Field experiments are designed and carried out in order to gain a better understanding of turbulent processes in the atmospheric boundary layer (especially subgrid-scale physics). High resolution measurements, using sonic anemometers (Campbell Scientific CSAT3), of velocity and temperature provide data that are used to test SGS models and develop improved parameterizations that are key to the success of LES. This is done by spatially distributing our sensors in large arrays, we can mimic the information available at grid points in a LES. We have collaborated recently in several SGS field experiments with the groups from Johns Hopkins University and from the National Center for Atmospheric Research (NCAR). Our most recent collaboration was on the salt flats of western Utah (see figure 1).

The data from these field experiments can be filtered and used to calculate SGS stresses and fluxes which require modeling in LES but can be used here to determine the necessary properties that such models require. In particular, the transfer rate of resolved-scale turbulent kinetic energy and scalar variance can be studied to understand how local flow dynamics effect the SGS dissipation rates, see figure 2. Most SGS models are designed to be fully dissipative but results from our field measurements have shown that significant occurrences of negative SGS energy transfer, or backscatter, are present. Further understanding of the role of these transfers and local coherent structures in the ABL will help improve parameterizations of the SGS physics.

Field experiments are currently being performed over frozen lakes which are common in Minnesota for a good portion of the year. Current circulation models of lakes use wind measured at the nearest National Weather Service monitoring station (often 5-10 miles away) as input for their lake surface boundary condition. However, the sheltering effect on lakes due to surrounding topography and structures (e.g., trees and houses) may drastically change the shear stress at the surface of the lake from that experienced at the weather station. Large arrays of cup anemometers, distributed along the wind direction, are used to quantify the sheltering effect over several archetypal lakes. A preliminary study took place in a freshly mown field behind a large cornfield, see figure 3. The analysis of these measurements will provide an advanced understanding of how sheltering effects the shear stress experienced by the lake’s surface, ultimately leading to more accurate lake circulation models.