Peter Sullivan, National Center for Atmospheric Research (NCAR), Colorado, USA


Fluxes and structures in stable boundary layers: Results from LES


Atmospheric and oceanic boundary layers are populated by myriad coherent structures that are largely responsible for momentum and scalar transport.  For example, buoyant plumes dominate the daytime convective boundary layer while turbulent Langmuir cells are abundant in the ocean mixed layer. Boundary layer coherent structures vary with the external forcing and are particularly sensitive to the level of background stratification.  Here focus is on the coherent structures in the stably stratified atmospheric boundary layer generated by high resolution large-eddy simulation (LES); the simulations use meshes with 1024^3 gridpoints. One of the most intriguing features in these simulations is the presence of ubiquitous temperature fronts. Animations show that these sharp fronts are tilted in the downstream direction, exhibit spatial spanwise and vertical coherence and propagate in time as organized entities. The front tilt and vertical separation is determined by the balance between the background stratification and the amplitude of the turbulent fluctuations.  The fronts are internally generated by the dynamical interaction between turbulence and a stably stratified temperature field - the surface boundary conditions in the LES are horizontally homogeneous.  Based on conditional averaging, the flow fields near a temperature front appear to be controlled by pairs of upstream and downstream vortices. Guided by the LES findings, we are able to identify qualitatively similar patterns in temperature profiles collected during the CASES-99 field campaign.


Tuesday May 7, 11:15


Rossbysalen C609, Arrhenius laboratory, Svante Arrhenius väg 16C, 6th floor