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Richard Rotunno and Manuela Lehner

Abstract

Observations and models of nocturnal katabatic winds indicate strong low-level stability with much weaker stability aloft. When such winds encounter an embedded depression in an otherwise smooth sloping plane, the flow responds in a manner that is largely describable by the inviscid fluid dynamics of stratified flow. Building on earlier work, the present study presents a series of numerical simulations based on the simplest nontrivial idealization relevant to the observations: the height-independent flow of a two-layer stratified fluid past a two-dimensional valley. Stratified flow past a valley has received much less attention than the related problem of stratified flow past a hill. Hence, the present paper gives a detailed review of existing theory and fills a few gaps along the way. The theory is used as an interpretive guide to an extensive set of numerical simulations. The solutions exhibit a variety of behaviors that depend on the nondimensional input parameters. These behaviors range from complete flow through the valley to valley-flow stagnation to situations involving internal wave breaking, lee waves, and quasi-stationary waves in the valley. A diagram is presented that organizes the solutions into flow regimes as a function of the nondimensional input parameters.

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Manuela Lehner and C. David Whiteman

Abstract

The Weather Research and Forecasting model is used to perform large-eddy simulations of thermally driven cross-basin winds in idealized, closed basins. A spatially and temporally varying heat flux is prescribed at the surface as a function of slope inclination and orientation to produce a horizontal temperature gradient across the basin. The thermal asymmetry leads to the formation of a closed circulation cell flowing toward the more strongly heated sidewall, with a return flow in the upper part of the basin. In the presence of background winds above the basin, a second circulation cell forms in the upper part of the basin, resulting in one basin-sized cell, two counterrotating cells, or two cells with perpendicular rotation axes, depending on the background-wind direction with respect to the temperature gradient. The thermal cell near the basin floor and the background-wind-induced cell interact with each other either to enhance or to reduce the thermal cross-basin flow and return flow. It is shown that in 5–10-km-wide basins cross-basin temperature differences that are representative of east- and west-facing slopes are insufficient to maintain perceptible cross-basin winds because of reduced horizontal temperature and pressure gradients, particularly in a neutrally stratified atmosphere.

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Manuela Lehner, Richard Rotunno, and C. David Whiteman

Abstract

Idealized two-dimensional model simulations are performed to study the frequent nocturnal occurrence of downslope-windstorm-type flows in Arizona’s Meteor Crater. The model topography is a simplified representation of the Meteor Crater and its surroundings, with an approximately 1° mesoscale slope upstream and downstream of the crater basin. A strong surface-based inversion and a katabatic flow develop above the mesoscale slope as a result of radiational cooling. The temperature and flow profiles are evaluated against observations over low-angle slopes from two field campaigns, showing that the model’s turbulence parameterization has a strong impact on the near-surface conditions. The interaction of the katabatic flow with the basin topography leads to the formation of waves and hydraulic jumps over the basin. The simplified two-dimensional simulations show good qualitative agreement with observations of downslope-windstorm-type flows from the Meteor Crater. The sensitivity of the flow solution over the basin to basin depth, basin width, and background wind speed is investigated. The resulting flow regimes include a sweeping of the basin atmosphere, a wake over the upstream crater sidewall, waves over the basin with one or two wave crests, and a hydraulic jump. The regimes are discussed in the context of stratified flow over mountains.

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Manuela Lehner, C. David Whiteman, and Sebastian W. Hoch

Abstract

Cross-basin winds produced by asymmetric insolation of the crater sidewalls occur in Arizona’s Meteor Crater on days with weak background winds. The diurnal cycle of the cross-basin winds is analyzed together with radiation, temperature, and pressure measurements at the crater sidewalls for a 1-month period. The asymmetric irradiation causes horizontal temperature and pressure gradients across the crater basin that drive the cross-basin winds near the crater floor. The horizontal temperature and pressure gradients and wind directions change as the sun moves across the sky, with easterly winds in the morning and westerly winds in the evening. A case study of 12 October 2006 further illustrates the obtained relation between these parameters for an individual day. The occurrence of an elevated cross-basin flow on 23 October 2006 is shown to relate to the presence of an elevated inversion layer.

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Thomas Haiden, C. David Whiteman, Sebastian W. Hoch, and Manuela Lehner

Abstract

Observations made during the Meteor Crater Experiment (METCRAX) field campaign revealed unexpected nighttime cooling characteristics in Arizona’s Meteor Crater. Unlike in other natural closed basins, a near-isothermal temperature profile regularly develops over most of the crater depth, with only a shallow stable layer near the crater floor. A conceptual model proposed by Whiteman et al. attributes the near-isothermal stratification to the intrusion, and subsequent detrainment, of near-surface air from outside the crater into the crater atmosphere. To quantify and test the hypothesis, a mass flux model of the intrusion process is developed. It is found that the observed temperature profile can be reproduced, providing confirmation of the conceptual model. The near-isothermal stratification can be explained as a result of progressively cooler air entering the crater and detraining into the atmosphere, combined with the finite time of ascent in the compensating rising motion. The strength of detrainment largely determines the characteristics of the cooling process. With weak detrainment, most of the cooling arises from an adiabatic rising motion (“filling-up” mode). Stronger detrainment leads to reduced rising motion and enhanced cooling at upper levels in the crater (“destabilization” mode). Of interest is that the detrainment also reduces the total cooling, which, for a given intrusion mass flux, is determined by the temperature difference between the intruding air and the crater atmosphere at rim height.

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C. David Whiteman, Sebastian W. Hoch, Manuela Lehner, and Thomas Haiden

Abstract

Observations are analyzed to explain an unusual feature of the nighttime atmospheric structure inside Arizona’s idealized, basin-shaped Meteor Crater. The upper 75%–80% of the crater’s atmosphere, which overlies an intense surface-based inversion on the crater’s floor, maintains a near-isothermal lapse rate during the entire night, even while continuing to cool. Evidence is presented to show that this near-isothermal layer is produced by cold-air intrusions that come over the crater’s rim. The intrusions are driven by a regional-scale drainage flow that develops over the surrounding inclined Colorado Plateau. Cold air from the drainage flow builds up on the upwind side of the crater and splits around the crater at low levels. A shallow layer of cold air, however, spills over the 30–60-m-high rim and descends partway down the crater’s upwind inner sidewall until reaching its buoyancy equilibrium level. Detrainment of cold air during its katabatic descent and compensatory rising motions in the crater atmosphere destabilize the basin atmosphere, producing the observed near-isothermal lapse rate. A conceptual model of this phenomenon is presented.

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C. David Whiteman, Manuela Lehner, Sebastian W. Hoch, Bianca Adler, Norbert Kalthoff, and Thomas Haiden

Abstract

The interactions between a katabatic flow on a plain and a circular basin cut into the plain and surrounded by an elevated rim were examined during a 5-h steady-state period during the Second Meteor Crater Experiment (METCRAX II) to explain observed disturbances to the nocturnal basin atmosphere. The approaching katabatic flow split horizontally around Arizona’s Meteor Crater below a dividing streamline while, above the dividing streamline, an ~50-m-deep stable layer on the plain was carried over the 30–50-m rim of the basin. A flow bifurcation occurred over or just upwind of the rim, with the lowest portion of the stable layer having negative buoyancy relative to the air within the crater pouring continuously over the crater’s upwind rim and accelerating down the inner sidewall. The cold air intrusion was deepest and coldest over the direct upwind crater rim. Cold air penetration depths varied around the inner sidewall depending on the temperature deficit of the inflow relative to the ambient environment inside the crater. A shallow but extremely stable cold pool on the crater floor could not generally be penetrated by the inflow and a hydraulic jump–like feature formed on the lower sidewall as the flow approached the cold pool. The upper nonnegatively buoyant portion of the stable layer was carried horizontally over the crater, forming a neutrally stratified, low–wind speed cavity or wake in the lee of the upwind rim that extended downward into the crater over the upwind sidewall.

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Bianca Adler, C. David Whiteman, Sebastian W. Hoch, Manuela Lehner, and Norbert Kalthoff

Abstract

Episodic nighttime intrusions of warm air, accompanied by strong winds, enter the enclosed near-circular Meteor Crater basin on clear, synoptically undisturbed nights. Data analysis is used to document these events and to determine their spatial and temporal characteristics, their effects on the atmospheric structure inside the crater, and their relationship to larger-scale flows and atmospheric stability. A conceptual model that is based on hydraulic flow theory is offered to explain warm-air-intrusion events at the crater. The intermittent warm-air-intrusion events were closely related to a stable surface layer and a mesoscale (~50 km) drainage flow on the inclined plain outside the crater and to a continuous shallow cold-air inflow that came over the upstream crater rim. Depending on the upstream conditions, the cold-air inflow at the crater rim deepened temporarily and warmer air from above the stable surface layer on the surrounding plain descended into the crater, as part of the flowing layer. The flow descended up to 140 m into the 170-m-deep crater and did not penetrate the approximately 30-m-deep crater-floor inversion. The intruding air, which was up to 5 K warmer than the crater atmosphere, did not extend into the center of the crater, where the nighttime near-isothermal layer in the ambient crater atmosphere remained largely undisturbed. New investigations are suggested to test the hypothesis that the warm-air intrusions are associated with hydraulic jumps.

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Daniel Martínez Villagrasa, Manuela Lehner, C. David Whiteman, Sebastian W. Hoch, and Joan Cuxart

Abstract

The late afternoon upslope–downslope flow transition on the west inner sidewall of Arizona’s Meteor Crater, visualized by photographs of smoke dispersion, is investigated for 20 October 2006 using surface radiative and energy budget data and mean and turbulent flow profiles from three towers, two at different distances up the slope and one on the basin floor. The bowl-shaped crater allows the development of the upslope–downslope flow transition with minimal influence from larger-scale motions from outside and avoiding the upvalley–downvalley flow interactions typical of valleys. The slow downslope propagation of the shadow from the west rim causes a change in the surface radiation budget and the consequent loss of heat from the shallow atmospheric layer above the western slope at a time when the sun still heats the crater floor and the inner east sidewall. The onset of the katabatic flow is visualized by the dispersion of the smoke, and the onset occurs at the same time at the two slope towers. The katabatic flow arrives later at the crater floor, cooling the air and contributing to the stabilization of a shallow but strong inversion layer there. A wavelet analysis indicates that the initial upslope current is driven by crater-size scales, whereas the later downslope flow is influenced by the thermal gradient between opposing sidewalls generated by their different cooling rates. A comparison with other days suggests that the timing of the transition is also influenced by the presence of convective eddies in addition to the local energy balance.

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Manuela Lehner, C. David Whiteman, Sebastian W. Hoch, Derek Jensen, Eric R. Pardyjak, Laura S. Leo, Silvana Di Sabatino, and Harindra J. S. Fernando

Abstract

Observations were taken on an east-facing sidewall at the foot of a desert mountain that borders a large valley, as part of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) field program at Dugway Proving Ground in Utah. A case study of nocturnal boundary layer development is presented for a night in mid-May when tethered-balloon measurements were taken to supplement other MATERHORN field measurements. The boundary layer development over the slope could be divided into three distinct phases during this night: 1) The evening transition from daytime upslope/up-valley winds to nighttime downslope winds was governed by the propagation of the shadow front. Because of the combination of complex topography at the site and the solar angle at this time of year, the shadow moved down the sidewall from approximately northwest to southeast, with the flow transition closely following the shadow front. 2) The flow transition was followed by a 3–4-h period of almost steady-state boundary layer conditions, with a shallow slope-parallel surface inversion and a pronounced downslope flow with a jet maximum located within the surface-based inversion. The shallow slope boundary layer was very sensitive to ambient flows, resulting in several small disturbances. 3) After approximately 2300 mountain standard time, the inversion that had formed over the adjacent valley repeatedly sloshed up the mountain sidewall, disturbing local downslope flows and causing rapid temperature drops.

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