SGS00: Sub-Gridscale Studies, 2000 (aka HATS)

Summary

A field experiment is proposed to study the response of small turbulent eddies to large turbulent motions and to develop better subgrid-scale (SGS) models for large eddy simulation (LES). LES is a popular numerical tool for studying turbulence; in this technique, a turbulent flow field is divided between resolved-scale eddies (which are also referred to as the filtered field or large eddies) and unresolved eddies (also referred to as the SGS or small eddies). The large eddies are explicitly calculated, while the effect of SGS eddies on the resolved-scale flow field is parameterized. Developing such a SGS model requires data that separate the turbulent motion into two components: filtered and SGS fields.

A method of measuring both filtered and SGS motions using a two-level array of 3-component sonic anemometers was recently proposed by Tong et al. (1999). A primary line consisting of nine sonic anemometers provides both filtered and SGS fields (velocity and temperature) at five points and also the SGS stresses at the center point; and a second line consisting of five more sonic anemometers is used to determine the strain rate of the resolved-scale field. This array allows us to examine relationships between the SGS and the resolved-scale motions in the atmospheric surface layer. In the proposed research, we will focus on the SGS response to stability effects, and also test two specific SGS models.

The proposed work is our first step towards a long-term, collective effort among LES practitioners, turbulence researchers and micrometeorology experimentalists to address complicated problems such as diffusion and chemical reactions in stable boundary layers where turbulence is intermittent; interactions between boundary-layer turbulence and the Earth's surface (both over ocean and land); and entrainment across strongly-stratified capping inversions where small-scale turbulent motions may dominate. To gain insights into these problems, we wish to take advantage of both field experiments and state-of-the-art numerical techniques: using field data to develop a proper SGS model for LES, and then applying LES to systematically tackle the processes listed above.