Large-Eddy Simulation Intercomparison Case Setup for GABLS3

In the year 2003, the first large-eddy simulation (LES) intercomparison of stable boundary layers was organized under the auspices of the GEWEX atmospheric boundary layer study (GABLS). More than ten LES modeling groups from around the world participated in this intercomparison study. They modeled a weakly stable barotropic boundary layer utilizing several LES subgrid-scale (SGS) models. This intercomparison study highlighted that LES of weakly stable boundary layers is quite feasible. However, some of the key turbulence statistics were found to be sensitive to SGS models even at relatively fine resolutions, which is not desirable.

Quite a few novel SGS models have been proposed in the atmospheric boundary layer (ABL) literature in the past few years. The existing SGS models have also evolved quite dramatically during this period. The GABLS community believes that the time is opportune for another LES intercomparison in order to re-evaluate the capabilities of the SGS models - in the context of stably stratified flows. This exercise will also revisit and perhaps resolve some of the outstanding issues (e.g., resolution sensitivity) of the first GABLS LES intercomparison.

In this presentation, we will describe the setup of this newly proposed GABLS LES intercomparison case along with preliminary results. This time, the focus will be on a moderately stratified, baroclinic, mid-latitude nighttime boundary layer. The boundary layer was observed over Cabauw, Netherlands on July 1st, 2006. The initial condition for the LES intercomparison is created by synthetically merging the observed 200-m Cabauw tower data and a high-resolution 00 UTC sounding from DeBilt. Time-height-dependent geostrophic wind forcings are derived from a network of sounding stations in Netherlands combined with analysis of a 3D weather-forecast model.

Contemporary ABL literature has shown that in order to represent the moderate to very stable regime in an ABL model, accurate surface temperature prescription or prediction is needed, and the use of surface heat flux as a boundary condition should be avoided. Ideally, for reliable simulations one should use a LES code equipped with a land-surface model (LSM). At present, however, only a handful of LES modeling groups have LSM schemes coupled to their LES models. Thus, in order to enhance the participation of LES modeling groups in this intercomparison study, as well as to reduce the complexity of the LES setup, we decided to prescribe near-surface air temperature and specific humidity as lower boundary conditions.

During this presentation, we will discuss the performances of several dynamic and static (stability corrected) SGS models in capturing the characteristics of this moderately stable boundary layer. We will put strong emphasis on the evolution of low-level jets and early morning transition.