Estimations of the dissipation of turbulent kinetic energy using sonics, 3-d hot-films, and Doppler lidar during TREX

During March and April of 2006, a field campaign (the Terrain Induced Rotors Experiment – TREX) was conducted in Owens Valley, California to study flow over mountains, in particular the formation of rotors and lee waves. A large number of both in situ and remote sensing instruments were deployed in the valley to investigate the powerful and turbulent rotational motions and downslope wind storms associated with periods of strong wave activity over the valley. Turbulence intensities were inferred, inter alia, using turbulent kinetic energy dissipation rate, which is determined by the work of the large-scale turbulent motions that converts into turbulence and cascades down the scales. In addition to inferring turbulence intensity, dissipation is important for theoretical studies on the atmospheric boundary layer, the turbulent kinetic energy budget and is considered a fundamental quantity of interest in flows past bluff bodies and air-water exchanges. Dissipation is difficult to measure directly, however, because of the required very high spatial resolution (a fraction of the Kolmogorov scale), and thus indirect methods have been developed which use the measurement of large-scale quantities. Comparisons between various methods for dissipation estimation for different instruments are given. Establishing the consistency of results is crucial for the future use of these instruments for dissipation measurements.

The direct measurements of the dissipation spectrum can be made with hot film anemometers (utilizing Taylor's hypothesis), and the indirect evaluations are made either using inertial subrange of the spectra (by fitting Kolmogorov spectra as in the case of sonic anemometers) or via empirical correlations (lidar-based estimations). Arizona State University's coherent Doppler lidar was placed in the valley within 1.5 km of a 30m tower outfitted with 5 sonic anemometer operated at 60hz (10, 15, 20, 25 and 30 m agl), and also within 1.5 km of a 5m tower fitted with three NCAR OTIHS (Outdoor Three-dimensional in-situ calibrated Hot-film anemometry System) and 4 sonic anemometers. Each OTIHS consists of three-dimensional hot-film anemometers that adaptively orient with the mean wind direction, operated at 2000Hz nested within a sonic anemometer. OTIHS were placed at 0.8 m, 1.4 m and 2.4 m above the local displacement height (of 0.6 m as determined from the small shrubs).

This paper presents an intercomparison of various methods of dissipation measurement/estimation, and, in particular, offers a unique opportunity to assess the efficacy of lidar dissipation retrieval in the presence of strongly anisotropic near-surface atmosphere.