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Persistent Cold-Air Pool Study

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Co-PIs: C. David Whiteman1, John Horel1 and Sharon Zhong2
1University of Utah; 2Michigan State University

A three-year research grant has been received from the National Science Foundation to investigate the processes leading to the formation, maintenance and destruction of persistent mid-winter temperature inversions (cold-air pools) that form in the Salt Lake basin. These cold-air pools occur frequently in the western US and throughout the world and their initiation and breakup are quite difficult to forecast. Air pollution can reach unacceptably high levels in urban cold-air pools and that pollution can be transported away during their breakup leading to regional-scale air pollution episodes.  In addition, fog and stratus can build up in the cold pool, which can contribute to hazardous episodes of persistent freezing rain, drizzle or fog affecting ground transportation and aviation.
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The objectives of the proposed research are 1) to identify the meteorological processes that lead to the buildup, maintenance and breakup of persistent inversions, 2) to determine the consequences of these processes on air pollution transport and diffusion in urban basins, and 3) to determine how meteorological models can be improved to provide more accurate simulations of them.

Image: Salt Lake Valley cloudy cold pool as viewed from Little Cottonwood Canyon

Our research project includes observations, analysis, and modeling. Analyses of past cold-air pool events and the development of a climatology of such events will be conducted in parallel with numerical simulations of past events to investigate and identify key physical mechanisms that lead to inversion formation and destruction.

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A field experiment will be conducted in the Salt Lake Basin from 1 Dec 2010 to 7 Feb 2011 using field research equipment provided in part by the U.S. National Center for Atmospheric Research. This PCAPS field experiment will help to provide the meteorological data needed to improve understanding of these events through further analyses and model simulations.

Undergraduate and graduate students will be heavily involved in the planning and conduct of the research program, which will provide field experience and other educational opportunities. The research program will be integrated into instruction in several classes during the 2010-2011 academic year. 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 inversions.

PCAPS Field Research Equipment

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Seven NCAR Integrated Surface Flux System (ISFS) 30 ft flux towers will be located at locations in the Salt Lake Valley, with radio communication to a central command post on the University of Utah campus.

Each tower will be instrumented as follows:

  • All components of the surface energy budget will be measured including sensible and latent heat fluxes, net all-wave radiation and ground heat flux.
  • Temperature, humidity, and wind will be measured.
  • A separate mounting system will be used to measure net radiation, with some towers measuring the individual solar and longwave radiation components.
  • Spatial averages of soil parameters will be obtained from soil heat flux sensors, soil temperature sensors, soil moisture sensors, and soil thermal properties sensors.

An NCAR Integrated Sounding System (ISS) will be located near the valley center to measure:

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  • Six-hourly atmospheric soundings of temperature, humidity, pressure and winds during persistent inversions using free-flying baloons carrying GPS rawinsondes
  • continuous vertical wind and temperature profiles to heights of 1.5 km (Radar Wind Profiler with Radio Acoustic Sounding System)
  • continuous vertical wind and temperature profiles to heights of 200 m AGL (Mini-SODAR with Radio Acoustic Sounding System)
  • surface conditions from a weather station measuring temperature, humidity, pressure, wind, radiation, and precipitation.

A variety of instrumentation will be deployed by the Mountain Meteorology Group of the UU Department of Atmospheric Sciences, including:

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  • a mini-SODAR to measure continuous vertical wind soundings to heights of 200 m AGL
  • portable weather stations to measure temperature, humidity, and wind at critical locations around the valley
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  • a large array of near-surface air temperature sensors to measure temperature profiles on Oquirrh and Wasatch Mtn slopes
  • video cameras to provide high resolution time-lapse video images of the Salt Lake Valley
  • two portable rawinsonde sets to measure vertical profiles of wind, temperature and humidity using free-flying helium-filled balloons

Additionally, PCAPS will involve efforts from numerous other departments, agencies, buisnesses and citizens:

The UU Biology Department will assist in providing information on particulate deposition to the winter snowpack.

The UU Mechanical Engineering Department will provide additional information on heat and moisture fluxes from fixed instrument sites within the valley.

The Utah Division of Air Quality, with assistance from UU's Chemical Engineering Department, will deploy a line of volumetric filter samplers to collect vertical profiles of particulate matter (PM2.5) on one of the valley sidewalls.

The Utah Department of Transportation will deploy additional weather stations in the valley.

Weber State University will fly a tethered balloon system to obtain information on the temperature structure of cold-air pools over the Great Salt Lake.

Dugway Proving Ground will provide vertical wind soundings from a radar wind profiler operated at Dugway, and will supply numerical model simulations of the Salt Lake Valley area.

The NOAA National Weather Service Forecast Office at Salt Lake City will assist with weather forecasting of cold-air pools.

Many public-spirited business and private citizens will assist in data collection and in providing operating sites for research equipment.
Other information about PCAPS

The following are a collection of public information regarding PCAPS, including presentations, previous research, and links to related or involved resources. For any additional information, please contact one of PCAPS' Co-PIs

Contact us for more information:

C. David Whiteman  John Horel and Sharon Zhong