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

Angevine, Wayne M.

Bange, Jens

Beare, R.J.

Beyrich, Frank

Canut, Guylaine

Couvreux, Fleur

Delbarre, Hervé

Gibert, Fabien

Jonker, Harm J. J.

Lenschow, Donald

Lohou, Fabienne

Lothon, Marie

Mahrt, Larry

Pardyjak, Eric

Pino González, David

Sorbjan, Zibgniew

Vilà-Guerau de Arellano, Jordi

English

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

UPCommons

Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)

UPCommons. Portal del coneixement obert de la UPC

P1.1 Studying the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST)David Pino1, Marie Lothon2, W. Angevine3, J. Bange4, B. Beare5, F. Beyrich6, G. Canut2, F. Couvreaux7, H. Delbarre8,F. Gibert9, B. Gioli10, H. H. Jonker11, D. H. Lenschow12, F. Lohou2, L. Mahrt13, E. Pardyjak14, Z. Sorbjan15, and J.Vila`-Guerau de Arellano161Applied Physics Department, Technical University of Catalonia, and Institute for Space Studies of Catalonia (IEEC), Barcelona,Spain2Universite´ de Toulouse, Laboratoire d’ Ae´rologie – CNRS, Toulouse, France3CIRES, University of Colorado, and NOAA ESRL, Boulder, Colorado, USA4Institute for Geoscience, Universita¨t Tu¨bingen, Germany5University of Exeter, Exeter, United Kingdom6Deutscher Wetterdienst (DWD), Lindenberg, Germany7Me´te´o-France, CNRM/GAME, CNRS, Toulouse, France8Laboratoire de Physico-Chimie de l’Atmosphe`re, Dunkerque, France9Laboratoire de Me´te´orologie Dynamique, IPSL, Paris, France10Ibimet CNR, Firenze, Italy11Delft University of Technology, Delft, The Netherlands12National Center for Atmospheric Research, Boulder, Colorado, USA13Oregon State University, Corvallis, Oregon, USA14University of Utah, Salt Lake City, Utah, USA15Marquette University, Milwaukee, Wisconsin, USA16Wageningen University, Wageningen, The NetherlandsABSTRACTAt the end of the afternoon, when the surface heatfluxes start to sharply decrease, the CBL turns from aconvective well-mixed layer to an intermittently turbulentresidual layer overlying a stably-stratified boundary layer.This transition raises several observational and modelingissues. Even the definition of the boundary layer duringthis period is fuzzy, since there is no consensus on whatcriteria to use and no simple scaling laws to apply. Yet itplays an important role in such diverse atmospheric phe-nomena as transport and diffusion of trace constituentsor wind energy production.This phase of the diurnal cycle remains largely un-explored, partly due to the difficulty of measuring weakand intermittent turbulence, anisotropy, horizontal hetero-geneity, and rapid time changes.The Boundary Layer Late Afternoon and SunsetTurbulence (BLLAST) project is gathering about thirtyresearch scientists from the European Union and theUnited States to work on this issue. A field campaign(BLLAST-FE) is planned for spring or summer 2011 in Eu-rope. BLLAST will utilize these observations, as well asprevious datasets, large-eddy and direct numerical simu-lations, and mesoscale modeling to better understand theprocesses, suggest new parameterizations, and evaluateforecast models during this transitional period.We will present the issues raised by the late after-noon transition and our strategy to study it.1. ISSUES AND BACKGROUNDIn the late afternoon, the surface heat fluxes start todecrease until they become negative and turn the surfacelayer from unstable to stable. Meanwhile, the convectiveboundary layer (CBL) is still active but starts to decouplefrom the surface and the turbulent kinetic energy (TKE)decays. Transitory processes like the late afternoon tran-sition (LAT) are still difficult to observe, describe and rep-resent in models. However, this regime plays a role in thetransport and diffusion of trace constituents like pollutantsor water vapour, and it also precedes the development ofnocturnal jet, sea to land breeze reversal, and valley andslope winds.1.1 Turbulence decaySeveral authors have previously studied the tran-sition regimes of turbulence through laboratory experi-ments (e. g. Cole and Fernando, 1998). They foundthat during the decay, the turbulence maintains the initialisotropy, with the energy decay following a power law tnof time t. The first study with large eddy simulation (LES)of the decaying atmospheric convective mixed layer wasperformed by Nieuwstadt and Brost (1986). They an-alyzed an idealized case of the shearless, clear mixedlayer, in which turbulence decayed as a result of a sud-den shut-off of the upward surface heat flux. The processwas also described in terms of time power laws.Sorbjan (1997) considered a more realistic gradualchange of the heat flux with time. The evolution of the de-caying shearless mixed layer was found to be governedby two time scales: a scale linked with the decreasing rateof the heat flux and a convective time scale t∗ = Zi/w∗,where Zi is the CBL depth and w∗ is the convective ve-locity:w∗ =(gθ〈wθ〉0Zi)1/3. (1)In equation (1), g is gravity, T is the mean CBL tempera-ture, and 〈wθ〉0 is the surface buoyancy flux. During thelate afternoon transition, the buoyancy flux vertical profileturns from linear to a weak ‘S-shaped’ profile (Fig. 1).FIGURE 1: After Sorbjan (1997): The linear vertical profileof buoyancy flux turns to S-shape of weak fluxes in the late afternoon (t/t*=0 when the surface flux starts to decrease).The decay of convective turbulence has then beenfurther analyzed by use of theoretical models (e. g.Goulart et al., 2003), LES (e. g. Acevedo and Fitzjarrald,2001; Pino et al., 2006), single column model (Edwards,2006) and Direct Numerical Simulations (DNS) (Shawnand Barnard, 2002). In those studies, the decay dependsto a large extent on the way that the decrease of the sur-face fluxes is prescribed.Although literature reports some observations of thedecay (e. g. Grant, 1997), the studies based on measure-ments remain rare.Thus, the TKE decay has been extensively studied,especially with numerical models and surface-layer ob-servations. But the decay of turbulence up to the top ofthe mixed or residual layer is poorly documented, and hasto be related in the real world, not only to a decreasingsurface heating, but also to competing processes suchas cloud evolution, radiation, shear, and advection.1.2 Characteristic length-scaleIn the late afternoon, the surface buoyancy flux be-comes too small to maintain turbulent mixing. Yet verticalmotions of about 1 m s−1 extending horizontally over sev-eral km have been observed (Pagen and Bryden, 1982).The reason for this large-scale uplift remains unclear;Possibilities to explore include horizontal variations in sur-face heating and orography, both of which can inducemesoscale circulations. The few available observationssuggest that the scale of these updrafts during the transi-tion are larger than the turbulent scales of vertical transferduring the middle of the day (less than, or of the order of1 km).There appears to be a lack of agreement in the liter-ature on the evolution of the characteristic length scalesduring the late afternoon transition, partly due to the diffi-culty of observing and/or modelling this period. By usingLES, Nieuwstadt (1986) found that the wavelength of themaximum of spectral density of vertical velocity multipliedby frequency remains constant during the decay process.In contrast, Grant’s (Grant, 1997) observations showedthat this scale decreases during the LAT. Sorbjan (1997)found that small eddies had a tendency to decay earlierthan large eddies. These results were later confirmedby DNS (Shawn and Barnard, 2002). Pino et al. (2006)have shown that a characteristic length scale based on aweighted integral of the zonal wind component spectrum,can increase or decrease during the decay, depending onthe shear (Fig. 2).FIGURE 2: After Pino et al. (2006): Evolution of the scale ofthe wind zonal component during four different LESs. NS1 andNS5 have no-shear, SH1 and SH5 have a 1 ms−1 wind speedjump across the PBL top. NS5 and SH5 have a larger potentialtemperature jump.Thus the scale issue remains unclear and only partlyunderstood.1.3 TransportSpatial distributions of scalars such as gases andaerosols vary rapidly during the LAT, at least in the sur-face layer. Recent studies (e. g. Vila`-Guerau de Arellenoet al., 2004; Casso-Torralba et al., 2008) have shown thatmorning and afternoon transitions are indeed importantfor the vertical exchange of species. In the evening, watervapour and pollutants, which were emitted at the surfaceand diluted into the convective layer during the day canbe incorporated into the free troposphere as the resid-ual layer forms over the developing stably-stratified sur-face layer. As part of the free troposphere, they can thusbe transported horizontally over long distances. Verticalvariations in timing of the afternoon/evening reduction ofturbulence modifies the vertical rate of scalar transport(Fig. 3). There is a great amount of knowledge still to begained on this topic.FIGURE 3: Evolution of potential temperature and carbondioxide concentration during an entire day, at several levels ofthe instrumented tower of Cabauw, NE. Extracted and enlargedto cover the whole day evolution from Casso-Torralba (2008).2. FURTHER CHALLENGES2.1 Definitions and scalingDue to its transitional aspect, this phase puts severalbasic boundary layer definitions into question. The pe-riod that we are considering, which lasts 2 to 3 hours,starts as soon as the surface buoyancy flux begins tosharply decrease (late afternoon transition), and it coversthe change of sign of the surface buoyancy flux (eveningtransition). Within this context, the mixed layer, the resid-ual layer and the surface layer are non-stationary. Asa consequence: The surface layer cannot be defined inthe same way from the start to the end of this few-hourtransitory phase (from super- to sub- adiabatic surfacelayer). The mixed layer evolves from a well-mixed (adia-batic) layer with vigorous turbulence, to the decoupling ofa stable layer overlaid by a residual layer. The residuallayer during the night will be a weakly stable layer with in-termittent turbulence. The mixed and residual layers eachhave different interactions with the entrainment zone andcapping inversion above and the surface layer below.During the day, in convective conditions, most of themoments can usually be scaled with w∗. This scaling isthe basis for a robust parameterization in bulk models.On the other hand, the stable boundary layer scaling isusually based on the surface wind stress. During the LAT,the surface buoyancy fluxes are small, and other smallforcing processes come into play. So, neither the con-vective scaling, nor the stable boundary layer scaling aredominant.Other parameterizations are put into question duringthis phase, like that of entrainment. It is usually parame-terized through the ratio of the buoyancy flux at the ABLtop to the surface buoyancy flux. Because the latter be-comes very small during the LAT, this ratio is no longerrelevant then (Fig. 4).FIGURE 4: Ratio β of the buoyancy entrainment flux to sur-face flux as a function of the shear at the top inversion, duringseveral LESs made over daytime with various initial condition ofthe mean vertical structure of the CBL, and similar surface sen-sible heat flux. (Here, a sinusoidal law is used to represent thedaytime evolution of the surface flux.) β increases with increas-ing shear. Due to very weak fluxes at the end of the simulation,β starts to be an unreliable parameter for the study and parame-terization of entrainment.2.2 Observational issuesThere are also some issues raised when trying toprobe the boundary layer during the LAT. For example,weak and intermittent turbulence is difficult to measurewith any in situ (aircraft, towers) or remote sensing de-vice. Also if the turbulent characteristic length scales arelarger, they require larger samples to be well probed.Linked to the definition issues, there is also a chal-lenge in interpreting remote sensing observations dur-ing the LAT, either by UHF wind profiler or aerosol lidar,due to the change of the vertical distribution of the echosources.3. STRATEGYAs sensed from the state of the art drawn up previ-ously, the role of several key elements will be important tostudy, including: (1) entrainment, (2) surface heterogene-ity, (3) baroclinicity and advection, (4) clouds, (5) radia-tion, (6) gravity waves.A new field experiment is planned in June 2011 overtwo weeks, in the vicinity of the measurement tower sitedin the Observatorie des Midi Pyrenees in Lannemezan(France) and will combine aircraft, Unmanned Aerial Ve-hicles (UAVs), remote sensing instruments, radiosound-ings, tethered balloons, and tracer releases in the vicin-ity of a measurement tower. Several sonic anemometerswill be deployed over different surfaces in the surround-ing area, to measure the differences in the structure andevolution of the transition among different vegetated sur-faces. A network of UHF and sodar wind profilers will givecontinuous profiles of the mean wind and CBL depth, andDoppler lidar(s) will give the fine-scale structure of the ra-dial velocity component, with high resolution in time andspace for turbulence statistics studies and entrainmentzone exploration. The surface layer and entrainment zonewill be probed by one or two tethered balloons. Ra-diosoundings will be conducted throughout the day, withincreased density of launches during the LAT. Concur-rently, airplanes of various airspeeds and capacities willprobe an area of several tens of kilometers across, withhorizontal legs at different levels within and just above theCBL mainly for flux measurement. The results obtainedfrom analysis of previous as well as the BLLAST fielddataset will be combined with results from LES, direct nu-merical simulations (DNS), and mesoscale simulations inorder to better understand the processes, suggest newparameterizations, and evaluate forecast models duringthis transitional period.AcknowledgementsBLLAST has been supported by CNRS-INSU/LEFEand the University of Paul Sabatier. The research of D.Pino was partially funded by the Spanish Ministry of Sci-ence and Innovation (MICINN, CGL2009-08609).REFERENCESAcevedo, O. C., and Fitzjarrald, D. R.: 2001, TheEarly Evening Surface–Layer Transition: Temporal andSpatial Variability, J. Atmos. Sci., 58, 2650–2667.Casso-Torralba, P. J., J. Vila`-Guerau de Arellano,Bosveld.F., M. R. Soler, A. Vermeulen, C. Werner,and E. Moors, 2008: Diurnal and vertical variability ofthe sensible heat and carbon dioxide budgets in theatmospheric surface layer, J. Geophys. Res., 113,DOI:10.1029/2007JD009583.Cole, G. S. and H. J. S. Fernando, 1998: Some as-pects of the decay of convective turbulence, Fluid Dyn.Res., 23, 161-176.Edwards, J. M., R. J. Beare, and A. J. Lapworth,2006: Simulation of the observed evening transition andnocturnal boundary layers: single column modelling,Quart. J. Roy. Meteorol. Soc., 132, 61-80.Goulart, A., G. Degrazia, U. Rizza, and D. Anfossi,2003: A theoretical model for the study of convective tur-bulence decay and comparison with Large-Eddy Simula-tion data, Boundary-Layer Meteor., 107, 143-155.Grant, A. L. M., 1997: An observational study of theevening transition boundary-layer, Quart. J. Roy. Meteo-rol. Soc., 123, 657-677.Nieuwstadt, F. T. M. and R. A. Brost, 1986: The de-cay of convective turbulence, J. Atmos. Sci., 43, 532-546.Pagen, D. and B. Bryden, 1982: Understanding theSky, Sport Aviation Pubs., 200 pp.Pino, D., H. J. J. Jonker, J. Vila`-Guerau de Arellano,and A. Dosio, 2006: Role of shear and the inversionstrength during sunset turbulence over land: character-istic length scales, Boundary-Layer Meteorol., 121, 537-556.Shaw, W. J. and J. C. Barnard, 2002: Scales of turbu-lence decay from observations and direct numerical sim-ulations, Proc. of the 15th Symposium on Boundary Lay-ers and Turbulence, pp. 15-19 July 2002,Wageningen,The Netherlands.Sorbjan, Z., 1997: Decay of convective turbulencerevisited, Boundary-Layer Meteorol., 82, 501-515.Vila`-Guerau de Arellano, J., B. Gioli, F. Miglietta, H.Jonker, H. Klein Baltink, R. Hutjes, and A. Holtslag, 2004:Entrainment process of carbon dioxide in the atmosphericboundary layer, J. Geophys. Res., 109, 148-227.

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Studying the Boundary Layer Late Afternoon nd Sunset Turbulence (BLLAST)

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