De Roo, F.

Huq, S.

Mauder, M.

Zhang, S.

English

KITopen

Large-eddy simulationof the energy balance closurein fully heterogeneous terrainFrederik De Roo, Sadiq Huq, Sha Zhang & Matthias MauderKarlsruhe Institute of Technology – Campus Alpinfrederik.deroo@kit.edu — ID 247611 Poster nr. 39Energy balance closure problemEddy-covariance: H + LE < Rn −G• storage terms (e.g. canopy)• instrumental errors• missing flux in low-frequenciesStrong imbalance in complex terrain [5]→ quasi-stationary secondary circulationsThe classic LES-method for the imbalance [3]uses domain-averaging “[[·]]” and is limited toregular terrain. The imbalance is explained byadvection.Objective: investigate an alternative character-ization of the energy balance that can be ex-tended to virtual eddy-covariance tower mea-surements in heterogeneous terrain. The ques-tion of the near-surface imbalance has to awaittechnical advancements in the LES model [2].Virtual control volumesin fully heterogeneous terrainAdvantages:• Account for local advective fluxes• Imbalance around specific location• “Footprint” of the tower includedImpression of a control volume, from [1]Simulations for agricultural field site [4]without significant topographySimulation design• 12002 × 200 grid points• 12.5 m resolution• driven by variable surface heat fluxMethods• The classic computation of the energy imbal-ance in homogeneous terrain [3] is based on[[w̄T̄ ]] = [[w′sT′s]] − [[w′tT ′t ]] with the missing fluxfrom advection being equal to the total flux at themeasurement height minus the turbulent flux.• In our approach the advection is accountedfor by subtracting a base temperature from thevirtual measurements: [wT ]c = [wT ] (zm) −[w(zm)] [Tb]. The (corrected) turbulent flux isdetermined analogously: [w′T ′]c = [wT ]c −[w̄]([T̄ ]− [T̄b]). The base temperature is deter-mined from the horizontal advection into the con-trol volume, with “[·]” area-averaging.Comparison in homogeneous terrain• For an ensemble of scattered control volumes,the ensemble mean of [wT ]c is within 5 % of the[[w′sT′s]] of [3]. The turbulent fluxes are clearlysmaller.• For 1024 control volumes that cover the wholedomain, the domain-averaged corrected turbu-lent flux is within 10 % of the turbulent flux foundby [3], but further study is needed to clarify thedifferences between both methods.Virtual heat fluxes for tower 13Fluctuations of the instantaneous virtual heatflux at the measurement height (red). The avail-able energy (black) is taken as the surface heatflux. The storage term (blue) matters due to thesignificant measurement height. The model non-conservation residual (green) is close to zero butfluctuates more strongly if the area of the controlvolume is smaller.Energy balance for tower 13Standard turbulent flux [w]′[T ]′ (T) and storagechange (S)Corrected turbulent flux [w′T ′]c (T) and storagechange (S)Conclusions• To suppress fluctuations in ∆, the control volumes have to comprise a minimal amount of grid points: in the horizontal about 102. Yet the area may not becometoo large, because for each c.v. area-averaged quantities are being calculated.• The standard turbulent term is rather small, which could be caused by an underestimation of the high-frequency turbulent flux (related to the grid length), orby a suppression of fluctuations due to the area-averaging over the upper lid of the control volume. We conclude that a separate virtual measurement of theturbulent flux should be added. Then the primary use of the control volume becomes to clarify the cause of the imbalance.References[1] J. J. Finnigan et al. A re-evaluation of long-term flux measurement techniques Part I. Boundary-LayerMeteorology, 107:148, 2003.[2] S. Huq, F. De Roo, S. Raasch, and M. Mauder, 2014. Presentation at AMS Symposium Leeds.[3] M. Kanda, A. Inagaki, M. O. Letzel, S. Raasch, and T. Watanabe. LES study of the energy imbalanceproblem with eddy covariance fluxes. Bound-Lay Meteorol, 110(3):381–404, 2004.[4] M. Mauder, R. L. Desjardins, E. Pattey, Z. Gao, and R. van Haarlem. Measurement of the sensible eddy heatflux based on spatial averaging of continuous ground-based observations. Bound-Lay Meteorol, 128:151–172, 2008.[5] P. C. Stoy et al. A data-driven analysis of energy balance closure across FLUXNET research sites: Therole of landscape-scale heterogeneity. Agr. Forest Meteorol., 171-172:137–152, 2013.AcknowledgementsWe thank the PALM group at Leibniz University Hannover for their open-source PALM code. This work was con-ducted within the Helmholtz Young Investigator Group “Capturing all relevant scales of biosphere-atmosphereexchange — the enigmatic energy balance closure problem”, which is funded by the Helmholtz-Associationthrough the President’s Initiative and Networking Fund, and by KIT.

Large-eddy simulation of the energy balance closure in fully heterogeneous terrain

https://publikationen.bibliothek.kit.edu/220098435/3816727

Large-eddy simulation of the energy balance closure in fully heterogeneous terrain

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