Large-eddy simulations of the nighttime boundary layer during CASES-99

The nighttime stable boundary layer (SBL) is associated with various atmospheric processes including low-level jets, internal gravity waves, Kelvin-Helmholtz (KH) shear instabilities, and other forms of intermittent turbulent events. Since turbulence is greatly suppressed under stratified conditions, these phenomena are difficult to represent in numerical models, and often require very high grid resolution. In this study, large-eddy simulations (LES) are performed to model and investigate a strongly stable night over the Great Plains. The observation site is located near Leon, Kansas, where the Cooperative Atmospheric-Surface Exchange Study 1999 (CASES-99) field program took place. Oct 5-6 during CASES-99, also known as IOP2, is a well- documented intermittently turbulent night. Tower observations revealed prevailing quiescent conditions, except for two short turbulent periods triggered by a propagating gravity current and a KH shear instability event. To properly model both events, simulations are initialized with North American Regional Reanalysis (NARR) data on 32 km grids, and one-way nested down to a very fine grid with 16 m horizontal spacing and 2 m near-surface vertical spacing. High resolution USGS topography (~10m) and land cover (~30m) are used to represent the complex terrain.

The gravity current event occurred during the daytime-evening transition period. It is resolved with great temporal accuracy (within o(1min)) at the finest LES domain. Improved grid resolution is found to be the key factor for modeling this gravity current event. The late night KH shear instability event induced strong turbulence within the SBL, resulting in a rapid transition from quiescent to turbulent flows. Prediction of this event depends strongly on the turbulence closure model. The conventional TKE-1.5 turbulence model is found to be overly dissipative. It failed to trigger turbulent transition under strongly stable conditions, resulting in a false “laminar” flow field. The dynamic reconstruction model (DRM) is more suitable for representing stratified flows. It has been shown to out-perform conventional eddy-viscosity models in idealized simulations. In this study, LES with the DRM is able to capture the KH shear instability event. The amplitude and dominant frequency of the KH billows compare well with LIDAR observations. Finally, the 3D LES addresses the question of whether the observed KH billows at the tower during IOP2 are due to temporal evolution at the local site or spatial translation of the flow field. From the simulations, the observed billows were generated and transported from an upwind location where a critical Richardson number was reached.