Abstract
Abstract. Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso- or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations.
Highlights
At interface between the earth’s surface and the atmosphere, the planetary boundary layer (PBL) is a critical component of the earth system
One of the main strengths of Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) project and field campaign is its focus on a well-defined issue: turbulence decay during the afternoon over land
The large collaborative efforts that enabled assembling almost all the observational platforms that are useful for probing the PBL, as well as a complete hierarchy of modelling tools have resulted in a rich data set for the study of the changing characteristics of turbulence during the afternoon transition (AT) and evening transition (ET), throughout 12 intensive observation periods (IOPs) days
Summary
At interface between the earth’s surface and the atmosphere, the planetary boundary layer (PBL) is a critical component of the earth system. During the night, the radiatively cooled surface stratifies the air above, which forms a stable nocturnal boundary layer Both midday and nocturnal periods, when in a stationary state, have been relatively successfully modelled, even if several issues remain open (see the reviews by Angevine, 2008; Cuxart, 2008; and Holstlag et al, 2013). Several recent studies have attempted to simulate the entire diurnal cycle both with large-eddy simulation (LES) and singlecolumn parameterized models (SCM) These include Kumar et al (2006), Basu et al (2008) or Svensson et al (2011), who made use of realistic conditions based on the Horizontal Array Turbulence Study (HATS, Horst et al, 2004), Wangara and CASES-99 campaigns, respectively. This general manuscript introduces the deeper analyses made on specific issues that are made in the other articles of the special issue
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