A priori field study of the subgrid-scale heat fluxes and dissipation in the atmospheric surface layer

F. Porté-Agel1,3, M. Parlange1,3, C. Meneveau2,3, and W.E. Eichinger4
1Department of Geography and Environmental Engineering, 2Department of Mechanical Engineering 3Center for Environmental and Applied Fluid Mechanics
The Johns Hopkins University, Baltimore MD 21218 4Iowa Institute for Hydraulic Research, Department of Civil and Environmental Engineering, University of Iowa, Iowa City IA 52242

ABSTRACT: Field measurements are carried out to study statistical properties of the subgrid-scale (SGS) heat fluxes and SGS dissipation of temperature variance in the atmospheric surface layer, and to evaluate the ability of several SGS models to reproduce these properties. The models considered are the traditional eddy-diffusion model, the non-linear (gradient) model and a mixed model which is a linear combination of the other two. High-resolution wind velocity and temperature fields are obtained using twelve three-dimensional sonic anemometers placed forming arrays in the surface layer. The basic setup consists of two horizontal parallel arrays (7 sensors in the lower array and 5 sensors in the upper array) at different heights (2.4m and 2.9m, respectively). Data from this setup are used to compute the SGS heat flux and dissipation of temperature variance by means of two dimensional filtering in horizontal planes, invoking Taylor's hypothesis. Model coefficients are measured from the data by requiring the real and modeled time-averaged dissipation rates to match. Various other setups that differ mainly in the separation between the sensors are utilized to show that filter size has a considerable effect on the various model coefficients near the ground. For the basic setup, conditional averaging is used to study the relation between large-scale coherent structures(sweeps and ejections) of the flow and the SGS quantities. It is found that under unstable conditions, negative SGS dissipation, indicative of backscatter of temperature variance from the subgrid scales to the resolved field, is most important during the onset of ejections transporting relatively warm air upwards. Large positive SGS dissipation of temperature variance is associated with the end of ejections (and/or the onset of sweeps) characterized by strong drops in temperature and vertical velocity under unstable conditions. These results are also supported by conditionally sampled two-dimensional (streamwise and vertical) velocity and temperature distributions corresponding to large (positive and negative) SGS dissipations, obtained using an additional setup consisting of the twelve anemometers placed in a vertical array. Among the models, the eddy diffusion model yields SGS fluxes and dissipation whose statistics (mean values and probability density functions) are significantly different from the statistics of the measured SGS variables. Also, as is well known, this model is not able to provide backscatter (negative SGS dissipation), which is found in the measurements. Models based on scale-similarity arguments (e.g. non-linear and mixed model) reproduce the observations better. In particular, they capture backscatter and yield conditional SGS fluxes and dissipation that are in closer agreement with the measurements.

J. Atmos. Sci. 18, 2673–2698 (2001)

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