Researchers shed new light on fog formation in mountainous areas

Of the world’s various weather phenomena, fog is perhaps the most mysterious, forming and dissipating close to the ground with fluctuations in air temperature and humidity interacting with the terrain itself.

Although fog poses a major danger to transportation safety, meteorologists have not yet figured out how to forecast it with the accuracy they have achieved for precipitation, wind and other storm events.

Indeed, the physical processes leading to the formation of fog are extremely complex, according to Zhaoxia Pu, professor of atmospheric sciences at the University of Utah.

“Our understanding is limited. In order to accurately forecast fog, we need to better understand the process that controls fog formation,” said Pu, who led a study of fog in a valley in northern Utah.

In a recent paper published by the American Mogenic Society, Pu and colleagues reported results from the Cold Fog Amongst Complex Terrain (CFACT) project, designed to study the life cycle of cold fog in mountain valleys.

Several other members of the University’s Department of Atmospheric Sciences, including Gannet Hallar and Sebastian Hoch, as well as Eric Pardyjak of the Department of Mechanical Engineering, a group of scientists from the National Center for Atmospheric Research (NCAR), also worked on the project. and Dr. Ismail Gultepe of Ontario Technological University, Canada.

Because it reduces visibility, fog presents serious dangers to the traveling public. For example, fog is the second leading cause of plane crashes after high winds. This leads to car accidents and disrupts ferry operations.

Between 1995 and 2004, 13,720 people in the United States died in fog-related accidents.

Improving fog forecasting would make travel safer, Pu said.

Today, most forecasting uses a computer model called numerical weather prediction (NWP), which processes massive weather observations with computer models to produce forecasts about precipitation, temperature, and all sorts of other weather elements. However, the current computer model does not work well for fog, and Pu’s team hopes improvements can be made using the masses of data collected over seven weeks during the winter of 2022 at several sites across the valley by Heber.

“Fog involves many physical processes, so it requires a computational model that can better represent all of these processes,” Pu said. “As fog consists of clouds close to the ground, it requires a high-resolution model to resolve it. So we need models at a very fine scale, which are very computationally expensive. Current models (resolution relatively coarser) are not capable of resolving fog processes, and we need to improve models for better fog forecasting.

Located about 50 miles southeast of Salt Lake City, the Heber Valley is nestled behind the Wasatch Mountains and framed by two major reservoirs on the Provo River.

This picturesque basin is a typical mountain valley, bordered by Mount Timpanogos and other high peaks, with the reservoirs serving as a source of moisture. The seven-week study window covered the time of year when the Heber Valley is at its foggiest.

Valley fog is a perfect example of how topography and atmospheric processes converge to create a distinctive weather phenomenon.

The ground cools overnight as denser, cooler air descends from mountain peaks and collects in valleys, in a phenomenon known as “cold air drainage.” Cooled by the ground, the falling air temperature can approach the dew point, and if there is enough moisture in the air, fog begins to form, becoming densest as it rises from the sun, when surface temperatures are lowest.

Winter nights create favorable conditions for different forms of fog, such as cold air sheet fog, ephemeral mountain valley fog, and radiative ice fog.

The Heber Valley Project focused on cold air fog that forms at freezing temperatures below zero degrees Celsius, according to Pu. However, by observing how these different types of fog form and dissipate, researchers continue to learn more about the weather conditions and physical processes governing fog formation.

For the CFACT study, the NCAR and U team set up two main data collection stations, one near Deer Creek Reservoir and another a few miles up the Provo River. These are low points in the valley, about 5,450 feet above sea level, that experience the densest fog. These sites were equipped with 100 foot towers to support an array of instruments capturing various weather data associated with humidity, wind, visibility, temperature, even snow depth and humidity of the ground. The recordings were made using in situ and remote sensing platforms.

Additionally, the team recorded fewer data points at nine satellite sites.

During the seven-week CFACT field campaign, nine intensive observation periods (IOPs), each conducted over 24-hour periods, produced a dataset including high-frequency radiosonde profiles, balloon profiles captive, remotely sensed thermodynamic and wind profiles, surface meteorological observations. and microphysical and aerosol measurements.

In addition to fog IOPs, the variety of non-fog IOPs have provided valuable observations for understanding near-surface inversion, ice crystal formation, moisture advection and transport, and boundary layers. stable in complex terrain, all of which are essential factors linked to fog formation. Extensive studies are underway to better understand cold fog in complex terrain.

The study was published on November 15 in the Bulletin of the American Meteorological Society. U researchers involved in the study included Zhaoxia Pu, Sebastian Hoch, A. Gannet Hallar, Rebecca Beal, Geraldo Carrillo-Cardenas, Xin Li and Maria Garcia of the Department of Atmospheric Sciences and Eric Pardyjak and Alexei Perelet of the Department of mechanical Engineering.