Testing subgrid parametrizations for LES through experiments

Micrometeorologists have traditionally used the framework of the ensemble-mean and fluctuating (average over all scales) decomposition in studying turbulence spectra, Reynolds-flux budgets, surface exchange relations, and the universal Monin-Obukhov similarity within the "constant flux" layer (Wyngaard and Peltier, 1996). More recently, the growth of supercomputers and computational fluid dynamics has stimulated micrometeorological applications of Large-Eddy Simulations (LES). LES uses a different framework, one based on the resolvable, subgrid-scale (average over small scales) decomposition. Since 1 December 1999 an experiment is performed in Cabauw, The Netherlands which will address the subgrid scale parameterization. The goal of this experiment is to perform turbulence measurements in order to study subgrid models in LES and flux-profile relationships (integral length scale, etc.). We will focus on the Stable Boundary Layer (SBL) and use a typical LES resolution of 1m. We will apply ideas of the co-spectrum similarity in the surface layer (Wyngaard and Coté, 1972) to improve the subgrid models used in LES. We use 13 thermocouples in an orthogonal array (x, y and z) with the sonic in the centre. The sonic is placed 3 m above the surface. In the three orthogonal directions the distance between the thermocouples will be about 1 m. This is a typical resolution of an LES for the stable boundary layer. The position of the thermocouples can thus be interpreted as LES gridpoints. A hot wire (in co-operation with the group of Frans Nieuwstadt, TUD) is located just beside the sonic and will measure u and w with 10 kHz. From the hot wire observations viscous dissipation rates will be determined in order to validate some of the parameterization issues.