CO2 at the sea surface is important, due to a
lack of detailed understanding of the controlling processes it is currently not very
accurately described. The exchange of CO2 between the ocean and the atmosphere
is controlled by the air-sea difference in partial pressure of CO2 (ΔpCO2) at the
surface and of the efficiency of the transfer processes. The partial pressure at the
water surface is controlled by biological, chemical and physical processes in the
water. The efficiency of the transfer processes is determined by the resistance to the
transfer in the atmosphere as well as in the ocean. Since the diffusivity of CO2 is
much greater in the atmosphere than in the water, the largest resistance to the CO2
transfer is by molecular diffusion and turbulent mixing in the aquaeous boundary
layer. The efficiency of the transfer processes is expressed by a transfer velocity, k.
The transfer velocity is usually described by a quadratic or cubical wind speed
dependence.
Direct flux measurements using the eddy-covariance method shows that the CO2 transfer
velocity is also a function of the mixed-layer depth of the water and, to a lesser degree, of the
stratification of the atmosphere. The transfer velocity is significantly enhanced by a large
mixed-layer depth, the enhancement increasing also as the surface cooling increases. This
means that for strongly convective conditions in the water the transfer velocity of carbon
dioxide increases significantly. The impact of mixed layer depth is expressed by
the convective velocity scale of the water (analogous to atmospheric convective
scaling).
Enhancement due to convection is important to capture for the diurnal cycle of air-sea
fluxes. But large air-sea temperature differences (indicating the presence of water-side
convection) also occurs due to advection of air masses. Specifically in small, or mid-size
water basins, in the vincinity of a coast or in areas with large horisontal temperature
gradients. We here investigate the importance for intermediate wind speeds. The impact
during higher wind speeds is not analysed due to lack of data, it is possible that water-side
convection is important also for higher winds-speeds.
The addition due to convection can be added to the traditional transfer velocity (kU) as
k = kU + kc, where kc here is expressed as a function of the water-side convective velocity.
In the presence of water-side convection, the resistance in the water is small. Then the relative
importance of the atmospheric resistance increases and atmospheric processes must be taken
into account. |
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