SGS-2000 Experiment

September 2000
San Joaquin Valley, CA

MMM Participants: Chin-Hoh Moeng (moeng@ucar.edu); Peter Sullivan (pps@ucar.edu); Don Lenschow (lenschow@ucar.edu); Jeff Weil (weil@ucar.edu)

ATD Participants: Tom Horst, Steve Oncley

Project 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.

Objective and Motivation

LES is increasingly used for studying PBL turbulence. It has been treated as "ground truth" in many applications, such as developing PBL parameterization schemes (e.g., Moeng and Wyngaard 1989). The premise of the LES technique is that large eddies, which contain most of the energy and carry most of the fluxes, are properly resolved, while small eddies (the unresolved motions) are parameterized. This is justified in the bulk of the PBL where energy-containing eddies are large and well-resolved and there is a wide spectral gap between the energy-containing eddies and the SGS eddies; there LES solutions have been shown to be insensitive to the SGS treatment (e.g., Nieuwstadt, et al. 1993).

However, in near-surface or stably-stratified regions, energy-containing eddies are small, and hence the SGS turbulence becomes important for the transport of heat, momentum and other constituents. It is in these applications that LES requires a carefully calibrated SGS model. Our need is well summarized by Wyngaard (1998): "Given the sociological indications about the dominant role of LES in our community, I suggest we pragmatically accept today's leading experimental challenge as that of testing and calibrating LES. LES involves resolvable-scale and subgrid-scale variables...so it requires an entirely different mindset of the experimentalist. (SGS) fluxes in LES are random variables, not expected values... This poses a new set of experimental problems for micrometeorologists." We need this kind of new experimental approach to improve SGS models, since only with improved SGS models can the LES technique be used to address the complicated geophysical turbulence problems discussed here.

Experimental Setup

http://www.atd.ucar.edu/sssf/projcets/sgs2000/index.html

We will adopt the newly-developed array technique proposed by Tong et al. (1999). The array consists of a primary line of nine equally-spaced sonic anemometers at one height, close to the ground, and a second line of five sonic anemometers slightly above (or below) the primary line (although Tong et al. have not yet used the second line in their study). Tong et al. (1998) evaluated the performance of this two-dimensional (x and y) filter with data from a high-resolution LES of a convective PBL and showed that "the two and three-dimensionally filtered data are essentially indistinguishable" if the cutoff lies on the peak of the w-spectrum.

The array is ideally aligned to be perpendicular to the mean wind direction, such that the filtered and SGS fields along the wind can be obtained by time filtering and Taylor's hypothesis and those across the wind can be retrieved from the nine sensor line as described in Tong et al. (1999). However, it is unlikely that the wind direction will be exactly normal to the array for a significant period of the field measurements. Nevertheless, moderate deviations from the ideal wind direction can be accommodated by again using Taylor's hypothesis and lagging the data from each of the individual anemometers to analyze the data in a reference frame normal to the wind. This will effectively reduce the crosswind spacing of the anemometers by the cosine of the wind direction deviation angle and can even be an advantage by providing data from a (limited) range of crosswind spacings.

In addition to the three components of velocity, sonic anemometers also measure the speed of sound and hence an acceptable approximation of virtual temperature. Thus with the primary line of sensors, one can easily compute the horizontal gradients of both filtered velocity and temperature fields. But the vertical gradients of these filtered fields require a second line of five sonic anemometers just above or below the primary line.

In this first experiment, we will try three different array setups. We plan to keep the experiment running both day and night so that we can collect data over a wide range of stability.

Potential Collaborations

We have discussed this proposal with many PBL researchers outside NCAR, and they all expressed a great interest in using the field data or/and a close collaboration in improving SGS models for LES. They include John Finnigan (from CSIRO, Australia, now visiting MMM) who has been involved in our discussions on field design; John Wyngaard (Penn State U) and Marac Parlange (Johns Hopkins University) who started and remain strong advocates of this new experimental approach; Jeff Weil (visitor from CIRES/CU) who would like to use the data set for his investigation of dispersion in the very stable PBL; Bjorn Stevens (UCLA) who has been an LES practitioner for many years and is particularly interested in SGS problems; and Berengere Dubrulle (visitor from CNRS, France) who developed the new dynamic SGS model that we would like to test in the field.

REFERENCES

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