Abstract
The nearly circular Meteor Crater, Arizona, is located on an extensive, slightly sloping plain, above which a south-westerly katabatic flow forms during undisturbed, clear-sky nights. For the katabatic flow over the upstream crater rim, the resulting flow regime in the lee depends on the upstream wind speed. For a shallow katabatic flow with a wind-speed maximum of about 5 m s^{-1} or less at a height of about 20 m above the ground, the flow decelerates as it approaches the crater. Cold-air intrusions form, that is, cold air spills over the crater rim and runs down the inner south-west sidewall. For a deep katabatic flow with a wind-speed maximum located above the 50-m high measurement tower and comparatively higher wind speeds, the flow accelerates towards the crater. The flow separates in the immediate lee of the crater rim, forming a wake over the south-west crater sidewall. The wake can either be small, affecting only the upper part of the sidewall, or large, affecting the entire crater sidewall or even the crater floor. When flow separation occurs, the wake region over the crater sidewall is characterized by low wind speeds and potentially a return circulation near the surface. Particularly for large wakes, stability in the crater atmosphere is reduced and relatively high wind speeds occur at the crater floor, which is otherwise submerged in a strong surface-based inversion. Turbulent kinetic energy at the crater sidewall is typically higher during cold-air intrusions than is the case during flow separation, but high values can occur at the floor when a large wake forms.
Highlights
As airflow interacts with topography, that is, as air is either forced to flow over mountains or depressions or to flow around them, the resulting flow regimes and processes depend strongly on flow characteristics and on topographic characteristics (e.g., Vosper 2004; Jackson et al 2012; Lehner et al 2016a; Rotunno and Lehner 2016)
Observations have been presented from the Meteor Crater Experiment (METCRAX) II field campaign in the approximately 170-m deep and 1.2-km wide Meteor Crater in Arizona that show that wakes form above the inner south-west crater sidewall
As the south-westerly katabatic wind flows over the approximately 30–50-m high crater rim, the flow either runs down the south-west sidewall because the air is colder than the ambient crater atmosphere or it separates from the surface, forming a wake over the inner sidewall
Summary
As airflow interacts with topography, that is, as air is either forced to flow over mountains or depressions or to flow around them, the resulting flow regimes and processes depend strongly on flow characteristics (i.e., stability and speed) and on topographic characteristics. Doyle and Durran (2002) defined a nondimensional pressure-gradient threshold criterion for post-wave flow separation based on idealized simulations Applying this criterion to observations in the lee of the Medicine Bow Mountains, Wyoming, Strauss et al (2016) found, that flow separation occurs for even smaller pressure gradients. One goal is to use the one-month long continuous measurements at Meteor Crater and compare periods with and without flow separation to identify the upstream flow conditions leading to flow separation and wake formation These results are compared to the flow-separation criterion by Baines and Hoinka (1985), which is based on laboratory experiments, to show that the criterion is useful in identifying flow separation in the atmosphere.
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