A three-year research project will be conducted to investigate processes leading to the formation, maintenance and destruction of persistent, multi-day, mid-winter temperature inversions or cold-air pools that form in the Salt Lake basin (SLB). These persistent inversions occur frequently as well in other regions throughout the western US and throughout the world, and their initiation and breakup are quite difficult to forecast. Air pollution can reach unacceptably high levels during persistent inversions in urban basins, and when the inversion breaks the breakup can carry pollution vertically to produce regional-scale air pollution and climate impacts. Because of poor mixing, fog and stratus can build up in the inversion leading to hazardous episodes of persistent freezing rain, drizzle or fog and interfering with ground transportation and aviation.

The objectives of the project are to 1) identify the meteorological processes that lead to buildup, maintenance and breakup of persistent inversions, 2) determine the consequences of these processes on air pollution transport and diffusion in urban basins, and 3) determine how meteorological models can be improved to provide more accurate simulations of such persistent inversions.

The project includes observations, analysis, and modeling. The initial focus will be modeling of an event in the winter of 2000-2001 using existing meteorological data. The analyses of this recent event study will identify the key physical mechanisms leading to persistent inversion formation and destruction in this case.

A field experiment will be conducted in the SLB from 1 Dec 2010 to 7 Feb 2011 to obtain the detailed meteorological data on several more cases needed for further model simulations and analyses. A mesoscale meteorological model will be used to interpret the observations, verify hypotheses, provide further insight into physical mechanisms, and evaluate the air pollution implications of the inversion processes. The simulations will take account of the often-observed fog and stratiform clouds that form and dissipate within the SLB inversions.

Intellectual Merit: The project will advance knowledge and understanding of the physical processes that affect persistent inversions. This increase in 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 uses innovative approaches, employs conceptual models of relevant physical processes resulting from earlier research, and uses a combination of analyses of prior data and collection of new data and numerical modeling to gain understanding.

Broader Impacts: Broader societal impacts are promoted through the integration of the research into university teaching, through the support of undergraduate and graduate students and through the promotion of investigator/student diversity. Project results will be widely disseminated through peer-reviewed scientific publications and presentations. The results have potential benefits to society through improved understanding of cold pools with potential applications for better understanding air pollution dispersion, weather forecasting and climate.

Project Report

The Persistent Cold-Air Pool Study (PCAPS) investigated the meteorology of multi-day wintertime stable layers that form in Utah's Salt Lake Valley. Because the stable layers are persistent and have the effect of reducing the vertical mixing of valley air with air above the surrounding mountains, particulate air pollutants and moisture collect within the valley, causing persistent air pollution and the occasional buildup of fogs and stratiform clouds. The trapping of cold air in valleys can also lead to freezing rain and drizzle events when warm rain falls into the cold valley. The reduced visibility and freezing precipitation cause transportation problems and the murky air affects the health and outlook of the residents. PCAPS began with the planning and execution of a wintertime meteorological study in Utah's Salt Lake Valley that ran from 1 December 2010 through 7 February 2011 and involved participants from many research institutions and federal and state agencies. Local residents provided photographs and comments through social media, and local landowners provided land for installation of the extensive set of meteorological instruments. Graduate students in the Atmospheric Sciences department ran the field program, with supervision from faculty. Many university students gained their first field experience when they volunteered to participate in this project, which had a high level of interest from the local population and media. Atmospheric graduate students planned special instrument deployments to answer specific scientific questions and used the resulting data in analyses that led to graduate degrees. Post-experiment analyses found that the depth and strength of the valley aerosol and stable layers was governed by a range of physical processes. Cold stable air in the lower valley often was initiated as a nocturnal inversion, often supplemented by cold air advection down the sloping sidewalls and inflow from the tributary canyons, or as cold air left in the valley by cold frontal passages. More frequently, however, the depth and strength of the stable layer in the valley were affected by differential advection. The stable layer strengthened when warm air was advected over the valley at mountaintop level, usually with the approach of a high-pressure ridge or center. Stable layers also occasionally subsided into the valley from middle levels of the troposphere. Cold air advection aloft, usually associated with the approach of a trough or low-pressure center, decreased inversion strength. Local-scale processes such as along-valley advection, evaporation from the ground, and sensible heat flux convergence and divergence provided a daily modulation of stable layer strength. Weak drainage flows at night carried the cold air down the valley and out over the Great Salt Lake to the north of the valley. The next day, if the air over the land surrounding the lake was heated by convection to become warmer than the air over the lake, the cold air over the lake, with its burden of pollutants, would be carried back up the valley. The lake breeze was not anticipated to be a key feature of winter meteorology and was a key new finding. A second key finding was the partial breakup of the valley cold-air pool when strong south winds, usually as a precursor to a trough approaching from the west, came into the valley over the Traverse Mountains at the valley's south end. A wave that forms in the lee of the Traverse Mountains brings warm air to the ground at the south end of the valley, pushing the cold-air pool in the valley to the north and exposing the south end of the valley to warm, clean air. A collaborator from the Chemical Engineering department led an investigation showing that atmospheric fine particulate concentrations generally decrease with height in the valley. A collaborator from the Biology Department found that cold-air pool aerosols were deposited into the snowpack on the valley's east sidewall. A climatological investigation used 40 winters of meteorology and air quality data. A vertically integrated bulk measure of atmospheric stability in the valley was most closely related to daily fine particulate concentrations. Vertically integrated wind speed, temperature, and relative humidity and snow cover were also important. Air quality varies greatly from winter to winter depending on the relative frequency of large-scale weather patterns. No statistically significant long-term trends in inversion strength were found over the 40 year period of record. The multi-day fine particulate episodes are triggered by an increase in stability past a certain threshold, with the pollutant concentration increasing day by day thereafter. Numerical simulations of the wintertime cold-air pools, southerly flow stable layer destruction and lake breeze recycling have been performed successfully by co-PI Sharon Zhong and University of Utah students. Results from this research project have been published in peer-reviewed journals, and have been presented at conferences, local workshops and in public forums. Webpages also introduce the research program and its findings.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Bradley F. Smull
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University of Utah
Salt Lake City
United States
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