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Person# Eric Richard Pardyjak

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Turbulence

In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows

Boundary layer

In physics and fluid mechanics, a boundary layer is the thin layer of fluid in the immediate vicinity of a bounding surface formed by the fluid flowing along the surface. The fluid's interaction wi

Measurement

Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events.
In other words, measurement is a process of determining how large or

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Holly Jayne Oldroyd, Eric Richard Pardyjak, Marc Parlange

Generally, the more an underlying terrain deviates from being flat, uniform and homogeneous, the less that classical theories and models of the atmospheric boundary layer hold. At times this leads to high uncertainties in turbulent flux predictions, and at other times the models completely break down. Hence, to better understand, model and subsequently predict surface turbulent exchanges over non-idealized terrain, an important objective of many recent field campaigns has been to investigate the near-surface turbulence structure. We present observations of momentum fluxes in nocturnal slope flows over steep (35.5 degree), alpine terrain in Val Ferret, Switzerland. Under clear-sky conditions, we observe two distinct flow regimes with mean winds directed down the slope: (1) buoyancy-driven, ‘katabatic flow', for which an elevated velocity maximum (katabatic jet peak) is observed and (2) ‘downslope winds', for which larger-scale forcing prevents formation of a katabatic jet. In downslope wind cases, the velocity profile is quite similar to a logarithmic profile often observed over flat terrain, and the corresponding momentum fluxes roughly resemble a constant-flux surface-layer. In stark contrast, the velocity profiles in the katabatic regime exhibit a jet-like shape. The katabatic jet strongly modulates the corresponding momentum fluxes, which show steep gradients over the shallow katabatic layer and typically change sign near the jet peak as the velocity gradients change sign. However, frequently a counter-gradient momentum flux is observed near the jet peak (and at times at higher levels), suggesting significant non-local turbulent transport within the katabatic jet layer. We compare and contrast this behavior with katabatic flow theories and observational studies over shallow-angle slopes, and use budget and co-spectral analyses to better understand the non-local transport dynamics. In addition, we show that as a consequence of the counter-gradient momentum fluxes, even local stability can be difficult to characterize because a counter-gradient momentum flux represent a sink in the shear term of turbulence kinetic energy budget equation. These results have broad implications for stability-based modeling of katabatic flows.

2016Wolf Hendrik Huwald, Holly Jayne Oldroyd, Eric Richard Pardyjak, Marc Parlange

In recent studies of atmospheric turbulent surface exchange in complex terrain, questions arise concerning velocity-sensor tilt corrections and the governing flow equations for coordinate systems aligned with steep slopes. The standard planar-fit method, a popular tilt-correction technique, must be modified when applied to complex mountainous terrain. The ramifications of these adaptations have not previously been fully explored. Here, we carefully evaluate the impacts of the selection of sector size (the range of flow angles admitted for analysis) and planar-fit averaging time. We offer a methodology for determining an optimized sector-wise planar fit (SPF), and evaluate the sensitivity of momentum fluxes to varying these SPF input parameters. Additionally, we clarify discrepancies in the governing flow equations for slope-aligned coordinate systems that arise in the buoyancy terms due to the gravitational vector no longer acting along a coordinate axis. New adaptions to the momentum equations and turbulence kinetic energy budget equation allow for the proper treatment of the buoyancy terms for purely upslope or downslope flows, and for slope flows having a cross-slope component. Field data show that new terms in the slope-aligned forms of the governing flow equations can be significant and should not be omitted. Since the optimized SPF and the proper alignment of buoyancy terms in the governing flow equations both affect turbulent fluxes, these results hold implications for similarity theory or budget analyses for which accurate flux estimates are important.

2016, , ,

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.