| The dynamics of the subgrid-scale (SGS) stress and scalar flux in the convective atmospheric surface layer are studied using field measurements from the Advection Horizontal Array Turbulence Study (AHATS). We extend the array technique previously used to evaluate the SGS velocity and temperature to include measurements of the fluctuating pressure, enabling separation of the resolvable- and subgrid-scale pressure and allowing for the first-ever observations of the pressure covariance terms and the full SGS budgets. Non-dimensional forms of the budget terms are analysed as functions of the surface-layer stability parameter and the ratio of the wavelength of the spectral peak of the vertical velocity to the filter width, the latter a measure of the large-eddy simulation (LES) fidelity. Analyses of the mean SGS turbulence kinetic energy budget show a balance among the production, transport, and dissipation. The mean SGS shear stress and SGS temperature flux budgets, meanwhile, are dominated by the production and pressure destruction, with the latter causing return to isotropy. The budgets of the normal components of the SGS stress are more complex. Most notably the pressure-rate-of-strain includes two competing processes, return to isotropy and generation of anisotropy, the latter due to ground blockage of the large-scale convective eddies. For neutral surface layers, return to isotropy dominates. For unstable surface layers return to isotropy dominates for small filter widths, whereas for large filter widths the ground blockage effect dominates, resulting in strong anisotropy.;Analyses of the terms in the budgets of the conditional mean SGS stress and SGS scalar flux, which must be correctly predicted by the SGS model in order for LES to reproduce the resolvable-scale velocity and temperature probability density functions, further reveal the complex dependence of the SGS pressure-rate-of-strain on the updrafts generated by buoyancy, downdrafts associated with the returning flow, and wall blocking effects. Under conditions of strong convective instability, the results most notably show conditional pressure redistribution from the (smaller) vertical to the (larger) horizontal velocity components during downdrafts, resulting in generation of anisotropy. The conditional mean pressure transport, meanwhile, is a significant source of energy during updrafts as a result of the near-wall pressure minima. The vertical advection also plays an important role in the transfer of SGS energy. The results in the present study, particularly for the pressure-rate-of-strain, provide important insights into the near-wall SGS dynamics. We demonstrate with a scaling-based similarity model that it is possible to predict a priori both return-to-isotropy and generation-of-anisotropy behaviours observed for the normal (redistributive) components of the SGS pressure-rate-of-strain. The work has important implications for modelling the SGS stress using its transport equation in the convective atmospheric boundary layer. |
