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.
The stable boundary layer over mountainous terrain is one of the most challenging subjects in the atmospheric sciences. The goal of this research project is to determine the sensitivity of stable boundary layer evolution in a closed basin to various physical processes, including drainage flows, turbulence, radiative transfer, and larger-scale ambient flows. The current project is a follow up to an earlier project that successfully collected a rich meteorological dataset on boundary layer evolution in Arizona’s Meteor Crater, a small, bowl-shaped basin. Meteor Crater is surrounded by a rim 30-60 m above the level of the surrounding plain of the Colorado Plateau, and is about 1200 m wide at the rim level and 160 m deep. Analysis of the observational data, augmented by fine-scale numerical modeling using the Advanced Regional Prediction System (ARPS), has succeeded in providing insight into stable boundary layer development inside the closed basin and the disturbing effect of the basin and the rim on mean and turbulent flows inside the basin and over the surrounding plain. Some of the major scientific findings are highlighted below: Development of an isothermal layer – a characteristic feature of the nocturnal boundary layer in a small basin An important feature of the observed thermodynamic structure of the nocturnal boundary layer in a small basin is the development of an isothermal layer over much of the basin depth, in contrast to a temperature inversion in a nocturnal boundary layer over flat terrain. Fine-scale numerical modeling using the ARPS model showed that a cold-air intrusion into the basin and compensating adiabatic ascent are key factors in the development of the isothermal layer. Meanwhile, radiative loss at the basin floor leads to the development of a strong, shallow surface inversion beneath the isothermal layer. Terrain features as small as the 30-m-high rim may significantly reduce mean wind speed in areas upstream and downstream of the terrain. Fine-scale model simulations carried out with and without the rim indicate that the rim can reduce the mean wind speed by 10-20% several hundred meters upstream and downstream of the basin. Although such a small reduction in wind speed may not be important for weather predictions, it can be significant for applications such as wind turbine siting where the power extracted from the wind is a function of the third power of wind speed. Hence, even in the areas of relatively flat terrain, such as the Great Plains of the United States, the effects of small terrain features should be considered for wind turbine siting. Small terrain features, such as the crater basin and its rim, can serve as powerful turbulence generators. Comparisons of turbulence observed inside and outside the basin revealed a considerably higher turbulence level inside the basin compared to over the adjacent plain on days with moderate to strong winds. While the mean wind speed inside the basin is found to be less than half the speed of the approaching flow, the turbulent kinetic energy inside the basin is double or triple what is observed over the plain. In other words, the basin, although small, is efficiently sheltered from mean winds, and is a powerful generator of turbulent motions. ARPS numerical simulations also revealed a strong production of turbulence in the wake of the basin by the rim. Empirical/semi-empirical formulae derived from data over flat terrain and commonly used in weather and climate models are inaccurate for describing turbulence in the surface layer within and downstream of the basin The mean and turbulence data taken from the flux towers inside the basin did not support the well-known empirical/semi-empirical relationships commonly used to describe the surface boundary layer in numerical weather prediction and climate models. The errors are especially large for stable conditions. This may explain the relatively poor performance of numerical weather prediction and climate models over areas of complex terrain, particularly for near surface variables and under stable conditions. Details of these and other findings are summarized in three peer-reviewed publications and a number of conference proceeding papers. The results have also been incorporated into a book chapter. In addition to the intellectual merits, the project also enriched the research experience of two Ph.D. students and two M.S. students including students from under-represented groups in sciences and from disciplines other than the atmospheric sciences (geography and statistics). The project also helped initiate international collaborations between Michigan State University and the University of Canterbury (New Zealand) and Lanzhou University (China). The project contributed to ARPS model improvements, including added routines for process analyses and turbulence analyses and a new stable boundary layer parameterization scheme. The results from this project have also been used in the curriculum of two advanced graduate courses (GEO890 and GEO892, Topic: Weather and Climate Modeling) at Michigan State University.
