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.
The campaign will combine in situ measurements made with towers, balloons and airplanes with the remote sensing capability. The measurements will be intensified during the late afternoon transition.
Two sites (hereafter « super-site 1 and 2 ») will concentrate the ground-based instruments and intensive flying over operations. They are associated with two different observational strategies: (1) vertical structure and (2) spatial heterogeneity, respectively.
(1) In super-site 1, a sodar, UHF and VHF wind profilers, a microwave radiometer, a ceilometer, a backscatter lidar and radiosoundings will give a complete view of the mean vertical structure of the troposphere. In addition, a vertically-pointing Doppler lidar will give the structure of the vertical wind, with a resolution in time and space high enough for turbulence statistics studies and entrainment zone exploration. In situ measurement of turbulence will be made on the 60-m high tower and with a tethered-balloon-borne probe. The radiation divergence in the surface layer will be estimated on another 10-m high tower.
(2) In super-site 2, several sonic anemometers will be deployed over three adjacent surfaces (a moorland, a maize field and a forest), in order to measure the differences in the structure and evolution of the transition among different vegetated surfaces. The surface layer above the moorland and the maize field will be extensively probed by two tethered balloons, while UASs will fly low over the three surfaces.
A network of two UHF radars and one sodar wind profilers (located on super sites 1 and 2, and on a third position that makes a triangle, see Fig. 3) will give continuous profiles of the mean wind for the study of the 3D atmospheric circulation.
Airplanes and unmanned aerial vehicles (UASs) will probe the atmosphere over both super-sites, focusing on either vertical structure or spatial variability. The two airplanes (Piper Aztec and Sky Arrow) will probe an area of a few tens of kilometres across centred around the super-sites (Fig. ), with horizontal legs at different levels within and just above the CBL. UASs will also fly over both super-sites, at low levels when combined with the manned airplanes, and up to 2 km height otherwise.
Over the 3.5 planned weeks, we expect 10 days during which the aircraft, the UASs and the balloons (tethered and radiosoundings) will be deployed intensively, while other instruments will work continuously during the whole period. Those richly documented days will constitute real cases on which the numerical simulation will be based.
Area and Time Frame
The 2011 BLLAST field campaign is planned in early summer, from 14 June to 8 July 2011 in France, near the Pyrénées Mountains. The site is called « Plateau de Lannemezan », a plateau of about 200 km2 area, nearby the Pyrénées foothills, at equal distance from the Mediterranean sea and from the Atlantic ocean (about 200 km), and aligned with a main S-N oriented valley which starts to the south (« Vallée d’Aure »). The surface is covered by heterogeneous vegetation: grasslands, meadows, crops, forest.
The most favourable situations for the experiment correspond to either anticyclonic conditions, or dry post-frontal conditions. In the first case, the low troposphere is governed by the mountain-plain circulation, with a north-easterly flow over the Plateau. The CBL is either clear, or with a few rare cumulus clouds. Post-frontal conditions correspond to north-westerly winds (wind direction ranges from W to N, depending on the importance of the valley-wind relatively to the meteorological wind). They are associated with more cumulus clouds, from nicely paved (cloud streets) sky to more active post-frontal situations (with more and/or deeper clouds). Fair weather can also be encountered in foehn situation (south-westerly flow over the Pyrénées mountains), but this situation is more complex and less favourable than the two other typical situations.