High-Resolution Modeling of Heterogeneous Boundary Layers Using LES and Eddy Seeding

Topographic and land-surface features on scales of a kilometer or less can have a significant impact on the structure and evolution of the atmospheric boundary layer; however, the scales of these features can be of the same order of magnitude as the scales of the largest boundary layer eddies, at least in convective conditions. The most effective current means of modeling the structure of these eddies is the large eddy simulation (LES) method, in which the largest eddies are explicitly represented, and the downscale transport of energy by smaller eddies is parameterized. Thus, the effect of the largest eddies on the evolution of the atmospheric boundary layer and the turbulent fluxes and transport of momentum, heat, and scalars such as toxic agents can be directly examined. But when LES models are initialized from mean atmospheric conditions, typically they require a spinup period to develop eddies with realistic structures. Use of periodic lateral boundary conditions (LBCs) allows this spinup to occur uniformly within the domain given sufficient time, but these LBCs are precluded when realistically heterogeneous atmospheric fields and surface properties are used. However, the use of non-periodic LBCs can lead to deficient or unrealistic eddy structures near inflow boundaries.

Here we will show how the use of an ‘eddy seeding' method can be used to improve turbulent statistics and eddy structure throughout the domain of an LES that uses nonperiodic LBCs. This method employs a relaxation technique and an appropriate scaling of pre-computed eddy structures to mitigate the problem with unrealistic eddies near non-periodic lateral boundaries. We will then show how the method can be applied to an LES for realistic case studies over Pennsylvania using the Weather Research and Forecast (WRF) model, when the LES is run as a nest within coarser domains that use Reynolds-averaged turbulence parameterizations. Cases exhibiting different vertical wind profiles and cloud patterns will be examined and contrasted. The impact of the eddy seeding technique and of other non-periodic LES configurations on the evolution of the resolved-scale variables and turbulent fluxes will be discussed.