Since the introduction of large eddy simulation (LES) into the atmospheric boundary layer (ABL) modelling practice by J. Deardorff in the 1970s, this numerical technique has been extensively used for studying different ABL flow types. Simulation of the unstable convective boundary layer (CBL) and neutral boundary layer has definitely been a success story.   The accurate simulation of the atmospheric SBL, on the other hand, is still associated with significant conceptual and computational challenges.  Conditions associated with the SBL and its transition from night-time to daytime conditions (ABL transitions) are closely coupled to the transport, mixing, and formation of atmospheric pollutants, the formation of low-level jets (LLJs), and similar phenomena. See Fig. 1 for a depiction the ABL and its transitions.

This project has focussed on the systematic investigation of the SBL and ABL transitions through coupled numerical and observational techniques.  In particular, the study has facilitated an examination of the turbulence structure of the atmospheric flow near the surface, dispersion under stable and transitional conditions, and the formation and development of LLJs in the ABL.  To realize these goals we have adopted an approach that combines sodar (acoustic radar), unmanned aerials systems (UAS), capable of monitoring the atmosphere, and numerical LES. Complementary to the actual sodar and UAS measurements, data from simulated versions of these instruments  (developed during the project) were incorporated into our analysis. Both virtual instruments produced simulated data streams based on atmospheric parameters taken from the LES. A representation of the sodar simulator can be seen in Fig. 2. Sample wind fields along with the sampling domain from the virtual sodar are shown in Fig. 3.

The sodar was deployed at the University of Oklahoma (OU) Kessler Atmospheric and Ecological Field Station (KAEFS) for routine monitoring of the atmosphere. During a few targeted experiments, the UAS was operated the OU KAEFS site. The LES was initialized and run to produce kinematic and thermodynamic fields corresponding to those periods. This allowed us to cross compare observations from the real and simulated sodar and real and simulated UAS and check how well they corresponded to the underlying “truth” data from the LES.  This allowed us to develop new methods of measuring atmospheric turbulence and validate some of the fundamental theories related to acoustic scatter and it relates to sodar observations.

Similarly the sodar was deployed at the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program Southern Great Plains (SGP) site in Lamont, Oklahoma for two extended field experiments called Lower Atmospheric Boundary Layer Experiment (LABLE).  The UAS was not operated. The DOE ARM SGP site hosts an array of atmospheric sensors, which were used during the study. Results confirmed that using the temperature lower boundary condition leads to larger and more realistic (as may be concluded from comparisons with observed turbulence characteristics at the ARM site) resolved turbulence kinetic energy values during transition and in the stably stratified boundary-layer flow phase. Generally, the modified subgrid exchange formulation provided higher resolved-turbulence levels under stable conditions compared to the original version of the code.

The consolidation of LES and sodar and UAS approaches has provided a unique opportunity to investigate the effectiveness and applicability of both LES and various other means of characterizing the CBL, the SBL, and transitions between the two.  These findings are anticipated to have significant impacts on SBL/CBL studies. The study has also helped to usher in improvements to LES code for stably stratified boundary layers and extend our under...

Please report errors in award information by writing to: awardsearch@nsf.gov.