The influence of idealized surface heterogeneity on virtual turbulent flux measurements
The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity, which affects the boundary-layer turbulence. To investigate the impact of surface variables on the partitioning of the energy b...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-04-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/5059/2018/acp-18-5059-2018.pdf |
| Summary: | The imbalance of the surface energy budget in eddy-covariance
measurements is still an unsolved problem. A possible cause is the presence
of land surface heterogeneity, which affects the boundary-layer turbulence.
To investigate the impact of surface variables on the partitioning of the
energy budget of flux measurements in the surface layer under convective
conditions, we set up a systematic parameter study by means of large-eddy
simulation. For the study we use a virtual control volume approach, which
allows the determination of advection by the mean flow, flux-divergence and
storage terms of the energy budget at the virtual measurement site, in
addition to the standard turbulent flux. We focus on the heterogeneity of the
surface fluxes and keep the topography flat. The surface fluxes vary locally
in intensity and these patches have different length scales. Intensity and
length scales can vary for the two horizontal dimensions but follow an
idealized chessboard pattern. Our main focus lies on surface heterogeneity of
the kilometer scale, and one order of magnitude smaller. For these two length
scales, we investigate the average response of the fluxes at a number of
virtual towers, when varying the heterogeneity length within the length scale
and when varying the contrast between the different patches. For each
simulation, virtual measurement towers were positioned at functionally
different positions (e.g., downdraft region, updraft region, at border
between domains, etc.). As the storage term is always small, the non-closure
is given by the sum of the advection by the mean flow and the
flux-divergence. Remarkably, the missing flux can be described by either the
advection by the mean flow or the flux-divergence separately, because the
latter two have a high correlation with each other. For kilometer scale
heterogeneity, we notice a clear dependence of the updrafts and downdrafts on
the surface heterogeneity and likewise we also see a dependence of the energy
partitioning on the tower location. For the hectometer scale, we do not
notice such a clear dependence. Finally, we seek correlators for the energy
balance ratio in the simulations. The correlation with the friction velocity
is less pronounced than previously found, but this is likely due to our
concentration on effectively strongly to freely convective conditions. |
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| ISSN: | 1680-7316 1680-7324 |