15 research outputs found
Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)
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.Peer ReviewedPostprint (published version
Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)
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.Peer ReviewedPostprint (published version
The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence
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.publishedVersio
La campaña experimental BLLAST 2011: Boundary Layer Late Afternoon and Sunset Turbulence
PresentaciĂłn realizada para las XXXII Jornadas CientĂficas de la AsociaciĂłn MeteorolĂłgica Española y 13Âş Encuentro Hispano-Luso de MeteorologĂa celebrados en Alcobendas (Madrid), del 28 al 30 de mayo de 2012
Transition Periods in the Diurnally-Varying Atmospheric Boundary Layer Over Land
International audienceThe atmospheric boundary layer undergoes transitions between stable and convective states. Over land, in undisturbed conditions, these transitions occur daily in the morning and late afternoon or early evening. Though less well studied and presenting more challenges than the fully stable and 2 fully convective states, such transitions have been the subject of growing interest over the last few decades. During transitions, all forcings are weak, and few simplifications are possible. Factors such as terrain, radiation, advection, and subsidence can seldom be safely neglected. In this paper, we review research on transitions over recent decades, with an emphasis on work published in Boundary-Layer Meteorology. The review is brief and inevitably reflects the interests and views of the authors
Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)
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.Peer Reviewe
Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)
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.Peer Reviewe
The BLLAST field experiment: Boundary-Layer late afternoon and sunset turbulence
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