An Investigation of the Eddy-Covariance Flux Imbalance in a Year-Long Large-Eddy Simulation of the Weather at Cabauw

Schalkwijk, J. C.; Jonker, Harm J. J.; Siebesma, A. Pier

doi:10.1007/s10546-016-0138-9

Cited by 17 publications

(20 citation statements)

References 37 publications

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“…This result was reproduced by Eder et al (2015b) by means of a study combining Doppler wind lidar and EC tower data. The same correlation has also been noticed in a recent year-long large-eddy simulation (LES) by Schalkwijk et al (2016) and in an idealized LES study by Inagaki et al (2006). In addition, the study of Eder et al (2015b) could relate the energy balance residual to the mean gradients in the lower boundary-layer, thereby providing more evidence for the connection between the energy imbalance and the presence of quasi-stationary structures in the boundary layer.…”

Section: The Role Of Landscape Heterogeneity In the Energy Balance CLsupporting

confidence: 74%

“…This means that our findings for virtual EC towers cannot be directly transferred to real eddycovariance towers. Other LES studies of the energy balance closure point towards a larger imbalance at higher z-levels, e.g., Steinfeld et al (2007), Huang et al (2008), andSchalkwijk et al (2016). It remains an open question if we can scale the measurement height (as long as it is in the constant flux layer) with the boundary-layer depth and the scale of the heterogeneity.…”

Section: Virtual Tower Measurements For Landscape Heterogeneity Of Kimentioning

confidence: 97%

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The influence of idealized surface heterogeneity on virtual turbulent flux measurements

Roo

Mauder

2018

Atmos. Chem. Phys.

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Abstract. The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity, which affects the boundary-layer turbulence. To investigate the impact of surface variables on the partitioning of the energy budget of flux measurements in the surface layer under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, which allows the determination of advection by the mean flow, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. We focus on the heterogeneity of the surface fluxes and keep the topography flat. The surface fluxes vary locally in intensity and these patches have different length scales. Intensity and length scales can vary for the two horizontal dimensions but follow an idealized chessboard pattern. Our main focus lies on surface heterogeneity of the kilometer scale, and one order of magnitude smaller. For these two length scales, we investigate the average response of the fluxes at a number of virtual towers, when varying the heterogeneity length within the length scale and when varying the contrast between the different patches. For each simulation, virtual measurement towers were positioned at functionally different positions (e.g., downdraft region, updraft region, at border between domains, etc.). As the storage term is always small, the non-closure is given by the sum of the advection by the mean flow and the flux-divergence. Remarkably, the missing flux can be described by either the advection by the mean flow or the flux-divergence separately, because the latter two have a high correlation with each other. For kilometer scale heterogeneity, we notice a clear dependence of the updrafts and downdrafts on the surface heterogeneity and likewise we also see a dependence of the energy partitioning on the tower location. For the hectometer scale, we do not notice such a clear dependence. Finally, we seek correlators for the energy balance ratio in the simulations. The correlation with the friction velocity is less pronounced than previously found, but this is likely due to our concentration on effectively strongly to freely convective conditions.

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Section: The Role Of Landscape Heterogeneity In the Energy Balance CLsupporting

confidence: 74%

Section: Virtual Tower Measurements For Landscape Heterogeneity Of Kimentioning

confidence: 97%

The influence of idealized surface heterogeneity on virtual turbulent flux measurements

Roo

Mauder

2018

Atmos. Chem. Phys.

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Section: The Role Of Landscape Heterogeneity In the Energy Balance CLsupporting

confidence: 74%

Reply to reviewer 2

Roo¹

2017

Preprint

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The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity, which affects the boundary-layer turbulence. To investigate the impact of surface variables on the partitioning of the energy budget of flux measurements in the surface layer under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, which allows the determination of advection by the mean flow, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. We focus on the heterogeneity of the surface fluxes and keep the topography flat. The surface fluxes vary locally in intensity and these patches have different length scales. Intensity and length scales can vary for the two horizontal dimensions but follow an idealized chessboard pattern. Our main focus lies on surface heterogeneity of the kilometer scale, and one order of magnitude smaller. For these two length scales, we investigate the average response of the fluxes at a number of virtual towers, when varying the heterogeneity length within the length scale and when varying the contrast between the different patches. For each simulation, virtual measurement towers were positioned at functionally different positions (e.g., downdraft region, updraft region, at border between domains, etc.). As the storage term is always small, the non-closure is given by the sum of the advection by the mean flow and the flux-divergence. Remarkably, the missing flux can be described by either the advection by the mean flow or the flux-divergence separately, because the latter two have a high correlation with each other. For kilometer scale heterogeneity, we notice a clear dependence of the updrafts and downdrafts on the surface heterogeneity and likewise we also see a dependence of the energy partitioning on the tower location. For the hectometer scale, we do not notice such a clear dependence. Finally, we seek correlators for the energy balance ratio in the simulations. The correlation with the friction velocity is less pronounced than previously found, but this is likely due to our concentration on effectively strongly to freely convective conditions.

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“…These large‐scale turbulent eddies and secondary circulations may not be adequately sampled by an EC tower in a finite averaging period (e.g., 30 min); hence, an imbalance occurs. This phenomenon has been confirmed by many observational (Eder, Schmidt, et al, ; Gao et al, ; Panin et al, ; Stoy et al, ; Xu et al, ) and large‐eddy simulation (LES) studies (Eder, De Roo, et al, ; De Roo & Mauder, ; Huang et al, ; Inagaki et al, ; Kanda et al, ; Schalkwijk et al, ; Steinfeld et al, ; Zhou et al, ).…”

Section: Introductionmentioning

confidence: 68%

“…It should be pointed out that in some previous studies (Huang et al, ; Inagaki et al, ; Kanda et al, ; Schalkwijk et al, ; Steinfeld et al, ; Zhou et al, ), the spatiotemporally averaged vertical heat flux (

([], \overset{true¯}{italicwθ})

) at a given height is used as the true flux. This method is often used over homogeneous surfaces (e.g., Huang et al, ; Kanda et al, ; Schalkwijk et al, ; Steinfeld et al, ; Zhou et al, ) and, to a lesser extent, 1‐D heterogeneous surfaces (Inagaki et al, ; Zhou et al, ). When the spatially averaged vertical velocity [ w ] is zero, which is typical for LES runs with periodic boundary conditions, the spatiotemporally averaged vertical heat flux can be further estimated by the spatial turbulent heat flux

([], \overset{true¯}{w^{''} θ^{''}}) .

However, the use of spatiotemporally averaged vertical heat flux (

([], \overset{true¯}{italicwθ})

) as the true flux over heterogeneous surfaces is complicated by the existence of mean vertical motions (Zhou et al, ).…”

Section: Methodsmentioning

confidence: 99%

The Effects of Surface Heterogeneity Scale on the Flux Imbalance under Free Convection

Zhou

2019

JGR Atmospheres

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It is well known that the available energy (i.e., the net radiation minus the ground heat flux) is often 10-30% larger than the sum of turbulent fluxes measured by the eddy-covariance method. Although field observations and previous large-eddy simulation studies have shown that surface heterogeneity can induce flux imbalance, the relationship between the flux imbalance magnitude and the surface heterogeneity scale remains to be investigated in more detail. Here we examine the flux imbalance over landscapes characterized by different surface heterogeneity scales in a dry freely convective boundary layer. We reveal that the flux imbalance initially increases with increasing surface heterogeneity scale. However, when the surface heterogeneity scale becomes larger than the boundary layer height, the surface starts to behave locally homogeneous, which leads to a lower flux imbalance. Based on large-eddy simulation results, we propose a conceptual model to explain how the domain average flux imbalance is influenced by surface heterogeneity. The flux imbalance is found to be controlled by the ratio of the boundary layer height to the Obukhov length (−z i /L), the integral length scale of vertical velocity (l w ), the mean horizontal speed (U), and the time averaging interval (T). Among these four variables, l w determines the size of turbulent coherent structures (i.e., large eddies), whereas −z i /L affects the form of these large eddies. Meanwhile, the U and T determine how many these large eddies can be sampled by the eddy covariance. This finding indicates that it may be possible to diagnose the flux imbalance using these four variables under convective conditions. Key Points: • The relation between flux imbalance and surface heterogeneity scale is investigated using large-eddy simulations and a cospectral model • A diagnostic equation for flux imbalance is proposed • The qualitative relations between flux imbalance and various factors reported in the literature can be explained by this equation

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An Investigation of the Eddy-Covariance Flux Imbalance in a Year-Long Large-Eddy Simulation of the Weather at Cabauw

Cited by 17 publications

References 37 publications

The influence of idealized surface heterogeneity on virtual turbulent flux measurements

The influence of idealized surface heterogeneity on virtual turbulent flux measurements

Reply to reviewer 2

The Effects of Surface Heterogeneity Scale on the Flux Imbalance under Free Convection

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