LNM Academic Report
Modeling turbulent transport of momentum and scalars (heat, water vapor and pollutants) in atmospheric boundary layer (ABL) is of great importance to forecast weather, climate, air pollution, wind loads on structures, and wind energy resources. The ABL flow is a highly turbulent boundary-layer flow with a Reynolds number of O(10^8) or larger. It has a huge continuous range of turbulent eddy scales, ranging from kilometers (the integral scale) down to millimeters (the Kolmogorov viscous dissipation scale). Direct numerical simulation of the ABL flow is not possible with today's computer resources. Of special relevance are the seminal works of Smagorinsky and Deardorff, who introduced the technique that we now call large-eddy simulation (LES). The physical basis for LES is the separation of the flow into resolved and subgrid-scale (SGS) motions. In the context of ABL flows, early SGS models have revealed that the mean wind and temperature profiles in the surface layer differ from the Monin-Obukhov similarity forms. Specifically, the nondimensional vertical gradients of velocity and temperature could be overestimated by more than 20% in the surface layer. Moreover, the real ABL is strongly influenced by temporal variability of buoyancy effects associated with the diurnal cycle of net radiation at the land surface, and also there exist highly non-linear interactions between the complexity of the land surfaces and the ABL turbulence.
In spite of all of the difficulties, LES has become a powerful tool to study turbulent transport and mixing in the ABL. Not only the need for accurate weather prediction has provided much of the impetus for the development of numerical methods in turbulence research, but also LES studies of the interaction between ABL turbulence and wind turbines, and the interference effects among wind turbines have been carried out, in order to understand the impact of wind farms on local meteorology as well as to optimize the design (turbine siting) of wind energy projects. This presentation will give an overview of recent research efforts aimed at improving SGS parameterizations and making LES a more reliable tool to study land-atmosphere interactions.