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 modeling
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 modeling to better understand the
processes, suggest new parameterizations, 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.
