Atmospheric Boundary Layer

The atmospheric boundary layer (ABL) is the lowest part of the atmosphere which is in direct interaction with the Earth’s surface and responds to surface forcing with time scales of one hour or less. It is a highly turbulent boundary-layer flow with a Reynolds number of order ~ 10⁹. As a result, the ABL flow has a huge continuous range of turbulent eddy scales, ranging from the integral scale, on the order of the boundary-layer depth ~ O(1 km), down to the Kolmogorov viscous dissipation scale ~ O(1 mm).

Prediction of ABL flow is complicated by the often strong temporal and spatial variability of the land-surface characteristics (e.g., surface temperature and aerodynamic roughness). The real ABL is strongly influenced by temporal variability of buoyancy (positive and negative) effects associated with the diurnal cycle of net radiation at the land surface. For an interactive simulation of a typical diurnal cycle evolution of the ABL, the reader is referred to an Atmospheric Boundary Layer Simulator. Moreover, land surfaces are often characterized by complex topography, which is in many cases multifractal, as well as spatial heterogeneity of aerodynamic roughness and temperature associated with different land-cover types. This leads to highly non-linear interactions between the complexity of the land surfaces and the ABL turbulence.

Modeling ABL turbulent fluxes of momentum and scalars (heat, water vapor and pollutants) is of great importance to forecast weather, climate, air pollution, wind loads on structures, and wind energy resources. Indeed the need for accurate weather prediction has provided much of the impetus for the development of numerical methods in turbulence research. Prediction of these fluxes at regional scales, which is required in most atmospheric models, is a challenging task due to the highly non-linear interactions between ABL turbulence and the complex multi-scale structure of the land surface.