Turbulence vertical structure of the boundary layer during the afternoon transition.

We investigate the decay of planetary boundary layer (PBL) turbulence in the afternoon, from the time the surface buoyancy flux starts to decrease until sunset. Dense observations of mean and turbulent parameters were acquired during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment by several meteorological surface stations, sounding balloons, radars, lidars, and two aircraft flying extensively during the afternoon transition. We analyzed a case study based on some of those observations and Large-Eddy Simulation (LES) data focusing on the turbulent vertical structure throughout the afternoon transition. The decay of turbulence is quantified through the temporal and vertical evolution of (1) the turbulence kinetic energy (TKE), (2) the characteristic length scales of turbulence, (3) the shape of the turbulence spectra. A spectral analysis of LES data, airborne and surface measurements is performed in order to (1) characterize the variation of the turbulent decay with height and (2) study the distribution of turbulence over eddy size. This study points out the LES ability to reproduce the turbulence evolution throughout the afternoon. LES and observations agree that the afternoon transition can be divided in two phases: (1) a first phase during which the TKE decays with a low rate, with no significant change in turbulence characteristics, (2) a second phase characterized by a larger TKE decay rate and a change spectral shape, implying an evolution of eddy size distribution and energy cascade from low to high wavenumber. The changes observed either on TKE decay (during the first phase) or on the vertical wind spectra shape (during the second phase of the afternoon transition) occur first in the upper region of the PBL. The higher within the PBL, the stronger the spectra shape changes. [ABSTRACT FROM AUTHOR]