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| Titel |
A simple interpretation of energy partitioning at the land surface of three eddy covariance sites in Germany and effects after precipitation |
| VerfasserIn |
Maik Renner, Christian Bernhofer, Uta Moderow, Uwe Spank, Axel Kleidon |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098928
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| Publikation (Nr.) |
 EGU/EGU2014-14652.pdf |
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| Zusammenfassung |
| Absorbed solar radiation heats the surface while the emission of thermal long wave radiation
and the turbulent heat exchange with the atmosphere both cool the surface. Using a
highly simplified energy balance model and neglecting horizontal heat exchange the
radiative–turbulent partitioning can be predicted by assuming that the atmospheric heat
exchange operates at a limit of maximum power (Kleidon and Renner, 2013). At this state,
which corresponds to a state of strong surface–atmosphere interactions, both the turbulent
and the radiative heat exchanges are solely driven by absorbed solar radiation Rsn. This
model, established for climatological means, predicts that the radiative partitioning ratio
Rn-Rsn and the turbulent ratio (H + LE)-Rsn should be constant. Based on this
climatological reference we explore the deviations from observations of the partitioning ratios
at the diurnal to the seasonal time scale for three neighboring FLUXNET sites of TU Dresden
with different land use.
We find that the fixed ratios represent the average energy partitioning quite well. When
then evaluating the deviations, dominant diurnal and seasonal cycles in both ratios get
apparent. However, the ratios tend to be rather constant in summertime, when turbulent
exchange is strongest. A not so apparent, but consistent effect is found on days after
precipitation events. To test for effects of precipitation on the partitioning, we subset the data
into time steps with precipitation (> 0.1 mm/h) in the previous 24 hours into wet and all other
as dry. It is found that under wet conditions the radiative ratio is higher, whereas the turbulent
ratio is smaller compared to dry conditions. This finding is evident for diurnal to seasonal
time scales and at all three sites. Further analysis of ground based radiation data reveals that
precipitation events alter the long wave radiation emission of both the atmosphere and
the surface. The combination of both effects results in distinctly lower radiative
temperature differences between the surface and the atmosphere under wet conditions and
corresponds to more stable atmospheric conditions. As the temperature difference also
displays seasonal changes with lower values in wintertime this suggests that long wave
radiation regimes of both the surface and the atmosphere exert a strong second order
control on energy partitioning. We conclude that our simple thermodynamic approach
provides an useful reference to interpret the dominant effects on land surface energy
partitioning.
Kleidon, A. and Renner, M.: Thermodynamic limits of hydrologic cycling within the
Earth system: concepts, estimates and implications, Hydrol. Earth Syst. Sci., 17, 2873–2892,
2013. |
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