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The BLLAST field experiment: Boundary-Layer Late Afternoon and Sunset Turbulence
Lothon, M. ; Lohou, F. ; Pino, D. ; Vilà-Guerau De Arellano, J. ; Hartogensis, O.K. ; Boer, A. van de; Coster, O. de; Moene, A.F. ; Steeneveld, G.J. - \ 2014
Atmospheric Chemistry and Physics 14 (2014). - ISSN 1680-7316 - p. 10931 - 10960.
large-eddy-simulation - observed evening transition - doppler spectral width - low-level jets - convective turbulence - intermittent turbulence - surface-layer - kinetic-energy - drainage flow - length scales
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.
Sensitivity and uncertainty of analytical footprint models according to a combined natural tracer and ensemble approach
Boer, A. van de; Moene, A.F. ; Schüttemeyer, D. ; Graf, A. - \ 2013
Agricultural and Forest Meteorology 169 (2013). - ISSN 0168-1923 - p. 1 - 11.
flux measurements - sonic anemometer - carbon-dioxide - surface-layer - water-vapor - exchange - heat - validation - efflux - fetch
Evaluations of analytical footprint models using data from several stations located in different land use types are still scarce, but valuable for defining the spatial context of the measurements. Therefore, we evaluated two analytical footprint models by applying a ‘forward’ and an ‘inversion’ method. We used eddy covariance measurements from a flat agricultural landscape in western Germany in the summer of 2009, with seven eddy covariance systems over three different land use types with contrasting sensible heat fluxes. We found that the model of Hsieh et al. (2000. Adv. Water Resour. 23, 765–772) and of Kormann and Meixner (2001. Boundary Layer Meteorol. 99, 207–224) are both overestimating the distance of the peak contribution of the footprint. In our evaluation, the former model performs slightly better, independent of whether the crosswind dispersion was used from the latter model, or from the proposed model by Detto et al. (2006. Water Resour. Res. 42, 1–16).
Measuring H2O and CO2 fluxes at field scales with scintillometry: Part I – Introduction and validation of four methods
Kesteren, A.J.H. van; Hartogensis, O.K. ; Dinther, D. van; Moene, A.F. ; Bruin, H.A.R. de - \ 2013
Agricultural and Forest Meteorology 178-179 (2013). - ISSN 0168-1923 - p. 75 - 87.
small aperture scintillometer - monin-obukhov similarity - temperature-humidity correlation - energy-balance closure - stable boundary-layer - surface-layer - water-vapor - sensible heat - structure parameter - heterogeneous surface
This study introduces four methods for determining turbulent water vapour and carbon dioxide flux densities, the evapotranspiration and CO2 flux respectively. These methods combine scintillometer measurements with point-sampling measurements of scalar quantities and consequently have a faster statistical convergence than the eddy-covariance method. The scintillometer measures the friction velocity and stability averaged over space, allowing the time averaging to be a minute or less in homogenous conditions. This paper aims to thoroughly test the methods by analysing their sensitivity to the variables that go into the method and validate the methods with 30-min eddy-covariance data. Introduced are: the Bowen-variance method, the flux-variance method, the structure-parameter method, and the energy-balance method. Sensitivity analysis shows that each method is sensitive to the turbulence measurements of the scalar quantities that are specific to the method, as well as to the friction velocity. This demonstrates that the accuracy of the flux results from a correct representation of the turbulence variables used by the methods. Furthermore, a 30-min flux validation shows that the methods compare well to the independent eddy-covariance fluxes. We found that the structure-parameter method performs best – a low scatter (the correlation coefficient, r = 0.99) and a 5% underestimation were observed. Also the other methods perform well, although the energy-balance did not close, because storage terms and CO2 flux were neglected. Furthermore, during the night the variance methods were influenced by non-stationarity in the measurement signal. Finally, we suggest using the correlation coefficients between temperature and scalar quantities to acquire the sign of the fluxes. Data for this study were gathered in May–June 2009 over a wheat field near Merken, Germany, in the framework of the TransRegio32 program.
The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer
Wiel, B.J.H. van de; Moene, A.F. ; Jonker, H.J.J. - \ 2012
Journal of the Atmospheric Sciences 69 (2012)1. - ISSN 0022-4928 - p. 3097 - 3115.
large-eddy simulations - temperature-fluctuation method - low-level jet - intermittent turbulence - atmospheric models - surface-layer - land-surface - part i - regimes - cases-99
The mechanism behind the collapse of turbulence in the evening as a precursor to the onset of the very stable boundary layer is investigated. To this end a cooled, pressure-driven flow is investigated by means of a local similarity model. Simulations reveal a temporary collapse of turbulence whenever the surface heat extraction, expressed in its nondimensional form h/L, exceeds a critical value. As any temporary reduction of turbulent friction is followed by flow acceleration, the long-term state is unconditionally turbulent. In contrast, the temporary cessation of turbulence, which may actually last for several hours in the nocturnal boundary layer, can be understood from the fact that the time scale for boundary layer diffusion is much smaller than the time scale for flow acceleration. This limits the available momentum that can be used for downward heat transport. In case the surface heat extraction exceeds the so-called maximum sustainable heat flux (MSHF), the near-surface inversion rapidly increases. Finally, turbulent activity is largely suppressed by the intense density stratification that supports the emergence of a different, calmer boundary layer regime.
Comments on “An extremum solution of the Monin-Obukhov similarity equations"
Wiel, B.J.H. van de; Basu, S. ; Moene, A.F. ; Jonker, H.J.J. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2011
Journal of the Atmospheric Sciences 68 (2011)6. - ISSN 0022-4928 - p. 1405 - 1408.
atmospheric boundary-layer - flux-profile relationships - temperature-fluctuation method - stable conditions - surface-layer - models - turbulence
Exploring the possible role of small scale terrain drag on stable boundary layers over land
Steeneveld, G.J. ; Holtslag, A.A.M. ; Nappo, C.J. ; Wiel, B.J.H. van de; Mahrt, L. - \ 2008
Journal of Applied Meteorology and Climatology 47 (2008). - ISSN 1558-8424 - p. 2518 - 2530.
atmospheric gravity-waves - complex terrain - critical-level - surface-layer - general-circulation - contrasting nights - turbulent-flow - form drag - model - parameterization
This paper addresses the possible role of unresolved terrain drag, relative to the turbulent drag on the development of the stable atmospheric boundary layer over land. Adding a first-order estimate for terrain drag to the turbulent drag appears to provide drag that is similar to the enhanced turbulent drag obtained with the so-called long-tail mixing functions. These functions are currently used in many operational models for weather and climate, although they lack a clear physical basis. Consequently, a simple and practical quasi-empirical parameterization of terrain drag divergence for use in large-scale models is proposed and is tested in a column mode. As an outcome, the cross-isobaric mass flow (a measure for cyclone filling) with the new scheme, using realistic turbulent drag, appears to be equal to what is found with the unphysical long-tail scheme. At the same time, the new scheme produces a much more realistic less-deep boundary layer than is obtained by using the long-tail mixing function.
Wind profiles, momentum fluxes and roughness lengths at Cabauw revisited
Verkaik, J.W. ; Holtslag, A.A.M. - \ 2007
Boundary-Layer Meteorology 122 (2007)3. - ISSN 0006-8314 - p. 701 - 719.
grenslaagmeteorologie - windsnelheid - fluctuaties - experimenten - boundary-layer meteorology - wind speed - fluctuations - experiments - atmospheric boundary-layer - surface-layer - complex terrain - land-surface - parameterization - similarity - turbulence - footprint - location - exchange
We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (<40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.
Exploring Self-Correlation in Flux-Gradient Relationships for Stably Stratified Conditions
Baas, P. ; Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2006
Journal of the Atmospheric Sciences 63 (2006). - ISSN 0022-4928 - p. 3045 - 3054.
atmospheric boundary-layer - monin-obukhov similarity - surface-layer - convective conditions - turbulence - temperature - land - profiles - cases-99 - validity
In this paper, the degree of scatter in flux-gradient relationships for stably stratified conditions is analyzed. It is generally found that scatter in the dimensionless lapse rate phi(n) is larger than in the dimensionless shear phi(m) when plotted versus the stability parameter z/Lambda (where Lambda is the local Obukhov length). Here, this phenomenon is explained to be a result of self-correlation due to the occurrence of the momentum and the heat flux on both axes, measurement uncertainties, and other possibly relevant physical processes left aside. It is shown that the ratio between relative errors in the turbulent fluxes influences the orientation of self-correlation in the flux-gradient relationships. In stable conditions, the scatter in phi(m) is largely suppressed by self-correlation while for phi(n) this is not the case (vice versa for unstable stratification). An alternative way of plotting is discussed for determining the slope of the linear phi(m) function.
Regional Advection Perturbations in an Irrigated Desert (RAPID) Experiment
Debruin, H.A.R. ; Hartogensis, O.K. ; Allen, R.G. ; Kramer, J.W.J.L. - \ 2005
Theoretical and Applied Climatology 80 (2005)2-4. - ISSN 0177-798X - p. 143 - 152.
surface-layer - land
The RAPID field experiment took place in August - September 1999 at a site 25km south of Twin Falls, Idaho, USA. The experiment concerned micrometeorological observations over extensive, well-irrigated fields covered with the fast-growing crop alfalfa. During daytime, on a number of days the sensible heat flux was negative and the latent heat flux exceeded net radiation. The energy required for the latent heat flux to be larger than net radiation has to be advected from elsewhere. As the fields were large, we refer to this process as "regional advection". "Local advection", on the other hand, refers to advection effects, where the wet to dry transition is on a field scale. Evidence is presented that the RAPID data are subject to regional advection conditions. A simple model, based on Penman-Monteith, is derived that describes the regional-advection case rather well. The influence of wind speed under those conditions is illustrated using data and the model. The correlation coefficients between temperature and horizontal wind component appear to be good indicators for advection
Structure parameters for temperature and humidity from simultaneous eddy-covariance and scintillometer measurements
Beyrich, F. ; Kouznetsov, R. ; Leps, J.P. ; Lüdi, A. ; Meijninger, W.M.L. ; Weisensee, U. - \ 2005
Meteorologische Zeitschrift 14 (2005)5. - ISSN 0941-2948 - p. 641 - 649.
large-aperture scintillometer - flevoland field experiment - refractive-index - surface-layer - heterogeneous surface - land-surface - fluxes - scintillation - heat - turbulence
Line-averaged values of the structure parameters of temperature and humidity, CT2 and Cq2, were estimated from simultaneous measurements with an optical and a microwave scintillometer over a path of 4.7 km length at the Meteorological Observatory Lindenberg during the LITFASS-2003 experiment. By cross-correlating the detected signals of the two scintillometers, the temperature-humidity structure parameter, CTq, and the temperature-humidity correlation, rTq, were also derived directly from the measurements. Comparison with corresponding values obtained from local measurements with an eddy-covariance system on a meteorological tower show a consistent behaviour in time (with some exceptions especially for Cq2 and rTq during nighttime). The deviations are of a magnitude between 20 % and 35 % of the typical daytime values of the structure parameters.
The dispersion of chemically reactive species in the atmospheric boundary layer
Vilà-Guerau de Arellano, J. ; Dosio, A. ; Vinuesa, J. ; Holtslag, A.A.M. ; Galmarini, S. - \ 2004
Meteorology and Atmospheric Physics 87 (2004)1-3. - ISSN 0177-7971 - p. 23 - 38.
large-eddy simulation - surface-layer - concentration fluctuations - microscale turbulence - general-theory - fluxes - plume - chemistry - model - formulation
The role of turbulence in the dispersion of atmospheric pollutants that react with linear (decay) and nonlinear (second-order) chemical reactions is examined. The most relevant processes that drive the reactivity of species emitted in a surface area or released by a point source are studied by deriving the dimensionless scaling numbers from equations for the atmospheric turbulent reacting flow. The first number is the ratio of the time scale of turbulence to the time scale of the chemical reaction, namely the Damkohler number. The second number is the ratio of the concentrations of the species present in the chemical transformations. In this paper, model results and experimental studies of turbulent reacting flows in the atmospheric boundary layer are presented to show the modifications and control exerted by turbulence on the atmospheric chemistry as a function of these numbers and processes. We also discuss how the chemical transformation is affected when species are in a state of chemical equilibrium. By studying the plume dispersion of a reactant, that decays with a simple chemical reaction, one can analyse the dependence of concentration fluctuations on the Damkohler number. The study is extended to plumes that react nonlinearly. In such reacting systems, the large gradients and segregation of species result in a significant reduction in the reaction rates. Because of this modification, the chemistry of species related to NOx and HOx can be very different from the chemistry in conditions where the species are uniformly mixed. The lack of complete observational evidence is hampering our understanding of these processes and our evaluation of numerical modelling results. Finally, we discuss briefly how to represent, in the form of a parameterization, the effect that turbulence can have on the reactivity of species emitted by a point source or an area source.
Intermittent turbulence in the stable boundary layer over land. Part III. A classification for observations during CASES-99
Wiel, B.J.H. van de; Moene, A.F. ; Hartogensis, O.K. ; Bruin, H.A.R. de; Holtslag, A.A.M. - \ 2003
Journal of the Atmospheric Sciences 60 (2003)20. - ISSN 0022-4928 - p. 2509 - 2522.
surface-layer - oscillations - temperature - stability - model
In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere¿Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantitative features of the theoretical classification are, however, rather sensitive to (often uncertain) local parameter estimations, such as the bulk heat conductance of the vegetation layer. This sensitivity limits the current applicability of the theoretical classification in a strict quantitative sense, apart from its conceptual value
Evapotranspiration fluxes over mixed vegetation areas measured from large aperture scintillometer
Hemakumara, H.M. ; Chandrapala, L. ; Moene, A.F. - \ 2003
Agricultural Water Management 58 (2003). - ISSN 0378-3774 - p. 109 - 122.
surface-layer - sensible heat - scintillation - index
Routine measurement of spatially averaged surface fluxes of sensible heat (H) in river basins is now feasible. These fluxes, when combined with net radiation estimates, can be used to derive areally averaged actual evapotranspiration (ET). The scintillation method is shown to be promising method for estimating areally averaged sensible heat fluxes. The large aperture scintillometer (LAS) is an optical device used to monitor fluctuations in refractive index of the turbulent atmosphere over a relatively large area. The study reported here has estimated ET fluxes for an area of mixed vegetation at Horana, a field site about 40 km southeast of Colombo, Sri Lanka. ET estimates derived from the scintillometer and net radiometer were compared with estimates obtained from a remote sensing based surface energy balance algorithm for land (SEBAL). The SEBAL estimating of ET were derived using NOAA satellite images without any a priori calibration. The average deviation of ET estimates between SEBAL and LAS for 10-day periods was 17%. However, this deviation fell to 1% when monthly estimates were considered. This suggests regional ET values derived from remote sensing are reasonable estimates, however, the LAS was used in only one agro-ecosystem to validate the SEBAL model. The LAS is a low cost alternative to other methods of estimating heat fluxes for use in basin scale water use studies. (C) 2002 Elsevier Science B.V. All rights reserved.
Bridging the gap between atmospheric physics and chemistry in studies of small-scale turbulence
Vilà-Guerau de Arellano, J. - \ 2003
Bulletin of the American Meteorological Society 84 (2003)1. - ISSN 0003-0007 - p. 51 - 56.
convective boundary-layer - large-eddy simulation - surface-layer - chemical-reactions - fluxes - nitrogen - plume - ozone - deposition - diffusion
The current understanding of the influence of atmospheric turbulence on chemical reactions is briefly reviewed. The fundamentals of this influence and the consequences for the transport and mixing of the reactants are discussed. A classification of the turbulent reacting flows is proposed in terms of the values of dimensionless numbers. These numbers depend on the characteristic timescale of the dynamics and the chemistry. The main findings obtained by modeling studies of various atmospheric boundary layer flows and chemical mechanisms, in particular the ones done by means of the large eddy simulation technique, are summarized. Based on the conclusions of these studies, the need to carry out intensive and comprehensive atmospheric field campaigns and laboratory experiments to corroborate the numerical results is discussed. Specific open questions are posed to improve, by combining observational experiments and modeling, our knowledge of the role played by physical processes on the transformations of reactive species in the atmospheric boundary layer