scholarly journals The influence of idealized surface heterogeneity on virtual turbulent flux measurements

2018 ◽  
Vol 18 (7) ◽  
pp. 5059-5074 ◽  
Author(s):  
Frederik De Roo ◽  
Matthias Mauder
Keyword(s):  
Energy Budget ◽  
Turbulent Flux ◽  
Surface Heterogeneity ◽  
Surface Fluxes ◽  
Convective Conditions ◽  
Mean Flow ◽  
Flux Divergence ◽  
Length Scales ◽  
Flux Measurements ◽  
The Mean

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.

scholarly journals The influence of idealized surface heterogeneity on virtual turbulent flux measurements

2017 ◽  
Author(s):  
Frederik De Roo ◽  
Matthias Mauder
Keyword(s):  
Boundary Layer ◽  
Energy Budget ◽  
Turbulent Flux ◽  
Surface Heterogeneity ◽  
Surface Fluxes ◽  
Surface Energy Budget ◽  
Parameter Study ◽  
Mean Flow ◽  
Length Scales ◽  
Large Eddy

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. The influence of surface heterogeneities on the atmospheric boundary layer has been intensively investigated since two decades. Previous studies found that heterogeneities of the boundary-layer scale or larger are most effective in influencing the boundary layer turbulence. Subsequent large-eddy simulations showed that also the turbulent fluxes are influenced by large-scale organized structures in the boundary layer. However, the precise influence of the surface characteristics on the energy imbalance of measurements in the surface layer and its partitioning is still unknown. To investigate the influence of surface variables on all the components of the flux budget 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 heterogeneities of length scales of the kilometer scale, and length scales of one order of magnitude smaller. For heterogeneities of these two types, 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.). Furthermore, we seek correlators for the energy balance ratio and the energy residual in the simulations. Besides the expected correlation with measurable atmospheric quantities such as the friction velocity, boundary-layer depth and temperature and moisture gradients, we have also found an unexpected correlation with the temperature difference between sonic temperature and surface temperature.

scholarly journals Deterministic Model of the Eddy Dynamics for a Midlatitude Ocean Model

2021 ◽  
Author(s):  
Takaya Uchida ◽  
Bruno Deremble ◽  
Stephane Popinet
Keyword(s):  
Deterministic Model ◽  
North Atlantic Ocean ◽  
Ocean Model ◽  
Global Ocean ◽  
Mean Flow ◽  
Flux Divergence ◽  
Time Step ◽  
Weather System ◽  
Basin Scale ◽  
The Mean

Mesoscale eddies, the weather system of the oceans, although being on the scales of O(20-100km), have a disproportionate role in shaping the mean jets such as the separated Gulf Stream in the North Atlantic Ocean, which is on the scale of O(1000km) in the along-jet direction. With the increase in computational power, we are now able to partially resolve the eddies in basin-scale and global ocean simulations, a model resolution often referred to as mesoscale permitting. It is well known, however, that due to grid-scale numerical viscosity, mesoscale permitting simulations have less energetic eddies and consequently weaker eddy feedback onto the mean flow. In this study, we run a quasi-geostrophic model at mesoscale resolving resolution in a double gyre configuration and formulate a deterministic parametrization for the eddy rectification term of potential vorticity (PV), namely, the eddy PV flux divergence. We have moderate success in reproducing the spatial patterns and magnitude of eddy kinetic and potential energy diagnosed from the model. One novel point about our approach is that we account for non-local eddy feedbacks onto the mean flow by solving the eddy PV equation prognostically in addition to the mean flow. In return, we are able to parametrize the variability in total (mean+eddy) PV at each time step instead of solely the mean PV. A closure for the total PV is beneficial as we are able to account for both the mean state and extreme events.

scholarly journals Characterizing the Energetics of Vortex-Scale and Sub-Vortex-Scale Asymmetries during Tropical Cyclone Rapid Intensity Changes

2019 ◽  
Vol 77 (1) ◽  
pp. 315-336
Author(s):  
Saiprasanth Bhalachandran ◽  
Daniel R. Chavas ◽  
Frank D. Marks Jr. ◽  
S. Dubey ◽  
A. Shreevastava ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Kinetic Energy ◽  
Mean Flow ◽  
Length Scales ◽  
Asymmetric Structures ◽  
Multiple Length Scales ◽  
Precise Understanding ◽  
Intensity Changes ◽  
Resolution Model ◽  
The Mean

Abstract Our collective understanding of azimuthally asymmetric features within the coherent structure of a tropical cyclone (TC) continues to improve with the availability of more detailed observations and high-resolution model outputs. However, a precise understanding of how these asymmetries impact TC intensity changes is lacking. Prior attempts at investigating the asymmetric impacts follow a mean–eddy partitioning that condenses the effect of all the asymmetries into one term and fails to highlight the differences in the role of asymmetries at different scales. In this study, we present a novel energetics-based approach to analyze the asymmetric impacts at multiple length scales during periods of rapid intensity changes. Using model outputs of TCs under low and high shear, we compute the different energy pathways that enhance/suppress the growth of multiscale asymmetries in the wavenumber (WN) domain. We then compare and contrast the energetics of the mean-flow field (WN 0) with that of the persistent, coherent vortex-scale asymmetric structures (WNs 1 and 2) and the more local, transient, sub-vortex-scale asymmetries (WNs ≥ 3). We find in our case studies that the dominant mechanisms of growth/decay of the asymmetries are the baroclinic conversion from available potential to kinetic energy at individual scales of asymmetries and the transactions of kinetic energy between the asymmetries of various length scales, rather than the barotropic mean–eddy transactions as is typically assumed. Our case study analysis further shows that the growth/decay of asymmetries is largely independent of the mean. Certain aspects of eddy energetics can potentially serve as early-warning indicators of TC rapid intensity changes.

Estimating Bolus Velocities from Data—How Large Must They Be?

2009 ◽  
Vol 39 (1) ◽  
pp. 70-88 ◽  
Author(s):  
Peter D. Killworth
Keyword(s):  
North Pacific ◽  
Density Level ◽  
Eddy Flux ◽  
Mean Flow ◽  
Flux Divergence ◽  
The North ◽  
Resolution Model ◽  
Eddy Fluxes ◽  
The Mean ◽  
The North Pacific

Abstract This paper examines the representation of eddy fluxes by bolus velocities. In particular, it asks the following: 1) Can an arbitrary eddy flux divergence of density be represented accurately by a nondivergent bolus flux that satisfies the condition of no normal flow at boundaries? 2) If not, how close can such a representation come? 3) If such a representation can exist in some circumstances, what is the size of the smallest bolus velocity that fits the data? The author finds, in agreement with earlier authors, that the answer to the first question is no, although under certain conditions, which include a modification to the eddy flux divergence, a bolus representation becomes possible. One such condition is when the eddy flux divergence is required to balance the time-mean flux divergence. The smallest bolus flow is easily found by solving a thickness-weighted Poisson equation on each density level. This problem is solved for the North Pacific using time-mean data from an eddy-permitting model. The minimum bolus flow is found to be very small at depth but larger than is usually assumed near the surface. The magnitude of this minimum flow is of order one-tenth of the mean flow. Similar but larger results are found for a coarse-resolution model.

Trace gas flux measurements at the landscape scale using boundary-layer budgets

1995 ◽  
Vol 351 (1696) ◽  
pp. 357-369 ◽  
Keyword(s):  
Boundary Layer ◽  
Local Area ◽  
Surface Fluxes ◽  
Trace Gas ◽  
Significant Heterogeneity ◽  
Methane Fluxes ◽  
Flux Measurements ◽  
The Mean ◽  
Nocturnal Inversion ◽  
One Year

Methane effluxes from wetland areas of Scotland were estimated by using the boundary-layer budget method by collecting air samples with an aircraft upwind and downwind of an area of extensive peatland. Nocturnal local area methane fluxes were also estimated at a peat bog site, Loch More, located at 58° 24' N 03° 36' W, using the concentration build up under the nocturnal inversion and from profiles of methane concentration using a tethered balloon. The mean daytime flux for the Loch More case studies in 1993 was found to be 128 ± 57 μ mol m -2 h -1 for the NE region of Scotland, comparable to but generally larger than those obtained for the same region one year earlier. The fluxes are smaller than those obtained in Caithness by the same technique. In 1993 the nocturnal fluxes were found to be 38 ± 7 μ mol m -2 h -1 , significantly smaller than those found during 1992. The daytime fluxes measured by the aircraft were generally larger than fluxes measured by micrometeorological techniques at the same time. These differences can be explained in terms of the significant heterogeneity in surface fluxes that exist on scales of a few hundred metres or less and the possibility of additional sources other than peatland in this region.

scholarly journals Quantifying the Airflow Distortion over Merchant Ships. Part I: Validation of a CFD Model

2006 ◽  
Vol 23 (3) ◽  
pp. 341-350 ◽  
Author(s):  
Bengamin I. Moat ◽  
Margaret J. Yelland ◽  
Robin W. Pascal ◽  
Anthony F. Molland
Keyword(s):  
Wind Speed ◽  
Charles Darwin ◽  
Reverse Direction ◽  
Cfd Model ◽  
Mean Flow ◽  
Flow Distortion ◽  
Flux Measurements ◽  
Computational Fluid Dynamics Cfd ◽  
The Mean

Abstract The effects of flow distortion created by the ship’s hull and superstructure bias wind speed measurements made from anemometers located on ships. Flow distortion must be taken into account if accurate air–sea flux measurements are to be achieved. Little work has been undertaken to examine the wind speed bias due to flow distortion in wind speed reports from voluntary observing ships (VOS). In this first part of a two-part paper the accuracy of the computational fluid dynamics (CFD) code VECTIS in simulating the airflow over VOS is investigated. Simulations of the airflow over a representation of the bridge of a VOS are compared to in situ wind speed measurements made from six anemometers located above the bridge of the RRS Charles Darwin. The ship’s structure was ideal for reproducing the flow over VOS when the wind is blowing onto either beam. The comparisons showed VECTIS was accurate to within 4% in predicting the wind speed over ships, except in extreme cases such as wake regions or the region close to the bridge top where the flow may be stagnant or reverse direction. The study showed that there was little change in the numerically predicted flow pattern above the bridge with change in Reynolds number between 2 × 105 and 1 × 107. The findings showed that the CFD code VECTIS can reliably be used to determine the mean flow above typical VOS.

scholarly journals Stability and energy budget of pressure-driven collapsible channel flows

2011 ◽  
Vol 705 ◽  
pp. 348-370 ◽  
Author(s):  
H. F. Liu ◽  
X. Y. Luo ◽  
Z. X. Cai
Keyword(s):  
Pressure Drop ◽  
Kinetic Energy ◽  
Energy Budget ◽  
Neutral Curve ◽  
Channel Flows ◽  
Mean Flow ◽  
Mode 2 ◽  
The Mean ◽  
The Stability ◽  
Pressure Driven

AbstractAlthough self-excited oscillations in collapsible channel flows have been extensively studied, our understanding of their origins and mechanisms is still far from complete. In the present paper, we focus on the stability and energy budget of collapsible channel flows using a fluid–beam model with the pressure-driven (inlet pressure specified) condition, and highlight its differences to the flow-driven (i.e. inlet flow specified) system. The numerical finite element scheme used is a spine-based arbitrary Lagrangian–Eulerian method, which is shown to satisfy the geometric conservation law exactly. We find that the stability structure for the pressure-driven system is not a cascade as in the flow-driven case, and the mode-2 instability is no longer the primary onset of the self-excited oscillations. Instead, mode-1 instability becomes the dominating unstable mode. The mode-2 neutral curve is found to be completely enclosed by the mode-1 neutral curve in the pressure drop and wall stiffness space; hence no purely mode-2 unstable solutions exist in the parameter space investigated. By analysing the energy budgets at the neutrally stable points, we can confirm that in the high-wall-tension region (on the upper branch of the mode-1 neutral curve), the stability mechanism is the same as proposed by Jensen & Heil. Namely, self-excited oscillations can grow by extracting kinetic energy from the mean flow, with exactly two-thirds of the net kinetic energy flux dissipated by the oscillations and the remainder balanced by increased dissipation in the mean flow. However, this mechanism cannot explain the energy budget for solutions along the lower branch of the mode-1 neutral curve where greater wall deformation occurs. Nor can it explain the energy budget for the mode-2 neutral oscillations, where the unsteady pressure drop is strongly influenced by the severely collapsed wall, with stronger Bernoulli effects and flow separations. It is clear that more work is required to understand the physical mechanisms operating in different regions of the parameter space, and for different boundary conditions.

scholarly journals Statistical evidence of anasymptotic geometric structure to the momentum transporting motions in turbulent boundary layers

2017 ◽  
Vol 375 (2089) ◽  
pp. 20160084 ◽  
Author(s):  
Caleb Morrill-Winter ◽  
Jimmy Philip ◽  
Joseph Klewicki
Keyword(s):  
Boundary Layers ◽  
Reynolds Shear Stress ◽  
Turbulent Boundary Layers ◽  
Wall Turbulence ◽  
Sensor Data ◽  
Joint Analysis ◽  
Large Reynolds Number ◽  
Mean Flow ◽  
Length Scales ◽  
The Mean

The turbulence contribution to the mean flow is reflected by the motions producing the Reynolds shear stress (〈− uv 〉) and its gradient. Recent analyses of the mean dynamical equation, along with data, evidence that these motions asymptotically exhibit self-similar geometric properties. This study discerns additional properties associated with the uv signal, with an emphasis on the magnitudes and length scales of its negative contributions. The signals analysed derive from high-resolution multi-wire hot-wire sensor data acquired in flat-plate turbulent boundary layers. Space-filling properties of the present signals are shown to reinforce previous observations, while the skewness of uv suggests a connection between the size and magnitude of the negative excursions on the inertial domain. Here, the size and length scales of the negative uv motions are shown to increase with distance from the wall, whereas their occurrences decrease. A joint analysis of the signal magnitudes and their corresponding lengths reveals that the length scales that contribute most to 〈− uv 〉 are distinctly larger than the average geometric size of the negative uv motions. Co-spectra of the streamwise and wall-normal velocities, however, are shown to exhibit invariance across the inertial region when their wavelengths are normalized by the width distribution, W ( y ), of the scaling layer hierarchy, which renders the mean momentum equation invariant on the inertial domain. This article is part of the themed issue ‘Toward the development of high-fidelity models of wall turbulence at large Reynolds number’.

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