Investigating turbulence regimes in the weakly stably-stratified boundary layer with Large-Eddy Simulation

Large-Eddy Simulations (LES) using Weather Research and Forecasting (WRF) model are performed in order to study the turbulence in different flow regimes, changing the stability and the wind speed. A small hill is placed in the center of the domain to produce a disturbance in the flow and induce the interaction between gravity waves and turbulence leeward of it. Results show that turbulence is mainly produced by shear with the energy contained in large eddies close to the ground and small eddies higher up. This turbulence is intermittent in time, especially in the more stable regimes where signals of gravity waves are also detected.

As the simulated flow accounts for the intermittent turbulence, we explore the turbulence regimes found in observations of the nocturnal boundary layer in CASES99 (Sun et al. 2012). The simulations capture the overall pattern of the observed relationships between turbulence components and wind speed, mainly for horizontal component, where turbulence increases with the wind speed and changes at different heights. This is the regime above a threshold where the flow approaches near neutral stratification. Simulations also confirm that turbulence is independent of both local shear and local vertical gradients of potential temperature close to the ground, which was seen from observations. Strongly stratified atmospheric boundary layer flows characterized by low wind speeds and weak turbulence represent a challenge for LES. Using WRF-LES no clear threshold could be captured between the two regimes.