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The Energy Balance Experiment EBEX-2000. Part I: Overview and Energy Balance

The Energy Balance Experiment EBEX-2000. Part I: Overview and Energy Balance
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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/51995974 The Energy Balance Experiment EBEX-2000.Part I: Overview and energy balance  Article   in  Boundary-Layer Meteorology · January 2007 DOI: 10.1007/s10546-007-9161-1 CITATIONS 141 READS 104 17 authors , including: Some of the authors of this publication are also working on these related projects: ScaleX   View projectSpatially resolved quantification of the advection influence on the balance closure of greenhousegases   View projectEva van GorselAustralian National University 92   PUBLICATIONS   1,809   CITATIONS   SEE PROFILE Christian FeigenwinterUniversity of Basel 66   PUBLICATIONS   1,412   CITATIONS   SEE PROFILE Irene LehnerLund University 25   PUBLICATIONS   1,449   CITATIONS   SEE PROFILE Andrea PitaccoUniversity of Padova 66   PUBLICATIONS   593   CITATIONS   SEE PROFILE All content following this page was uploaded by Andrea Pitacco on 10 January 2017. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the srcinal documentand are linked to publications on ResearchGate, letting you access and read them immediately.  Boundary-Layer Meteorol (2007) 123:1–28DOI 10.1007/s10546-007-9161-1ORIGINAL PAPER The Energy Balance Experiment EBEX-2000.Part I: overview and energy balance Steven P. Oncley  ·  Thomas Foken  ·  Roland Vogt  · Wim Kohsiek  ·  H. A. R. DeBruin  · Christian Bernhofer  ·  Andreas Christen  ·  Eva vanGorsel  ·  David Grantz  ·  Christian Feigenwinter  · Irene Lehner  ·  Claudia Liebethal  ·  Heping Liu  · Matthias Mauder  ·  Andrea Pitacco  ·  Luis Ribeiro  · Tamas Weidinger Received: 13 December 2005 / Accepted: 22 January 2007 / Published online: 14 March 2007© Springer Science+Business Media B.V. 2007 Abstract  An overview of the Energy Balance Experiment (EBEX-2000) is given.This experiment studied the ability of state-of-the-art measurements to close the sur-faceenergybalanceoverasurface(avegetativecanopywithlargeevapotranspiration)where closure has been difficult to obtain. A flood-irrigated cotton field over uniformterrain was used, though aerial imagery and direct flux measurements showed thatthe surface still was inhomogeneous. All major terms of the surface energy balancewere measured at nine sites to characterize the spatial variability across the field.Included in these observations was an estimate of heat storage in the plant canopy.The resultant imbalance still was 10%, which exceeds the estimated measurementerror. We speculate that horizontal advection in the layer between the canopy topand our flux measurement height may cause this imbalance, though our estimates of  The National Center for Atmospheric Research is supported by the National Science Foundation.S. P. Oncley ( B )National Center for Atmospheric Research/ATD,P.O. Box 3000, Boulder, CO 80307-3000, USAe-mail: oncley@ucar.eduT. FokenUniversity of Bayreuth,Bayreuth, GermanyR. Vogt  ·  A. Christen  ·  C. Feigenwinter  ·  I. LehnerUniversity of Basel,Basel, SwitzerlandW. KohsiekRoyal Netherlands Meteorological Institute (KNMI),De Bilt, The NetherlandsH. A. R. DeBruinMeteorology and Air Quality Group, Wageningen University and Research Center,Wageningen, The Netherlands  2 S. P. Oncley et al. thistermusingourmeasurementsresultedinvalueslessthanwhatwouldberequiredto balance the budget. Keywords  Flux divergence · Latent heat flux · Spatial sampling · Sensible heat flux · Soil heating  ·  Surface energy budget 1 Introduction Energy must be conserved at the earth’s surface, a fundamental fact that is used inall weather and climate models. The major components of the surface energy budgetare net radiation  R net   (in both the visible and infrared part of the spectrum), sensibleheatflux H   (exchangeofheatbetweenthesurfaceandtheatmospherebyconductionand convection), latent heat flux  LE  (evaporation of water from the surface, whereL is the latent heat of vaporization), and heating  G  of materials on the surface (soil,plants, water, etc.), with a small fraction converted to chemical energy when plantsare present. Thus, R net   =  H   + LE + G . (1) C. BernhoferDresden University of Technology,Dresden, GermanyE. van GorselCSIRO,Canberra, AustraliaD. GrantzUniversity of California, Kearney Research Center,Parlier, CA, USAC. Liebethale-fellows.netMunich, GermanyH. LiuJackson State University,Jackson, MS, USAM. MauderAgriculture and Agri-FoodOttawa, Ont., CanadaA. PitaccoUniversity of Padova,Padova, ItalyL. RibeiroBragança Polytechnic Institute,Bragança, PortugalT. WeidingerEötvös Loránd University,Budapest, Hungary  The energy balance experiment EBEX-2000 3 Table 1  Recent energy balance observationsExperiment Reference Residual (%) Duration (days) SurfaceFIFE-89 Verma et al. (1992) 0–10 40 grass Vancouver Island-90 Lee and Black (1993) 17 9 16m deciduous forest TARTEX-90 Foken et al. (1993) 33 24 barley/bare soil KUREX-91 Panin et al. (1998) 38 27 agricultural LINEX-96/2 Foken et al. (1997) 21 3 medium grass LINEX-97/1 Foken (1998) 32 9 short grass LITFASS-98 Beyrich et al. (2002) 14 21 bare soil FIFE is the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experi-ment,TARTEXistheTartuExperiment,LINEXistheLindenbergExperiment,andLITFASSistheLindenberg Inhomogeneous Terrain – Fluxes between Atmosphere and Surface: a long-term study Several early measurements of the energy balance terms failed to achieve clo-sure (Elagina et al. 1973, 1978; Tsvang et al. 1987). The problem was more evident during the land surface experiments at the end of the 1980s such as the First Interna-tionalSatelliteLandSurfaceClimatologyProject(ISLSCP)FieldExperiment(FIFE)(Kanemasu et al. 1992). While the cause was first thought to be in the eddy-covari-ancemeasurements,laterstudiesdiscussedproblemswiththeradiationmeasurements(Koitzsch et al. 1988) and the storage in the canopy and the upper soil layer (Foken 1990; Bolle et al. 1993; Braud et al. 1993). The lack of closure of the energy balance in the land surface experiments given in Table 1 was interpreted by Foken (1998) as an effect of the fractional coverage of vegetation and the influence of the soil heatstorage. The imbalance was attributed by Panin et al. (1998) to the influence of heter- ogeneities in the surrounding area and by Twine et al. (2000) to unknown problems with the sensible and/or latent heat flux measurements. Finnigan et al. (2003) focused on the problem of inadequate temporal averaging (which would lose low-frequencycontributions)fortheturbulentfluxes,perhapsrelatedtonon-homogeneoussurfaces.It also is conceivable that a physical process exists that has not been included inEq. (1); however we only find one (Sect. 6.3 below) and show that it is negligible.The lack of energy balance closure has been found over all types of surfaces frombare soil to forests and the vast majority have found higher energy input by radiationfluxes than loss by turbulent fluxes and ground heat flux. Several overview papersare now available (Laubach and Teichmann 1996; Foken 1998; Aubinet et al. 2000; Wilson et al. 2002; Culf et al. 2004) that make the problem well-known. Energy bal- anceclosurehasbeenusedtocharacterizethequalityofeddy-covariancebasedfluxes,although other factors such as the choice of coordinate system, the footprints of eachof the budget terms, and mesoscale transport can influence the closure (Foken et al.2004).The error that is found is much larger than is usually expected for the measure-ments of any of the individual terms. It is important to determine the reasons for thisdeficit. Otherwise, it is impossible to use experimental data for, say, the evaluationof subgrid parameterizations of the soil-vegetation-atmosphere transfer schemes inclimate numerical models, because it is not known which components of the energybalance may be in error.The primary objective of the Energy Balance EXperiment (EBEX) was to deter-mine why measurements often cannot achieve closure. EBEX was the direct resultof a European Geophysical Society workshop (Foken and Oncley 1995), which listed  4 S. P. Oncley et al. bothinstrumentationandfundamentalproblemsinclosingtheenergybudget.EBEXaddressed these problems by:1. Studying a surface for which energy balance closure has been difficult to obtain,but still relatively easy to instrument—a closed canopy with high evapotranspira-tion.2. Measuring all terms of the energy budget directly at comparable scales. In par-ticular, deploying enough sensors to create an average of each term over an arealarge enough to encompass several flux “footprints”.3. Performing side-by-side intercomparisons of instruments from different manu-facturers.4. Comparing processing methods of different research groups, including filteringand flow distortion corrections in the eddy-covariance measurements, using areference dataset.In addition, temperature and wind profiles were measured at three locations to pro-vide information on site homogeneity—in particular the presence or lack of internalboundary layers over the site. It was not expected that point 2 above would be theprimarysourceofthesystematicimbalanceobservedinthepast,sincesamplingcouldcause  H   + LE + G  to be either larger or smaller than  R net  . 2 Site description We selected a flood-irrigated cotton field in the San Joaquin Valley of Californiasince the typically cloud-free skies resulted in quite high evapotranspiration, withmaximum values over 600Wm − 2 . The site was a field 1,600m × 800m approximately20km south-south-west of the town of Lemoore. The local overall topography wasquite flat with a slope of 0.1degree. The canopy grew during the project and alsovaried spatially, but was generally 1m high; the canopy closed at all but sites 9 and 10during the measurement period.Winds were quite steady from the north-north-west at upper levels, as shown inFig. 1, due to channelling of the synoptic flow by mountains surrounding the Val- ley. Figure 2 shows that these north-north-west winds are perfectly aligned with thetopography.Thenear-surfacewindsalsocamefromthenorth-eastduringthedayandfrom the west at night, probably due to drainage/upslope flows through CottonwoodPass, about 30km to the south-west. The influence of topography on the air flow inthis area is described in Tanrikulu et al. (2000). Most flux measurements were made at 4.7m above the ground (about 4m abovethe estimated displacement height) and thus had a fetch (producing 90% of the fluxfor slightly unstable conditions) of about 400m (Horst and Weil 1994). The layout of  the ten tower sites (Fig. 3) with tower spacing of 200m was chosen to have this foot- print totally within the cotton field and to have overlapping footprints from adjacenttowers to identify any sections of the field with significantly different fluxes. The rela-tiveorientationofthesetowerswassettotheexpectedmeanwinddirection(fromthenorth-north-west)sothatthesefootprintswouldoverlap.Althoughinternalboundarylayers were seen, temperature (and to a lesser extent, wind speed) profiles measuredat sites 7, 8, and 9 indicated that the measuring level for fluxes was above this layer atmost of the stations (except stations 3, 6, 9 for easterly winds).
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