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Marc B. Parlange - One of the best experts on this subject based on the ideXlab platform.
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Katabatic Flow a closed form solution with spatially varying eddy diffusivities
Boundary-Layer Meteorology, 2017Co-Authors: R. Grandi, M. G. Giometto, J. Fang, P. A. Monkewitz, Marc B. ParlangeAbstract:The Nieuwstadt closed-form solution for the stationary Ekman layer is generalized for Katabatic Flows within the conceptual framework of the Prandtl model. The proposed solution is valid for spatially-varying eddy viscosity and diffusivity (O’Brien type) and constant Prandtl number (Pr). Variations in the velocity and buoyancy profiles are discussed as a function of the dimensionless model parameters \(z_0 \equiv \hat{z}_0 \hat{N}^2 Pr \sin {(\alpha )} |\hat{b}_\mathrm{s} |^{-1}\) and \(\lambda \equiv \hat{u}_{\mathrm{ref}}\hat{N} \sqrt{Pr} |\hat{b}_\mathrm{s} |^{-1}\), where \(\hat{z}_0\) is the hydrodynamic roughness length, \(\hat{N}\) is the Brunt-Vaisala frequency, \(\alpha \) is the surface sloping angle, \(\hat{b}_\mathrm{s}\) is the imposed surface buoyancy, and \(\hat{u}_{\mathrm{ref}}\) is a reference velocity scale used to define eddy diffusivities. Velocity and buoyancy profiles show significant variations in both phase and amplitude of extrema with respect to the classic constant \(\textit{K}\) model and with respect to a recent approximate analytic solution based on the Wentzel-Kramers-Brillouin theory. Near-wall regions are characterized by relatively stronger surface momentum and buoyancy gradients, whose magnitude is proportional to \(z_0\) and to \(\lambda \). In addition, slope-parallel momentum and buoyancy fluxes are reduced, the low-level jet is further displaced toward the wall, and its peak velocity depends on both \(z_0\) and \(\lambda \).
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Buoyant Turbulent Kinetic Energy Production in Steep-Slope Katabatic Flow
Boundary-Layer Meteorology, 2016Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:We develop several critical concepts that should be considered when interpreting, modelling and designing future experiments for Flows over sloping terrain. Vertical buoyancy fluxes in Katabatic Flows can be positive and a source of turbulent kinetic energy (TKE) despite the statically stable, thermal stratification that drives these Flows. This phenomenon occurs when the ratio of along-slope to slope-normal kinematic heat fluxes is greater than the cotangent of the slope angle, suggesting a critical value of slope-angle steepness found in earlier studies. We provide field-data-based evidence that the along-slope heat flux may dominate the variables in this inequality, and therefore in generating buoyant TKE production or suppression over a steep slope. These data show the along-slope heat flux can be more variable and significantly larger in magnitude than the slope-normal component. The gradient Richardson number does not include the effects of the along-slope buoyancy; furthermore, none of the canonical stability parameters can properly reflect the TKE redistribution from turbulent transport divergence and the sink of TKE in cases of counter-gradient momentum fluxes, which we frequently observe near the peak of the Katabatic jet. In such cases, canonical stability parameters inadequately represent the physical mechanisms associated with stability. These results have broad implications related to accurately modelling turbulence and surface exchanges over sloping terrain and illustrate the need to more thoroughly investigate the along-slope heat flux and its drivers, the meaning and definitions of stability, and the effects of non-local turbulent transport.
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Buoyant turbulence kinetic energy (TKE) production in Katabatic Flow despite stable thermal stratification
2015Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:As micrometeorological research shifts to increasingly non-idealized environments, the lens through which we view classical atmospheric boundary layer theory must also shift to accommodate unfamiliar behavior. We present observations of Katabatic Flow over a steep (35.5 degree), alpine slope and draw comparisons with classical theory for nocturnal boundary layers (NBL) over flat terrain to delineate key physical differences and similarities. In both cases, the NBL is characterized by a strong, terrain-aligned thermal stratification. Over flat terrain, this temperature inversion tends to stabilize perturbations and suppresses vertical motions. Hence, the buoyancy term in the TKE budget equation acts as a sink. In contrast, the steep-slope Katabatic Flow regime is characterized by buoyant TKE production despite NBL thermal stratification. This buoyant TKE production occurs because streamwise (upslope) heat fluxes, which are typically treated as unimportant over flat terrain, contribute to the total vertical buoyancy flux since the gravity vector is not terrain-normal. Due to a relatively small number of observations over steep terrain, the turbulence structure of such Flows and the implications of buoyant TKE production in the NBL have gone largely unexplored. As an important consequence of this characteristic, we show that conventional stability characterizations require careful coordinate system alignment and interpretation for Katabatic Flows. The streamwise heat fluxes play an integral role in characterizing stability and turbulent transport, more broadly, in Katabatic Flows. Therefore, multi-scale statistics and budget analyses describing physical interactions between turbulent fluxes at various scales are presented to interpret similarities and differences between the observations and classical theories regarding streamwise heat fluxes.
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Evaluation and Advancement of Similarity Scalings for a Steep Alpine Slope
2014Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:Keywords: similarity scaling ; Katabatic Flow ; mountain meteorology Reference EPFL-CONF-206450 Record created on 2015-03-14, modified on 2017-12-10
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Momentum balance of Katabatic Flow on steep slopes covered with short vegetation
Geophysical Research Letters, 2014Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Gabriel G. Katul, Marc B. ParlangeAbstract:Katabatic Flows over alpine mountainous terrain differ from their forested or bare slope counterparts due to the presence of well-ventilated, short vegetation. The impact of a grass canopy and larger-scale pressure perturbations on the one-dimensional mean momentum balance is explored via theory and field measurements. The model presented here reproduces the measured velocity jet shape and turbulent flux gradients. These two features imply that even when Monin-Obuhkov similarity theory breaks down, its use for a stability adjusted mixing length remains effective to first order. Results reveal that outer layer pressure effects can be significant under low-speed wind conditions at the top of the thin Katabatic layer when larger variations in the wind direction are observed. An analytical expression to estimate the jet height, which can be utilized in large-scale weather prediction models, shows the importance of including canopy effects for the thin Katabatic Flow region above the vegetation.
Branko Grisogono - One of the best experts on this subject based on the ideXlab platform.
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COMPARISON OF THE PRANDTL MODEL WITH K(z) AND NON ZERO f WITH DOPPLER SODAR OBSERVATIONS
2015Co-Authors: Iva Kavčič, Branko Grisogono, Ian A. Renfrew, Philip S. Anderson, Željko Večenaj, Ivana StiperskiAbstract:Abstract: We present the analytical and numerical solutions for (U, V) depending on (z, t) in the Katabatic Flow, where U and V are the downslope and cross-slope wind components. The solutions for (U, V) from the recently extended analytical 1D Prandtl model are verified against observations (the 15-min averages) from a novel autonomous Doppler sodar wind profiling system in Antarctica. Two cases of the wintertime, primarily Katabatically driven, Flows with the different stability values are investigated. The analytical and numerical model parameters are calculated from the Doppler sodar and AWS (automatic weather station) data. Observations of the downslope component are generally better described by the model solutions, especially in the case of the stronger stability. The model solutions for V agree qualitatively with the observations, but are not accurate enough in determining the height and the speed of the V maximum. Therefore, further improvements of the Prandtl model will be amendable as well as recommendations for a better parameterization of Katabatic Flows in large-scale numerical models
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Katabatic Flow with coriolis effect and gradually varying eddy diffusivity
Boundary-Layer Meteorology, 2007Co-Authors: Iva Kavčič, Branko GrisogonoAbstract:Katabatic Flows over high-latitude long glaciers experience the Coriolis force. A sloped atmospheric boundary-layer (ABL) Flow is addressed which partly diffuses upwards, and hence, becomes progressively less local. We present the analytical and numerical solutions for (U ,V, θ) depending on (z, t) in the Katabatic Flow, where U and V are the downslope and cross-slope wind components and θ is the potential temperature perturbation. A Prandtl model that accounts for the Coriolis effect, via f, does not approach a steady state, because V diffuses upwards in time; the rest, i.e., (U, θ), are similar to that in the classic Prandtl model. The V component behaves in a similar manner as the solution to the 1st Stokes (but inhomogeneous) problem. A WKB approach to the problem of the sloped ABL winds is outlined in the light of a modified Ekman-Prandtl model with gradually varying eddy diffusivity K(z). Ideas for parameterizing these high-latitude persistent Flows in climate models are revealed.
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Including Coriolis effects in the Prandtl model for Katabatic Flow
Quarterly Journal of the Royal Meteorological Society, 2007Co-Authors: Ivana Stiperski, Iva Kavčič, Branko Grisogono, Dale R. DurranAbstract:Katabatic Flow down long glaciers in high latitudes experiences deflection due to the Coriolis force. If the Coriolis force is added to the classic Prandtl model for Katabatic Flow, the cross-slope wind component does not approach a true steady state, but rather diffuses upwards in time. On the other hand, the down-slope component and the potential temperature perturbations do reach stationarity on the same time-scale as in the classic Prandtl model. Numerical and approximate analytic solutions are presented describing this spatio-temporal behaviour. Both solutions are in accordance with physical intuition. The analytic approximate solution can be useful in boundary-layer parameterizations and data analysis. Copyright 2007 Royal Meteorological Society
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Comparison of the rotating Prandtl model with K(z) and a mesoscale numerical model
2007Co-Authors: Iva Kavčič, Ivana Stiperski, Branko Grisogono, Dale R. DurranAbstract:Deployed analytical approach extends Prandtl model for simple Katabatic Flow by using vertically varying eddy diffusivity and allowing for the Earth rotation. The related Flow pertains to long glaciers on Greenland and Antarctic, which are poorly resolved in climate and some numerical weather prediction (NWP) models. We compare the extended 1D analytical Prandtl model for (U, V, Theta) with a mesoscale numerical model. The MIUU mesoscale model employs a fine vertical resolution and a higher-order turbulence parameterization scheme. The analytical model slightly underestimates both the height and the speed of U maximum, and the variations in Theta However, it successfully describes the time evolution of the V component and its maximum, although somewhat overestimating the magnitude of V. Nevertheless, the overall tentative agreement of the analytical model with the MIUU model profiles is good. The differences arise from the hierarchy of the effects of the various assumptions made in the extended analytical Prandtl model. Hence, further improvements of the Prandtl model will be amendable as well as recommendations for a better parameterization of Katabatic Flows in large-scale numerical models.
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Katabatic Flow with coriolis effect
2005Co-Authors: Ivana Stiperski, Iva Kavčič, Branko GrisogonoAbstract:Katabatic Flows on long glaciers in high latitudes experience the Coriolis effect deflecting the Flow thus affecting turbulent transports in the boundary layer. Analytically Katabatic Flows have been best represented by Prandtl model. However, the classic Prandtl model does not take into account the effect of the Coriolis force. It is found that after a straightforward inclusion of this effect, the solution to the model is correct only up to a constant, and does not simultaneously satisfy all boundary conditions. Therefore, a modified analytic Prandtl model is examined. The modified approximate solution agrees with a numerical solution, and qualitatively, with the results of other related studies. Besides a theoretical significance, our approximate analytic solution can be useful for developing a better parameterization in climate models having a poor vertical resolution for such shallow but persistent Flows.
Eric R. Pardyjak - One of the best experts on this subject based on the ideXlab platform.
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The evolution and sensitivity of Katabatic Flow dynamics to external influences through the evening transition
Quarterly Journal of the Royal Meteorological Society, 2016Co-Authors: Derek D. Jensen, Daniel F. Nadeau, Sebastian W. Hoch, Eric R. PardyjakAbstract:Data collected over an arid shallow slope (2–4°) during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to study the Katabatic structure and onset of Katabatic Flow through the evening transition. An unprecedented suite of instrumentation, including a transect of five turbulence towers with 29 sonic anemometers, is used for the investigation. Fifteen transition periods with well-defined Katabatic Flow and relatively little synoptic forcing are used in the study. The Katabatic onset, jet velocity and jet height all show a large degree of interdiurnal and intersite variance. The slope-aligned budgets of momentum and potential temperature are used to define time scales that describe the evolution of the Katabatic Flow. Composite wind velocity time series are used to show that ≈ 30 min elapses from the time when the Katabatic Flow initializes at 0.5 m to the point of initialization at 20 m. A simple Katabatic model utilizing surface energy-budget modeling is developed and used to model the interdiurnal Katabatic variance. Finally, uni- and multi-variate statistical analysis are used to diagnose the influence of specific external variables. Valley wind speed, turbulence structure, soil moisture, and shadow front speed are all found to influence the Katabatic dynamics to varying degrees.
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Buoyant Turbulent Kinetic Energy Production in Steep-Slope Katabatic Flow
Boundary-Layer Meteorology, 2016Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:We develop several critical concepts that should be considered when interpreting, modelling and designing future experiments for Flows over sloping terrain. Vertical buoyancy fluxes in Katabatic Flows can be positive and a source of turbulent kinetic energy (TKE) despite the statically stable, thermal stratification that drives these Flows. This phenomenon occurs when the ratio of along-slope to slope-normal kinematic heat fluxes is greater than the cotangent of the slope angle, suggesting a critical value of slope-angle steepness found in earlier studies. We provide field-data-based evidence that the along-slope heat flux may dominate the variables in this inequality, and therefore in generating buoyant TKE production or suppression over a steep slope. These data show the along-slope heat flux can be more variable and significantly larger in magnitude than the slope-normal component. The gradient Richardson number does not include the effects of the along-slope buoyancy; furthermore, none of the canonical stability parameters can properly reflect the TKE redistribution from turbulent transport divergence and the sink of TKE in cases of counter-gradient momentum fluxes, which we frequently observe near the peak of the Katabatic jet. In such cases, canonical stability parameters inadequately represent the physical mechanisms associated with stability. These results have broad implications related to accurately modelling turbulence and surface exchanges over sloping terrain and illustrate the need to more thoroughly investigate the along-slope heat flux and its drivers, the meaning and definitions of stability, and the effects of non-local turbulent transport.
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Structure of Turbulence in Katabatic Flows Below and Above the Wind-Speed Maximum
Boundary-Layer Meteorology, 2016Co-Authors: Andrey A. Grachev, Eric R. Pardyjak, Laura S. Leo, Silvana Di Sabatino, Harindra J. S. Fernando, Christopher W. FairallAbstract:Measurements of small-scale turbulence made in the atmospheric boundary layer over complex terrain during the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program are used to describe the structure of turbulence in Katabatic Flows. Turbulent and mean meteorological data were continuously measured on four towers deployed along the east lower slope (2–4 $$^{\circ }$$ ∘ ) of Granite Mountain near Salt Lake City in Utah, USA. The multi-level (up to seven) observations made during a 30-day long MATERHORN field campaign in September–October 2012 allowed the study of temporal and spatial structure of Katabatic Flows in detail, and herein we report turbulence statistics (e.g., fluxes, variances, spectra, and cospectra) and their variations in Katabatic Flow. Observed vertical profiles show steep gradients near the surface, but in the layer above the slope jet the vertical variability is smaller. It is found that the vertical (normal to the slope) momentum flux and horizontal (along-slope) heat flux in a slope-following coordinate system change their sign below and above the wind maximum of a Katabatic Flow. The momentum flux is directed downward (upward) whereas the along-slope heat flux is downslope (upslope) below (above) the wind maximum. This suggests that the position of the jet-speed maximum can be obtained by linear interpolation between positive and negative values of the momentum flux (or the along-slope heat flux) to derive the height where the flux becomes zero. It is shown that the standard deviations of all wind-speed components (and therefore of the turbulent kinetic energy) and the dissipation rate of turbulent kinetic energy have a local minimum, whereas the standard deviation of air temperature has an absolute maximum at the height of wind-speed maximum. We report several cases when the destructive effect of vertical heat flux is completely cancelled by the generation of turbulence due to the along-slope heat flux. Turbulence above the wind-speed maximum is decoupled from the surface, and follows the classical local $$z$$ z -less predictions for the stably stratified boundary layer.
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Buoyant turbulence kinetic energy (TKE) production in Katabatic Flow despite stable thermal stratification
2015Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:As micrometeorological research shifts to increasingly non-idealized environments, the lens through which we view classical atmospheric boundary layer theory must also shift to accommodate unfamiliar behavior. We present observations of Katabatic Flow over a steep (35.5 degree), alpine slope and draw comparisons with classical theory for nocturnal boundary layers (NBL) over flat terrain to delineate key physical differences and similarities. In both cases, the NBL is characterized by a strong, terrain-aligned thermal stratification. Over flat terrain, this temperature inversion tends to stabilize perturbations and suppresses vertical motions. Hence, the buoyancy term in the TKE budget equation acts as a sink. In contrast, the steep-slope Katabatic Flow regime is characterized by buoyant TKE production despite NBL thermal stratification. This buoyant TKE production occurs because streamwise (upslope) heat fluxes, which are typically treated as unimportant over flat terrain, contribute to the total vertical buoyancy flux since the gravity vector is not terrain-normal. Due to a relatively small number of observations over steep terrain, the turbulence structure of such Flows and the implications of buoyant TKE production in the NBL have gone largely unexplored. As an important consequence of this characteristic, we show that conventional stability characterizations require careful coordinate system alignment and interpretation for Katabatic Flows. The streamwise heat fluxes play an integral role in characterizing stability and turbulent transport, more broadly, in Katabatic Flows. Therefore, multi-scale statistics and budget analyses describing physical interactions between turbulent fluxes at various scales are presented to interpret similarities and differences between the observations and classical theories regarding streamwise heat fluxes.
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Evaluation and Advancement of Similarity Scalings for a Steep Alpine Slope
2014Co-Authors: Holly J. Oldroyd, Eric R. Pardyjak, Chad W. Higgins, Marc B. ParlangeAbstract:Keywords: similarity scaling ; Katabatic Flow ; mountain meteorology Reference EPFL-CONF-206450 Record created on 2015-03-14, modified on 2017-12-10
Peter G. Duynkerke - One of the best experts on this subject based on the ideXlab platform.
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Turbulence in a Katabatic Flow
Boundary-Layer Meteorology, 1999Co-Authors: Ernst Van Der Avoird, Peter G. DuynkerkeAbstract:Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajökull in Iceland are described. The boundary layer, in which Katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions φ_m and φ_h as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where φ_m was linear as a function of z/L, in the Katabatically forced boundary layer, the dependence of φ_m on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions. Furthermore, measurements on the ice were used to construct the energy balance. These showed good agreement between observed melt and components contributing to the energy balance: net radiation (supplying 55% of the energy), sensible heat flux (30%) and latent heat flux (15%). Local sources and sinks in the turbulent kinetic energy budget are summed and indicate a reasonable balance in near-neutral conditions but not in more stable situations. The standard deviation of the velocity fluctuations σ_u, σ_v, and σ_w, can be scaled satisfactorily with the local friction velocity u_* and the standard deviation of the temperature fluctuation σ_θ with the local temperature scale θ_*.
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Turbulence in a Katabatic Flow
Boundary-Layer Meteorology, 1999Co-Authors: Ernst Van Der Avoird, Peter G. DuynkerkeAbstract:Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajokull in Iceland are described. The boundary layer, in which Katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions φm and φh as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where φm was linear as a function of z/L, in the Katabatically forced boundary layer, the dependence of φm on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions.
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turbulence in a Katabatic Flow does it resemble turbulence in stable boundary layers over flat surfaces
Boundary-Layer Meteorology, 1999Co-Authors: Ernst Van Der Avoird, Peter G. DuynkerkeAbstract:Turbulence measurements performed in a stable boundary layer over the sloping ice surface of the Vatnajokull in Iceland are described. The boundary layer, in which Katabatic forces are stronger than the large-scale forces, has a structure that closely resembles that of a stable boundary layer overlying a flat land surface, although there are some important differences. In order to compare the two situations the set-up of the instruments on an ice cap in Iceland was reproduced on a flat grass surface at Cabauw, the Netherlands. Wind speed and temperature gradients were calculated and combined with flux measurements made with a sonic anemometer in order to obtain the local stability functions O m and O h as a function of the local stability parameter z/L. Unlike the situation at Cabauw, where O m was linear as a function of z/L, in the Katabatically forced boundary layer, the dependence of O m on stability was found to be non-linear and related to the height of the wind maximum. Thermal stratification and the depth of the stable boundary layer however seem to be rather similar under these two different forcing conditions. Furthermore, measurements on the ice were used to construct the energy balance. These showed good agreement between observed melt and components contributing to the energy balance: net radiation (supplying 55% of the energy), sensible heat flux (30%) and latent heat flux (15%). Local sources and sinks in the turbulent kinetic energy budget are summed and indicate a reasonable balance in near-neutral conditions but not in more stable situations. The standard deviation of the velocity fluctuations σ u , σ υ , and σ w , can be scaled satisfactorily with the local friction velocity u * and the standard deviation of the temperature fluctuation σ θ with the local temperature scale θ * .
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Surface energy balance and Katabatic Flow over glacier and tundra during GIMEX-91
Global and Planetary Change, 1994Co-Authors: Peter G. Duynkerke, Michiel R. Van Den BroekeAbstract:The energy balance observed in the summer of 1991 during the Greenland Ice Margin EXperiment (GIMEX-91) is described. Several masts were erected along a transect perpendicular to the ice edge; from a point 90 km up the ice cap, through the ablation zone, down to a point 5.8 km within the tundra zone. The diurnal variation of the friction velocity, sensible heat flux and latent heat flux, both on a single day (22 July) and averaged over the complete observational period are discussed. The mean daily values of the sensible heat flux (positive upwards) over the ice were negative (about — 30 W/m2), as the ice was melting during most of the observational period. On average the latent heat flux was positive, which moistened the boundary layer air Flowing down the ice cap. Up the ice cap, more than about 20 km from the ice edge, the energy balance is driven by the net radiation which causes higher fluxes during the day than during the night. The smaller (more negative) sensible heat fluxes during the night accelerate the Katabatic Flow; this leads to a wind maximum late at night or early in the morning. This diurnal cycle in energy balance is very similar to that observed on the slopes of Antarctica. At the ice edge, the large difference between the radiative and thermal properties of the ice and the tundra exerts a strong thermal forcing on the Katabatic Flow, analogous to the land-sea breeze circulation. During day-time the thermal wind accelerates the Katabatic Flow, which leads to an enhancement of the turbulent exchange rate and turbulent fluxes.
Roger A. Pielke - One of the best experts on this subject based on the ideXlab platform.
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The Interaction of Katabatic Flow and Mountain Waves. Part II: Case Study Analysis and Conceptual Model
Journal of the Atmospheric Sciences, 2007Co-Authors: Gregory S. Poulos, James E. Bossert, Thomas B. Mckee, Roger A. PielkeAbstract:Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of Katabatic Flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation. The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by Flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage Flows develop. Simultaneously, Katabatic Flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, Flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based Katabatic Flow layer can be reflected in the Katabatic Flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing Katabatic Flow. Variability is found to occur on two scales, on the meso- due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking–induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of Katabatic Flow.
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The Interaction of Katabatic Flow and Mountain Waves. Part I: Observations and Idealized Simulations
Journal of the Atmospheric Sciences, 2000Co-Authors: Gregory S. Poulos, James E. Bossert, Thomas B. Mckee, Roger A. PielkeAbstract:Abstract The mutual interaction of Katabatic Flow in the nocturnal boundary layer (NBL) and topographically forced gravity waves is investigated. Due to the nonlinear nature of these phenomena, analysis focuses on information obtained from the 1993 Atmospheric Studies in Complex Terrain field program held at the mountain–canyon–plains interface near Eldorado Canyon, Colorado, and idealized simulations. Perturbations to Katabatic Flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by dynamic pressure effects. Based on a local Froude number climatology, case study analysis, and the simulations, the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and, through altered horizontal pressure gradient forcing, to the surface-based Katabatic Flows. It is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant p...
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The mutual evolution of mountain waves and Katabatic Flow
1996Co-Authors: Gregory S. Poulos, James E. Bossert, Thomas B. Mckee, Roger A. PielkeAbstract:Typically, Katabatic Flows have been studied in their least complicated or idealized state. Further, these Flows are generally regarded as having simple forcing and non-turbulent structure due to the strong atmospheric stability they are bedded within. Somewhat analogously, mountain waves and their effects have been mostly studied in their idealized state, i.e. for constant upstream Flow and stability. Even in the numerous cases where these two atmospheric phenomena have been studied in their realistic state, seldom has their mutual interaction been considered. One exception that includes numerical modeling is Gross (1990). The express purpose of this work is to examine how each of these phenomena interact with each other in an evolving nocturnal atmosphere. This work is motivated by observations from the Atmospheric Studies in Complex Terrain (ASCOT) Program which clearly indicate non-idealized behavior in Katabatic Flows. Although numerous idealized simulations were also completed, discussion here focuses on the most realistic simulations of the case night 3--4 September 1993. This night was dominated by clear skies and light near surface winds. A high pressure system to the southwest of Colorado caused northwesterly Flow at {approximately} 7 m s{sup {minus}1} upstream of the Rockies with a Froude number of 0.45 overnight.more » ASCOT observations indicated that Katabatic and mountain wave Flow were occurring.« less