Details
| Original language | English |
|---|---|
| Pages (from-to) | 381-404 |
| Number of pages | 24 |
| Journal | Boundary-Layer Meteorology |
| Volume | 110 |
| Issue number | 3 |
| Publication status | Published - Mar 2004 |
Abstract
The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the 'turbulent' heat flux at a grid point from the horizontally averaged 'total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks. The temporally averaged 'turbulent' flux based on a point measurement systematically underestimates the 'total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.
Keywords
- Convective boundary layer, Eddy covariance method, Imbalance problem, LES, Spatial representativeness of flux, Surface heterogeneity
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Atmospheric Science
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In: Boundary-Layer Meteorology, Vol. 110, No. 3, 03.2004, p. 381-404.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Les study of the energy imbalance problem with eddy covariance fluxes
AU - Kanda, Manabu
AU - Inagaki, Atsushi
AU - Letzel, Marcus Oliver
AU - Raasch, Siegfried
AU - Watanabe, Tsutomu
N1 - Funding Information: This research was supported by CREST (Core Research for Evaluation Science and Technology) of Japan Science and Technology Cooperation, the German National Merit Foundation and Grants 07ATF37-UH and 01LD0103 from the Federal Ministry of Education and Research, Germany. The calculations were performed on the SGI Origin 2000 of the Tokyo Institute of Technology, Japan. The authors wish to thank R. Moriwaki (Tokyo Institute of Technology, Japan) and J. Asanuma (Tsukuba University) for their useful scientific comments and K. Ketelsen (ZIB, Berlin, Germany) and Y. Jono (SGI, Tokyo, Japan) for their technical support.
PY - 2004/3
Y1 - 2004/3
N2 - The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the 'turbulent' heat flux at a grid point from the horizontally averaged 'total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks. The temporally averaged 'turbulent' flux based on a point measurement systematically underestimates the 'total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.
AB - The spatial representativeness of heat fluxes on the basis of single-tower measurements, and the mechanism of the so-called energy imbalance problem, are investigated through numerical experiments using large-eddy simulation (LES). LES experiments are done for the daytime atmospheric boundary layer heated over a flat surface, as a best-case scenario completely free of sensor errors and the uncertainties of field conditions. Imbalance is defined as the deviation of the 'turbulent' heat flux at a grid point from the horizontally averaged 'total' heat flux. Both the theoretical and numerical results of the present study suggest the limitation of single-tower measurements and the necessity of horizontally-distributed observation networks. The temporally averaged 'turbulent' flux based on a point measurement systematically underestimates the 'total' flux (negative imbalance). This is attributed to local advection effects caused by the existence of turbulent organized structures (TOS), whose time scale is much longer than that of thermal plumes. The temporal and spatial change of TOS patterns causes low-frequency trends in the velocity and temperature data resulting in large scatter of the flux estimates. The influences of geostrophic wind speed, averaging time, observation height, computational domain size and resolution on tower-measured fluxes are also discussed. Finally, it is suggested that a weak inhomogenity in surface heating may reduce the negative bias of flux estimates.
KW - Convective boundary layer
KW - Eddy covariance method
KW - Imbalance problem
KW - LES
KW - Spatial representativeness of flux
KW - Surface heterogeneity
UR - http://www.scopus.com/inward/record.url?scp=1142293079&partnerID=8YFLogxK
U2 - 10.1023/B:BOUN.0000007225.45548.7a
DO - 10.1023/B:BOUN.0000007225.45548.7a
M3 - Article
AN - SCOPUS:1142293079
VL - 110
SP - 381
EP - 404
JO - Boundary-Layer Meteorology
JF - Boundary-Layer Meteorology
SN - 0006-8314
IS - 3
ER -