A three-year meteorological research program will be conducted to investigate the diurnal development of the nocturnal stable boundary layer (SBL) in the closed basin of Arizona's Meteor Crater. An extensive data set was collected there by the Principal Investigators (PIs) under NSF funding during October 2006. The experimental design in this small, circular, and comprehensively instrumented experimental basin has parallels to laboratory experiments. The field campaign provided a data set uniquely suited to support innovative analyses to answer extant scientific questions about SBL evolution within topography. The goal of the research is to determine the physical processes leading to SBL evolution in a closed basin, with the objectives of identifying the roles of drainage flows, turbulence, radiative transfer, and larger-scale ambient flows in SBL evolution.
Novel and innovative concepts are featured with analyses and modeling informed by field experience and the PIs' initial analyses. The PIs will perform the first comprehensive investigation of SBL asymmetries during the transition periods when differential insolation occurs on the basin sidewalls. Hypotheses will be tested to determine the cause of unusual SBL thermal structure evolution in the crater. The topographic amplification factor concept will be tested for the first time with observational data. The relative roles of slope flows, radiative transfer and sensible heat flux in SBL development will be diagnosed. An analytical model of self-induced katabatic flow shutdown in basins will be developed. Exploratory investigations of turbulence characteristics will be made for the floor, sidewalls, and rim of a basin, as well as over the surrounding plain.
Intellectual Merit: The research will advance knowledge and understanding of the physical processes that affect SBLs in complex terrain, and this knowledge is expected to lead to improvements in models and, ultimately, in weather forecasts for the western U.S. and throughout the world. The work explores innovative approaches and concepts and uses a combination of analyses and numerical modeling to gain understanding. Separately funded collaborators from other institutions and countries will contribute to the research.
Broader Impacts: Potential benefits to society will accrue through improved understanding of SBL evolution with potential applications for air pollution dispersion, general and fire weather forecasting, and climate. Broader societal impacts are promoted through the infusion of the research into university teaching, the support of undergraduate and graduate students, the promotion of investigator/student diversity, and the development of training courses, workshops and seminars. Project results will be widely disseminated through peer-reviewed scientific publications, teaching modules, scientific presentations and web sites.
Temperature inversions and their evolution in basins and valleys have a strong effect on the activities, health and wellbeing of local residents. During inversion episodes, cold and dense air accumulates on the floors of valleys and basins and mixing is inhibited. In heavily populated basins this often leads to the accumulation of hazardous air pollutants and to unhealthy episodes of smog conditions. The frequent occurrence of inversions also influences the ecology of valley and basin sidewalls. This project investigated the evolution of nocturnal inversions and their post-sunrise breakup in a natural, small-scale, circularly symmetrical basin - the Barringer Crater (a.k.a. the Arizona Meteor Crater). The crater, formed by the impact of a 50 to 100-m diameter meteorite approximately 49000 years ago, is an especially apt location for research because of its simple shape. A one-month meteorological field study was conducted at the privately owned crater in October 2006 in a previously funded project. Funding for the current project has led to further analysis of these data, to the training of 3 M.S. and three Ph.D. students, to several international collaborations, and to the publication of a book chapter, a Ph.D. thesis, two M.S. theses that are presently in the final stages of writing, and 12 published journal articles, with one additional article submitted and one in preparation. Some of the major scientific findings are briefly enumerated below: - surface radiative and energy budgets play a strong role in inversion evolution. This study has expanded knowledge of these effects using observations and numerical models. A sophisticated numerical model of atmospheric radiative transfer developed by a German collaborator was, for the first time, applied to complex terrain on this scale, and provided new information on the role of radiative gradients in producing atmospheric cooling and heating over slopes and within basins. - cold air intrusions occur over the rim of Meteor Crater due to nighttime regional-scale drainage flows that are deeper than the rim. These intrusions cause distinct changes to the temperature structure and its evolution, producing generally weaker atmospheric stability except for a shallow layer on the basin floor. We developed simple conceptual and mathematical models (analytical and numerical) with an Austrian collaborator that have improved understanding of these effects. We collaborated with a different Austrian group in a parallel study for an Alpine basin that has enumerated eight categories of external influences that can affect the temperature structure in basins. - a cross-basin flow from the cold sidewall to the warm sidewall occurs near the floor of a basin during situations where one sidewall receives more solar radiation than the opposing sidewall, with a weaker return flow going towards the cold sidewall above the near-ground flow. These cross-basin or cross-valley flows are especially marked in the morning just after sunrise. We have observed cross-basin flows in the Meteor Crater and have modeled them with a sophisticated numerical flow model, greatly increasing the understanding of the mechanisms leading to these flows, and evaluating the flows in basins of different size and under different background wind speeds. - warm air intrusions occur intermittently on one sidewall of the basin leading to atmospheric flows that are similar to flows over projecting rocks in streams or rivers and lead to a phenomenon known as a hydraulic jump. We developed a conceptual model of these 'hydraulic flows'. We believe that further investigations will produce a better understanding of the physical mechanisms causing downslope windstorms. - moderate to strong background airflows approaching the crater produce winds that are weaker but much more turbulent inside the crater. This phenomenon appears to be a wake effect. Our analyses have quantified this important effect, which we believe will occur in other basins and valleys. In addition to the project's intellectual merits, the project had substantial broader impacts. The project provided funding and guidance to students working on advanced degrees and provided opportunities for other students to gain experience in field programs. It provided data sets, conceptual models, and analytical and numerical models that can be used to test and evaluate understanding of inversions in other locations and situations. The project funded new research equipment including a Doppler SoDAR (which measures wind profiles using sound) that can be used in future projects by other scientists and students. While not anticipated at the beginning of the project, the findings have been of interest to scientists investigating craters on Mars. Additionally, we have discovered that the Meteor Crater is a natural small-scale setting where downslope windstorm mechanisms could be investigated without having to use expensive research aircraft. The research findings have been broadly disseminated through scientific publications, seminars, workshops, conference presentations and graduate and undergraduate classes.
