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 -