At the end of the afternoon, when the surface heat
fluxes start to sharply decrease, the CBL turns from a
convective well-mixed layer to an intermittently turbulent
residual layer overlying a stably-stratified boundary
layer. This transition raises several observational and
modelling issues. Even the definition of the boundary
layer during this period is fuzzy, since there is no consensus
on what criteria to use and no simple scaling
laws to apply. Yet it plays an important role in such diverse
atmospheric phenomena as transport and diffusion
of trace constituents or wind energy production.
This phase of the diurnal cycle remains largely unexplored,
partly due to the difficulty of measuring weak
and intermittent turbulence, anisotropy, horizontal heterogeneity,
and rapid time changes.
The Boundary Layer Late Afternoon and Sunset Turbulence
(BLLAST) project is gathering about thirty research
scientists from the European Union and the
United States to work on this issue. A field campaign
(BLLAST-FE) is planned for spring or summer 2011 in
Europe. BLLAST will utilize these observations, as well
as previous datasets, large-eddy and direct numerical
simulations, and mesoscale modelling to better understand
the processes, suggest new parameterisations,
and evaluate forecast models during this transitional period.
We will present the issues raised by the late afternoon
transition and our strategy to study it.
