Turbulent kinetic energy decay in the late afternoon over heterogeneous surface

The Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST ) campaign took place in France, on the Lannemezan Plateau (700m of elevation), in June and July 2011. It focused on the transition from a well-developed daytime atmospheric boundary layer (ABL) to a residual layer overlying a stably stratified surface layer. One of the issues addressed in the BLLAST project is the role of surface heterogeneities on the late afternoon transition. According to the surface characteristics and vegetation cover, each landscape patch is characterized by a different flux partitioning between latent and sensible heat flux, a different canopy and ground storage. This may play a role in maintaining the balance between the atmosphere and the surface when the net radiation decreases.

During the BLLAST experiment a site composed of three large vegetation covers (moorland, maize field and forest) of about 1.5 km length and 500 m width was equipped to measure mean meteorological parameters and the surface energy balance terms. Surface characteristics, such as temperature, or humidity were also measured over the three adjacent patches. The surface layer above the moorland and the maize field was extensively probed by two tethered balloons, providing vertical profiles of temperature, humidity and wind. The atmosphere above was probed by two aircrafts measuring turbulence in the boundary layer during the late afternoon transition.

In a first step, the characteristics of the three surfaces are studied in terms of energy balance for a composite day gathering the eleven intensive observation periods. The type of soil being similar from one patch to another, the differences observed are mainly due to the vegetation cover. As expected, the forest is very different from corn and moor, but the corn appears to react as a vegetation cover at an intermediate position between moor and forest from many points of view (radiative component, ground storage, latent and sensible heat flux…).

The second step concerns the evolution of the turbulent kinetic energy from noon to the sunset. The turbulent kinetic energy decay over the three surface patches is compared and related to the surface characteristics. Such a comparison puts into question the use of a scaling law which will be discussed. Furthermore, thanks to surface and airborne measurements at different levels, the turbulent kinetic energy decay is considered simultaneously at the surface and in the whole boundary layer for different days of the BLLAST campaign.