Structure of the Atmospheric Boundary Layer from Large Eddy Simulation

We carry out here a detailed analysis of Monin -Obukhov (MO) similarity using large eddy simulations (LES) of the atmospheric boundary layer (ABL) under three stability states. We treat the issues of determining the appropriate normalizing scales and determining the functional dependence on the normalized variables, separately. Under the MO hypothesis, the MO-normalized profiles of statistical quantities in the surface layer obtained for the three ABL states are functions of only z/L and should collapse onto a single curve when plotted against z/L. We look for deviations from MO similarity by searching separately for departures from the assumption of a single normalizing scale, or from a collapse in the normalized functional relationship. For the close-to-neutral case, we also study the relative roles of grid resolution and subgrid-scale (SGS) parameterization on predictions of mean shear, mean temperature gradient and vertical velocity variance. The effect of grid resolution is analyzed using the technique of "embedded mesh" and the effect of SGS parameterization is studied by comparing two SGS parameterizations. The simulated temperature field satisfies the MO hypothesis and agrees well with observations. The velocity field, on the other hand, shows significant departures. Except for the horizontal variance, MO scales are the appropriate normalizing scales for the surface layer statistics. However, the boundary layer depth z_{i} has an "indirect" influence on all surface layer variables except temperature, and the MO-scaled variables exhibit a functional dependence on both z/L and z/z _{i}. Two-dimensional spectra of velocity and temperature fluctuations, however, indicate that while large scales deviate from MO similarity, inertial subrange scales are MO similar. We also study the local structure of the ABL using three-dimensional visualization. In order to understand the relative roles of buoyancy and shear, we analyze a wide range of stability states. It is shown that the near -wall shear-induced low-speed streaks play a significant role in the formation of horizontal roll vortices in a moderately convective boundary layer. The roll vortices originate in the near-wall low-speed streaky regions that tend to accumulate warmer fluid. Shear, therefore, modulates the buoyancy induced thermals into sheet-like updrafts that align with the mean wind.