UCAR Communications

staff notes monthly

May 2003

For some staff, the March 2003 storm provided more than a day off. Doug Wesley of COMET is examining the storm from a mesoscale modeling viewpoint in order to understand the processes that caused widespread snowfall of historic proportions. He’s working with Greg Poulos (Colorado Research Associates), Mike Meyers (National Weather Service), John Snook (Colorado Research Assciates), and Ed Szoke (NOAA). According to Doug, smaller-scale processes such as the influence of mountains were responsible for the local variations in wind and snow that left some people shoveling for days while others walked down clean sidewalks. The team’s modeling will incorporate several unique observations about the storm: snowfall six feet or deeper on both sides of the Continental Divide; a fetch of warm, moist air with a conveyor belt appearance that moved from the Gulf of Mexico into northeastern Colorado; and unexpectedly light accumulations in a few locations, such as Lyons. Doug hopes to share some preliminary insights into the storm at the UCAR/National Weather Service Winter Storm Symposium in late May in FL2.

During the Investigation of Sulfur Chemistry in the Antarctic Troposphere, a joint venture between NCAR and the Georgia Institute of Technology, ACD’s Fred Eisele and his team discovered a chemical surprise in the Antarctic. Hydroxyl radicals, the atmosphere’s primary cleansing agents, occur in exceptionally high concentrations in the lower troposphere (the lowest level of the atmosphere). These elevated levels of radicals above the South Pole are most likely due to large amounts of nitric oxide released by the interaction of sunlight and nitrates in snow—a process known as photolysis. The hydroxyl radicals prevent toxic buildups by oxidizing (essentially erasing) pollution and many naturally occurring chemicals in the air. Some of this oxidation also appears to be occurring in sections of the snowpack that are no more than about 10 years old and are shallow enough to be reached by sunlight. This fact could complicate measurements, particularly of nitrates, in ice core samples used to determine atmospheric conditions of the past. While the hydroxyl radicals have an immediate, local effect of cleaning the air, the global implications are still unknown. Fred’s research is continuing, and he hopes to use the NSF/NCAR C-130 research aircraft in 2005 to extend the investigation over much of the Antarctic Plateau.

Floods are the most expensive and the second deadliest natural hazard in the United States, and they remain a concern in Colorado even though parts of the state remain in a severe drought. In RAP, Hatim Sharif is working with David Yates and Ed Brandes to perform hydrometeorological analyses of flash floods in the Denver metropolitan area. The first area selected for analysis is Harvard Gulch, a highly urbanized catchment (an area, defined by topography, that drains to a common point). The sophisticated hydrologic model used in this study can include many physical details and incorporate high-resolution radar rainfall estimates to predict flash floods. The team, which includes RAP’s Rita Roberts, is implementing a computer forecasting system known as the NCAR Auto-nowcaster to provide short-term flash flood forecasts for organizations such as the Denver Urban Drainage and Flood Control District. These forecasts can predict lead times of up to 90 minutes for the Harvard Gulch study area and potentially even longer lead times for larger catchments. The study results are encouraging; in one case, the errors were less than 25% for flash floods predicted 70 minutes beforehand.

Flash flooding in Fort Collins.

While studying past volcanic eruptions, Caspar Ammann (CGD) and Philippe Naveau (CU’s Applied Math Department) came across an intriguing pattern: major tropical volcanic activity appears to follow a 76-year cycle. Caspar examined 600 years of volcanic activity, using both traces of volcanic sulfates in ice cores as well as an independent index of past volcanic activity based on volcanologic field data and eyewitness accounts of eruptions. In the ice data, he found indications of 61 explosive eruptions in the tropics that were powerful enough to leave sulfate traces in polar ice fields and thus had the potential to affect the atmosphere worldwide. According to a probabilistic model put together by Philippe, the periods of activity have waxed and waned every 76 years. If the finding holds up, it could have important climatic implications, since major eruptions send enough sulfates and other aerosols into the air to block sunlight and cool global temperatures, sometimes for several years. (The last such event occurred in 1991, when Mt. Pinatubo in the Philippines erupted.) However, it remains unclear what force could be creating such a 76-year cycle. Caspar’s next step is to peer farther into the past, seeing whether the cycle goes back 2,000 years.

Trees and other plants emit chemicals, such as isoprene, monoterpenes, and other volatile organic compounds, that have far-ranging impacts on climate and air quality. But not all plants emit the same levels of VOCs. A natural forest tends to produce a moderate amount of the chemicals because there are both high emitters and low emitters, but a tree plantation comprised of a single species is likely to produce either a very high or a very low level of emissions. Alex Guenther and his team in ACD’s Biosphere Atmosphere Interactions Group has been studying natural forests and tree plantations in the western United States, the Amazon Basin, and China to determine their regional impacts on ground-level ozone and greenhouse gases. To analyze changes in emissions, the team uses the new Model of the Exchange of Gases between the Atmosphere and Nature (MEGAN). The model incorporates Geographic Information Systems technology to track changes in climate and land cover. The group recently received a three-year EPA grant to expand its research throughout the United States, looking into how vegetation and wildfire emissions will respond to changes in climate and land management and how this will impact climate, air quality, and ground cover. For more information, see bai.acd.ucar.edu.

Also in this issue:

In the midnight hour: BAMEX takes aim at dangerous night storms

The long riders: How some staffers cope with epic commutes

Study finds lower atmosphere warming

An information divide

Building bridges for Latina students

Delphi Question: Publications on the Web

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