Evaluating Large-Eddy Simulations Using Volume Imaging Lidar Data

Abstract

The authors apply data analysis techniques that demonstrate the power of using volume imaging lidar observations to evaluate several aspects of large-eddy simulations (LESs). They present observations and simulations of an intense and spatially evolving convective boundary layer on 13 January 1998 during the Lake-Induced Convection Experiment (Lake-ICE). To enable comparison of observed and simulated eddy structure, aerosol scattering was estimated from LES output of relative humidity, a passive tracer, and liquid water. Spatial and temporal correlation functions of aerosol structure from horizontal planes reveal the mean and turbulent convective structure. The correlation functions of the observed and simulated aerosol backscatter are presented as a function of altitude and offshore distance. Best-fit ellipses to the closed contours encircling the origin of the correlation functions are used to obtain the mean ellipticity and orientation of the structures. The authors demonstrate that these two parameters are not sensitive to minor changes in the functional relationship between humidity and optical scattering. The lidar-derived mean wind field is used as a reference for evaluating the LES mean flow. The ellipses from lidar data indicate that structures near the surface tend to be aligned with the mean wind direction, while in the entrainment zone they are aligned perpendicular to the mean wind direction. In the middle of the mixed layer, convective plumes tended to be circular and, therefore, had no preferred orientation at small lags. At longer lags, however, the correlation functions from the middle of the mixed layer show that the observed convective plumes were organized into linear bands oriented perpendicular to the mean wind direction. The perpendicular bands suggest the important role of gravity waves in organizing convective structures. The study shows that the model generates reasonable coherent structures (open cells) where the LES technique is expected to perform poorly (near the surface) and fails to capture the wind-perpendicular organization of closed cells in the middle of the mixed layer where the LES technique is expected to be robust. The authors attribute this failure to the boundary conditions that limited the growth of waves above the mixed layer.