Manuela Lehner

The MATERHORN: Unraveling the Intricacies of Mountain Weather

AbstractEmerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multi-institutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platfor...

Nocturnal Cold-Air Intrusions into a Closed Basin: Observational Evidence and Conceptual Model

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 crat...

A Mass Flux Model of Nocturnal Cold-Air Intrusions into a Closed Basin

AbstractObservations 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 detrain...

A Case Study of the Nocturnal Boundary Layer Evolution on a Slope at the Foot of a Desert Mountain

AbstractObservations 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 la...

Diurnal Cycle of Thermally Driven Cross-Basin Winds in Arizona’s Meteor Crater

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.

The METCRAX II Field Experiment: A Study of Downslope Windstorm-Type Flows in Arizona’s Meteor Crater

AbstractThe second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona’s Meteor Crater. The experiment was designed to investigate nighttime downslope windstorm−type flows that form regularly above the inner southwest sidewall of the 1.2-km diameter crater as a southwesterly mesoscale katabatic flow cascades over the crater rim. The objective of METCRAX II is to determine the causes of these strong, intermittent, and turbulent inflows that bring warm-air intrusions into the southwest part of the crater. This article provides an overview of the scientific goals of the experiment; summarizes the measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results.

The Thermally Driven Cross-Basin Circulation in Idealized Basins under Varying Wind Conditions

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 surfaceasafunctionofslopeinclinationandorientationtoproduceahorizontaltemperaturegradientacrossthe 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 backgroundwind-inducedcellinteractwitheachothereithertoenhanceortoreducethethermalcross-basinflowandreturn flow.Itisshownthatin5‐10-km-widebasinscross-basintemperaturedifferencesthatarerepresentativeofeastand 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.

Instrument configuration for dual-Doppler lidar coplanar scans: METCRAX II

Abstract The second Meteor Crater Experiment (METCRAX II) was designed to study downslope-windstorm-type flows occurring at the Barringer Meteorite Crater in Arizona. Two Doppler wind lidars were deployed to perform a coplanar dual-Doppler lidar analysis to capture the two-dimensional (2-D) vertical structure of these flows in the crater basin. This type of analysis allows the flow to be resolved on a 2-D Cartesian grid constructed in the range height indicator scan overlap region. Previous studies have shown that the dominant error in the coplanar dual-Doppler analysis mentioned above is due to the under sampling of radial velocities. Hence, it is necessary to optimize the setup and choose a scan strategy that minimizes the under sampling of radial velocities and provides a good spatial as well as temporal coverage of these short-lived events. A lidar simulator was developed using a large Eddy simulation wind field to optimize the lidar parameters for METCRAX II field experiment. A retrieval technique based on the weighted least squares technique with weights calculated based on the relative location of the lidar range gate centers to the grid intersection point was developed. The instrument configuration was determined by comparing the simulator retrievals to the background wind field and taking into account the limitations of commercially available lidars.

Warm-air intrusions in Arizona's meteor crater

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 layerintheambientcrateratmosphereremainedlargelyundisturbed.Newinvestigationsaresuggestedtotest the hypothesis that the warm-air intrusions are associated with hydraulic jumps.

The Upslope-Downslope Flow Transition on a Basin Sidewall

AbstractThe 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. Th...