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

Field measurements are carried out to study statistical properties of the s ubgrid-scale (SGS) heat fluxes and SGS dissipation of temperature variance in the atmospheric surface layer, and to evaluate the ability of several SG S models to reproduce these properties. The models considered are the tradi tional eddy-diffusion model, the nonlinear (gradient) model, and a mixed mo del that is a linear combination of the other two. High-resolution wind vel ocity and temperature fields are obtained from arrays of 3D sonic anemomete rs placed in the surface layer. The basic setup consists of two horizontal parallel arrays (seven sensors in the lower array and five sensors in the u pper array) at different heights (2.4 and 2.9 m, respectively). Data from t his setup are used to compute the SGS heat flux and dissipation of temperat ure variance by means of 2D filtering in horizontal planes, invoking Taylor 's hypothesis. Model coefficients are measured from the data by requiring t he real and modeled time-averaged dissipation rates to match. Various other experimental setups that differ mainly in the separation between the senso rs are utilized to show that filter size has a considerable effect on the v arious model coefficients near the ground. For the basic setup, conditional averaging is used to study the relation between large-scale coherent struc tures (sweeps and ejections) and the SGS quantities. It is found that under unstable conditions, negative SGS dissipation, indicative of backscatter o f temperature variance from the subgrid scales to the resolved field, is mo st important during the onset of ejections transporting relatively warm air upward. Large positive SGS dissipation of temperature variance is associat ed 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 2D (streamwise and vertical) velocity and temperature distributions, obtained using an add itional setup consisting of the 12 anemometers placed in a vertical array. The nonlinear and mixed model reproduce the observations better than the ed dy-diffusion model.