A 4-yr research program has been designed to follow-up on a serendipitous discovery that we made in prior experiments (METCRAX I) conducted at Arizona's Meteor Crater. Arizona's Barringer Meteorite Crater (AKA Meteor Crater) is a nearly ideal axisymmetric basin formed by the impact of a meteorite 49,000 years ago (Kring 2007). The bowl-shaped crater, 175 m deep and 1.2 km in diameter, is located on a plain (Fig. 1) that is tilted up slightly (1° angle) to the southwest toward Arizona's Mogollon Rim. The crater rim, unbroken by any major passes or saddles, extends 30-50 m above the surrounding plain. The crater floor elevation is 1563 m MSL. A photograph of the crater is provided in Figure 2.
Figure 1. Topography of the Meteor Crater region. Southwest of the crater is the Mogollon Rim, a high altitude ridge of mesas. A mesoscale drainage flow forms on the slope between the Mogollon Rim and the Meteor Crater, with an inversion depth of about 30-50 m and a drainage flow extending to 100-250 m AGL with a jet maximum height of 20-35 m.
Figure 2. Meteor Crater, looking southwest.
During experiments conducted in Meteor Crater on clear, light-background-wind nights in October 2006, we experienced occasional unusual meteorological events on the west sidewall of the crater. There, short-lived high wind and turbulence events occurred, while other sites within the crater were completely unaffected. These downslope-windstorm-type flows (DWFs) were confined to the immediate lee of the upwind rim of the crater, and many such events occurred during the one-month observational period. The temporary and often short-lived instability leading to these discrete events is apparently produced by changes in the vertical temperature or wind structure in the regional-scale drainage flow that impinges on the crater's rim from the terrain southwest of the crater. This drainage flow has a jet maximum and an inversion height that correspond roughly to the height of the crater rim above the adjacent plain. This site thus provides an unusual opportunity to investigate the sensitivity of DWFs in a stably stratified atmosphere to temporal changes in the ambient approach flow structure. The flow over the crater rim appears similar to that of hydraulic flows over obstacles in stream channels, and is thought to include the development of a hydraulic jump in the lee of the crater's ridge (Fig. 3).
Figure 3. Conceptual model for warm air intrusions in Arizona's Meteor Crater (Adler et al. 2011) showing the instrument layout. Isentropes (black) are shown.
We have used this fortuitous topographic/meteorological situation to design a field experiment and modeling program to systematically study this 'katabatically driven hydraulic flow' phenomenon in a natural setting. This has not been possible before and has the potential to greatly improve understanding of analogous atmospheric flows such as those that occur in Bora, Foehn and Chinook windstorm events, and in cascades of air over passes and through gaps and channels. Potential benefits of the project include improved understanding, analysis, and prediction of atmospheric flows leading to downslope windstorm-type events, which have many important societal impacts.
The nighttime mesoscale drainage flow approaches the crater from the southwest. The meteorological measurements will come from 5 experimental locations, as indicated in Fig. 3 on a southwest to northeast cross section.
The five sites are described as follows:
The research is designed to answer several scientific questions about atmospheric downslope-wind-type flows that form in stratified airflow over topographic ridges. Scientific questions include:
• What is the three-dimensional structure of downslope-windstorm-type flow that develops behind the circular ridge of Meteor Crater? How do these three-dimensional flows evolve? What intermediate changes in flow structure occur as the approach flow changes? What are their characteristics and climatology? How does the crater atmosphere respond to the warm air intrusion associated with the DWF? What role does the downstream stability inside the crater play in determining the depth of penetration of this flow?
• What are the controlling upstream parameters (e.g., inversion depth and stability, wind speed and vertical shear) that cause the DWF to develop? How does a blocked flow layer upwind of the circular rim of the crater modify the inflow? How much fluid is drawn from the upwind blocked layer as the flow goes over the ridge? What meteorological mechanism produces pulses in the approach flow that tip the fluid structure into a full-fledged downslope-wind-type flow?
• Which of the existing theories on DWFs is responsible for DWFs at Meteor Crater?
• Can existing mesoscale models produce accurate simulations of the evolution of the DWFs at Meteor Crater and for other idealized basins and ridges of different size and shape? Will parametric studies with the models be successful in defining the parameter space in which such flows can be expected and assist in leading to improved understanding that will provide practical benefits for forecasting of downslope windstorm events?
Further detailed information on the project can be found in our National Science Foundation proposal.
The goal of the fall 2013 field experiments is to answer the scientific questions posed above by designing a laboratory-like experiment where the approach flow is carefully monitored along with the atmospheric response inside the crater. The data will be used to support detailed meteorological analyses to fully characterize these events and to develop and test conceptual and atmospheric numerical models regarding their causes and the role of changes in the approach flow that affect their evolution.
The actual locations of the sites where equipment was located in the Fall 2013 experiment are shown in Figure 4 below. These sites follow the general plan given above (i.e., Sites A through E), except that logistics considerations (communications, cell phone coverage, radio interference of RASSes, access to roads, etc.) required adjustments to the locations of some of the sites.
Figure 4. Map of Meteor Crater and vicinity, showing actual equipment locations during the 2013 experiments. a) Meteor Crater’s location in Arizona, b) Meteor Crater environs, and c) Meteor Crater.
PDF of METCRAX equipment locations
Paste the following latitude and longitude into Google Earth to see 2- and 3-D images of Meteor Crater: 35°01′38.92″ N 111°01′20.24″ W
Numerical simulations of the flow over the Meteor Crater have already been performed in advance of the field experiments to assist in the location of meteorological instruments for the Fall 2013 field experiments. Those simulations have been used as input to make a series of simulations of the lidar volume that will be scanned by the Site D and E lidars to assist in interpreting field data and to gain further knowledge about the downslope-windstorm-type flows.
Idealized two-dimensional (2-D) and three-dimensional (3-D) simulations will be performed for the Meteor Crater. The idealized setup will allow us to control the upstream and in-crater conditions (i.e. stability and winds) and, thus, to determine the conditions necessary for the formation of DWFs in the lee of the crater rim. We will focus on the controlling parameters individually, changing one parameter at a time, to determine the respective change in the flow regimes. A simple initial simulation of the flow over the Meteor Crater is shown in Figure 6.
Figure 5. Initial simulation of the formation of a DWF in the Meteor Crater.
With the comparison of 2-D and 3-D simulations we will examine whether the DWFs can be described adequately by simple 2-D flow over a ridge. The circular crater topography most likely produces 3-D phenomena, such as flow splitting around the crater in contrast to a blocking of the flow by the crater rim, which may affect or determine the resultant flow regime.
We will investigate the upstream and in-crater atmospheric conditions systematically in a parametric study where individual parameters will be systematically varied to determine their influences on the flow. These simulations will be guided by the field observations and thus will be focused on those parameters that can be identified in the observations as controlling the formation of hydraulic jumps. Based on the data from the previous field campaign at Meteor Crater, upstream temperature conditions are likely to include (i) the depth of the surface inversion that forms on the sloping plain outside the crater, (ii) the inversion strength, (iii) the absolute temperature difference between the air that comes over the rim and the air inside the crater, and (iv) the temperature profile of the stagnant air that pools up ahead of the crater rim in contrast to the temperature profile further upstream. Upstream wind parameters may include (v) the height of the jet in the drainage flow with respect to the rim height, (vi) the jet depth with respect to the rim height, (vii) jet strength, (viii) wind shear at the height of the rim, and (ix) the possible deformation or lifting of the drainage flow by the cold-air pool upstream of the crater rim. In-crater conditions may include (x) the stability of the crater atmosphere, and (xi) capping inversions on top of the crater atmosphere.
Model simulations will be used to evaluate the effect of the unique crater topography and to generalize the findings from Meteor Crater. The spatial scale of Meteor Crater is small compared to most locations where severe downslope windstorms and atmospheric hydraulic jumps have been observed. Furthermore, the crater is strongly asymmetric, with a height of 30-50 m above the surrounding plain and a depth of 175 m above the crater floor. Thus, an important question to be answered is how the results from Meteor Crater can be scaled to larger topography and applied to ridges with different ratios of upstream ridge height to downstream ridge height.
Publicly accessible quick-look un-QAed figures from the 5-min average data sets are available now from the NCAR/EOL METCRAX II website. Separate webpages containing ISS and ISFS field notes, notes on equipment locations, data processing and quality assurance information, and manufacturer's information on the meteorological equipment used can be reached from this homepage, which also leads to password-protected digital data portals that are accessible only to project participants. After one year the password protection will be removed and all METCRAX II data will become publicly accessible.
Non-NCAR data are, similarly, accessible from a password-protected website at the University of Utah through the link below. These data come from meteorological measurements that supplemented those made by NCAR and come from the University of Utah, Karlsruhe Institute of Technology, Arizona State University, University of Oklahoma, University of Basel, and Ruhr University of Bochum.
METCRAX II data archive at UU
What meteorological equipment was used in METCRAX II?
What publications and presentations have come from this research program?
For additional information on the crater including photographs and scientific results from the 2006 experiments at Meteor Crater, see the webpage for our previous experiments.