Effects of Mesoscale Surface Heterogeneity on the Afternoon and Early Evening Transition of the Atmospheric Boundary Layer

Abstract

Large-eddy simulation is used to examine the afternoon and early evening transition of the atmospheric boundary layer (ABL) over heterogeneous surfaces with respect to the decay of the turbulence kinetic energy (TKE). The heterogeneous surface sensible heat flux is prescribed based on the inverse transform of the Fourier spectrum whose slope is controlled to behave as $\kappa^{0}$, $\kappa^{ - 1}$, $\kappa^{ - 2}$, and $\kappa^{ - 3}$ (where $\kappa$ is the wavenumber) in the wavelength range from 18 to 0.2 km. The $\kappa^{0}$ slope spectrum corresponds to a randomly homogeneous field of surface flux, which is implicitly assumed in the grid cells of most large-scale models. The spectral slopes of negative exponents represent heterogeneous surface fields with a larger negative exponent describing stronger mesoscale heterogeneity. The results for a homogeneous surface flux are consistent with that of previous studies in which, during the transition period, the decay of the volume-averaged TKE normalized with the convective velocity scale follows the power law $\tau^{ - n}$ (where $\tau$ is time normalized by the eddy turnover time scale) for exponents n = 1, 2, and 6. While the $\tau^{ - 6}$ portion still remains over the surface with weak mesoscale heterogeneity, the $\tau^{ - 6}$ portion of the spectrum is replaced with a $\tau^{ - 2/3}$ portion over moderate heterogeneity, and the $\tau^{ - 2}$ and $\tau^{ - 6}$ portions are replaced with $\tau^{ - 2/3}$ and $\tau^{ - 1}$ portions of the spectrum over strong heterogeneity, resulting from the surface-heterogeneity-induced horizontal flow persisting after sunset. In fact, mesoscale features across the ABL become more apparent with the reduced number of turbulent eddies in the evening.