LES study of the energy imbalance problem with eddy covariance fluxes
Responsible: Manabu Kanda; Marcus Oliver Letzel
Project type: East-Asia Scholarship Programme of the DaimlerChysler AG and the ​German National Academic Foundation (Studienstiftung des deutschen Volkes)
Duration: 01/04/2001 - 31/03/2002

This is a joint project together with Manabu Kanda at the ​Department of International Development Engineering, Tokyo Institute of Technology, Japan. It includes a one-year research stay in Japan.

The spatial representativeness of heat fluxes obtained with the eddy covariance method from point measurements and the mechanism of the so-called energy imbalance problem are investigated through numerical experiments using PALM. LES experiments are performed for a convective boundary layer over a flat surface as a best case scenario completely free of sensor errors and the uncertainties associated with field mearurements.

The results of this study are as follows; (1) Point measurements systematically underestimate the heat flux (negative imbalances), which is attributed to local advection effects. (2) Imbalance bias and variance decrease with increasing wind speed. (3) Longer time averaging gives smaller bias but larger variance of imbalance. (4) Higher elevation gives larger bias and variance of imbalance. (5) Turbulent Organized Structures are shown to account for these statistical features of imbalance. (6) High-pass filters may reduce the variance of imbalance if properly designed. (7) Surface inhomogeneities can significanlty effect the imbalance. (See also the studies by Schröter, Inagaki and Steinfeld.)