Experimental and Numerical Investigations of Stably Statified Atmospheric Flows

Statically stable atmospheric flows are characterized by weak and highly anisotropic turbulence, gravity waves, instabilities, and meandering motions that are not observed in neutral or unstable flows. These features complicate both modeling and measurements in stable boundary layers. Nevertheless, these flows remain of considerable importance for a variety of problems including nighttime flow and transport and polar boundary layers and climates. This paper presents experimental and numerical studies of turbulence under stable conditions in the atmospheric boundary layer. INTRODUCTION Using experimental measurements over a glacier and numerical experiments using the Large Eddy Simulation (LES) technique, we seek to study the effects of varying stability on high Reynolds number flows in the atmospheric boundary layer (ABL). We focus on statically stable flows (Richardson number > 0) which are considerably complicated by numerous flow dynamics that are negligible or not present in unstable flows. The experimental results are from the Snow Horizontal Array Turbulence Study (SnoHATS) field experimental campaign which was performed over an extensive glacier in Switzerland from February to April 2006. The snow cover provided stable stratification of the flow over long periods. We perform a-priori analysis of the dynamics of small-scale turbulence and test the skills of existing subgrid scale models used in numerical simulation tools to represent the role of these scales. The numerical results are from ongoing large eddy simulation studies we are conducting to examine the impact of stability on the TKE budget in a stable ABL under a range of Richardson numbers. Some of the simulations are designed to reproduce the experimental conditions of wind tunnel Particle Image Velocimetry measurements to facilitate the validation of the simulation results and to cover a wider range of turbulent scales and Reynolds numbers. The aim is to study the effect of stability on turbulence structure and dynamics in the ABL. The experimental studies focus on small turbulent scales that cannot be resolved in large eddy simulations of high Reynolds number flows. These socalled subgrid scales play an important role and their proper parameterization is critical for adequate numerical modeling of stable ABL flows. SUMMARY OF RESEARCH The experimental studies use an array of 3D sonic anemometers measuring wind speed and temperature at 20 Hz to obtain turbulence measurements that resolve scales down into the inertial subrange. These measurements are subsequently filtered to separate the large scales, that would be resolved in LES, from the subgrid scales (SGS). The role and modeling of the subgrid scales and their interaction with the resolved can then be studied (for previous applications of this a-priori testing approach, see [1; 2; 3]). The data were analyzed