On the turbulence structure of deep katabatic flows on a gentle mesoscale slope

Abstract

A comprehensive analysis of the turbulence structure of relatively deep midlatitude katabatic flows (with jet maxima between 20 and 50 m) developing over a gentle (1 • ) mesoscale slope with a long fetch upstream of the Meteor Crater in Arizona is presented. The turbulence structure of flow below the katabatic jet maximum shows many similarities with the turbulence structure of shallower katabatic flows, with decreasing turbulence fluxes with height and almost constant turbulent Prandtl number. Still stark differences occur above the jet maximum where turbulence is suppressed by strong stability, is anisotropic and there is a large sub-mesoscale contribution to the flux. Detecting the stable boundary-layer top depends on the method used (flux-vs. anisotropy-profiles) but both methods are highly correlated. The top of the stable boundary layer, however, mostly deviates from the jet maximum height or the top of the near-surface inversion. The flat-terrain formulations for the boundary-layer height correlate well with the detected top of the stable boundary layer if the near-surface and not the background stratification is used in their formulations; however, they mostly largely overestimate this boundary-layer height. The difference from flat-terrain boundary layers is also shown through the dependence of size of the dominant eddy with height. In katabatic flows the eddy size is semi-constant with height throughout the stable boundary-layer depth, whereas in flat terrain, eddy size varies significantly with height. Flux-gradient and flux-variance relationships show that turbulence data from different stable boundary-layer scaling regimes collapse on top of each other showing that the dominant dependence is not on the scaling regime but on the local stability. K E Y W O R D S boundary-layer depth, low-level jet, scaling regimes, stable boundary layer This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Flow Separation in the Lee of a Crater Rim

“…This agrees with Whiteman et al (2018a) that flow intrusions occur with a shallow katabatic flow. The wind-speed and stability profiles for the wake cases agree better with the deep katabatic flow described by Stiperski et al (2018), with a continuous increase in wind speed throughout the height of the 50-m tower and comparatively lower stability. For the large wakes, the 90th percentiles show that cases with much higher wind speeds are possible.…”

“…A thorough analysis of a cold-air-intrusion-dominated night is presented by Whiteman et al (2018a), who also showed that cold-air intrusions (Fig. 13a, b) form when the upstream katabatic flow is shallow according to the classification by Stiperski et al (2018) and the flow decelerates towards the crater. As the lower part of the drainage flow is colder than the crater atmosphere, the cold air runs down the sidewall until it reaches its level of neutral buoyancy.…”

“…Wind speed at the crater rim plays a crucial role, with flow separation becoming the dominant regime for layer-averaged wind speeds > 5 m s −1 . The upstream wind profile in flow-separation conditions is typically characterized by a deep katabatic flow, according to the classification by Stiperski et al (2018), and the flow accelerates towards the crater, with the horizontal pressure gradient just upwind of the crater rim opposing the direction of the katabatic flow ( Fig. 13c, d).…”

“…Figure 5b shows that their occurrence is usually also accompanied by lower stabilities compared to intrusions, also in the upper, near-isothermal layer, which may thus facilitate flow separation. Stiperski et al (2018) analyzed vertical profiles upwind of the crater at the NEAR tower and distinguished two different types of katabatic flows approaching the crater. A shallow type with a wind-speed maximum at 15-25 m a.g.l.…”

Revealing the Dynamical Transition of Anisotropy Behind the HOST by Koopman Analysis

“…Nevertheless, if the gap exists, the energy injection rate about large to small scale motions must be small enough, and the effect of shear and rotation from large to small scale will be negligible (Pouquet et al, 1983). This condition might not be satisfied in the stable boundary layer for weak shear motion, leading to fuzzy crossing zero points and uncertainty of upper bounds on time intervals (e.g., Stiperski & Calaf, 2018;Stiperski et al, 2019Stiperski et al, , 2020Voronovich & Kiely, 2007). Meanwhile, the fluxes such as horizontal kinetic energy, vertical momentum, and sensible heat have different time scales, leading to the insufficiency in the application of any single flux spectral analysis to reveal the dynamic and thermodynamic coupled time length.…”