Part I - Introduction


Growth of the convective planetary boundary layer (CBL) over land in the middle of the day due to solar heating of the Earth's surface has been extensively observed and relatively successfully modelled. But the early morning transition - when the CBL emerges from the nocturnal boundary layer - and the late afternoon transition (LAT) - when the CBL decays to an intermittently turbulent residual layer overlying a stably-stratified boundary layer - are difficult to observe and model due to turbulence intermittency and anisotropy, horizontal heterogeneity, and rapid time changes. Even the definition of the boundary layer during these transitional periods is fuzzy, since there is no consensus on what criteria to use and no simple scaling laws to apply. Yet they play an important role in such diverse atmospheric phenomena as transport and diffusion of trace constituents, wind energy production, and convective storm initiation. The residual layer can be incorporated into the overlying free troposphere, so that water vapour and pollutants emitted at the surface and diffused throughout the CBL during the day can become isolated from the boundary layer and may be transported over long distances with no interaction with the surface.

At some point in the afternoon, the surface buoyancy flux is not large enough to maintain turbulent mixing throughout the CBL, especially for a deep CBL. Yet, vertical motions of up to 1 m s-1 extending horizontally over several km have been observed. The reason for this large-scale uplift is unclear; possibilities include surface variability and orography that can induce mesoscale circulations. The scale of these updrafts during the transition seems to be larger than the turbulent scales of vertical transfer during the middle of the day. Previous large-eddy simulation (LES) studies showed that during that period of the day, a decoupled residual layer, within which turbulence is still active, develops above the stably-stratified surface layer and is characterised by larger-scale updrafts than the mid-day eddies.

Quantitative observational evidence for this circulation is lacking, partly due to the difficulty of measuring weak turbulence and mean circulations in transitory conditions and at larger scales. Thus this phase of the diurnal cycle remains largely unexplored, from both modelling and observational perspectives.

The objective of the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) 2011 field experiment is to make more and better observations of the LAT, so as to better understand the physical processes that control it, and elucidate the role of the LAT on mesoscale and turbulence scale motions, and on species transport. This implies the study of entrainment across the CBL top, surface heterogeneity, baroclinicity, horizontal advection, clouds, radiation and gravity waves.

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