Large eddy simulations of boundary layer turbulence during late afternoon transition

The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign took place in France in June and July 2011 focusing on the evening collapse of the boundary layer. The BLLAST experiment gathered numerous different and complementary instruments to study the relatively unknown processes controlling the transition from a well-developed daytime atmospheric boundary layer (ABL) to a residual layer overlying a stably stratified surface layer. In an effort to guide the BLLAST instrument deployment and sampling strategies, the present numerical study aimed to answer basic questions such as: what initiates, determines, and controls the late afternoon transition, or which atmospheric layers exhibit the strongest response to the diminishing solar forcing? To address these questions, two Large Eddy Simulations (LES) codes – the NCAR LES, and Méso-NH from Laboratoire d'Aérologie and CNRM/GAME – simulate the daytime convective boundary layer observed on14 June 2002 during the International H2O Project (IHOP 2002) field experiment. Surface fluxes were prescribed as low boundary conditions. Besides guiding the experimental design, this numerical study had two additional goals: 1/ comparing the two codes and their ability to simulate this delicate period during which the surface buoyancy flux is decreasing, and 2/ studying the turbulence characteristics during this period.

The results show that the two LES codes produce nearly the same evolution of the mean ABL characteristics, such as ABL height and vertical profiles of the key parameters during the late afternoon. This comparison also investigates: 1/ the mixed layer scaling, 2/ the ‘S' shape of the buoyancy flux, 3/ the decay of turbulent kinetic energy (TKE) and, 4/ the time evolution of turbulent length scales.

We find that new scaling laws are necessary during the evening transition. During the transition, the dimensionless buoyancy flux profile becomes non-linear, gradually becoming 'S' shaped in accordance with Sorbjan (2007) and Pino et al (2006). We also find that the decay of the mean TKE in the ABL is governed by two time scales, the external time scale controlling the surface heat flux evolution, and the convective time scale, in agreement with Sorbjan (1997). The analysis of the time evolution of the TKE at different heights in the ABL points out that high TKE values descend from the top of the boundary layer to the bottom during the late afternoon, with entrainment gaining the upper hand over surface convective processes. Finally, the late afternoon boundary layer hosts larger horizontal turbulent scales than does the mid-day boundary layer.