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July 26, 2011 | Heavy rains in the Midwest and a deep Western snowpack led to floods across much of the country this spring and summer. The Lower Mississippi recorded its greatest flood in history, the Missouri River inundated towns and fields, and runoff-swollen Western rivers threatened to jump their banks.
Close to home, record spring snowfall led to raging rivers and flood advisories. Boulder Creek, which in mid-July flowed at nearly 1,000 cubic feet per second, spilled out over the adjacent bike path. Fort Collins shut down city access to the Cache La Poudre River, and restrictions were in place on other rivers across the state.
At UCAR/NCAR, scientists are studying many aspects of floods, from the weather and climate that cause them to how people respond when the waters rise. They’re placing sensors near Fourmile Creek, making forecasts for Bangladesh, exploring the relationship between climate change and floods, and training the next generation of forecasters.
La Niña and the Midwest floods
According to the National Climatic Data Center, the United States has never experienced a spring with as much of the country either extremely wet or extremely dry since record keeping began more than a century ago. This spring also ranked fifth most extreme in the past 102 years when factoring in both temperature and precipitation.
The vast floods that deluged the Midwest this spring—and the intense drought across much of the South and Southwest—are both consistent with La Niña, the counterpart to El Niño. This ocean-atmosphere pattern occurs when cooler-than-normal sea surface temperatures form in the central and eastern Pacific Ocean off the coasts of Peru and Ecuador.
La Niña alters the path of the jet stream, making the predominant storm track in winter traverse the Midwest and avoid the South. As a result, the pattern usually brings abnormally wet weather to the Midwest and dry weather to the South. La Niña’s impact on the jet stream typically fades in the spring, but this year the atmospheric part of the pattern stayed strong. Meanwhile, in April sea surface temperatures in the Gulf of Mexico heated up to their third warmest levels in more than a century, allowing more moisture than usual to evaporate there, flow north, then fall over the Midwest in La Niña–fueled storms.
“The pattern of the weather we’ve experienced has followed closely the La Niña pattern of dry in the south and wet up the Ohio River valley,” says Kevin Trenberth (NESL/CGD). “The areas that have been wet and dry are as expected, except perhaps for California, which had some extra rains this past winter.”
As the global climate warms, it’s expected to boost the hydrological cycle, triggering more intense rain and snow while also intensifying drought. As Kevin sees it, the magnitude of weather extremes experienced by Americans this spring—not just floods but also droughts, tornadoes, and wildfires—brings climate change into the equation. “The United States has recently experienced some of its worst climate extremes in history. When natural variability is compounded by human influences on climate, this is what we get,” he says.
Researchers at NCAR and in the community have been using the CESM (Community Earth System Model) to produce new simulations of global and regional climate in preparation for the next Intergovernmental Panel on Climate Change report, due in 2013. As part of a broad effort to look at model results, Synte Peacock (NESL/CGD) has examined projected changes in temperature, rainfall, and snowfall over North America. The findings, soon to be published in a special issue of Journal of Climate devoted to CESM results, show an increase in total precipitation over nearly all of North America except for the U.S. Southwest. Higher-resolution regional models may provide more detail in the future, Synte says.
Close to home
This spring’s classic La Niña pattern strongly influenced the West, with the northern states buried in snow and at risk of flooding while Arizona, New Mexico, and parts of eastern Colorado experienced drought and wildfires.
Here in NCAR’s backyard, Dave Gochis (RAL) is collaborating with the U.S. Geological Survey (USGS), Denver Urban Drainage and Flood Control District (UDFCD), and National Weather Service (NWS) to place three new, state-of-the-art weather stations that measure standard hydrometeorological conditions in the Fourmile Fire burn area just outside Boulder. A major fire can leave charred soil hydrophobic (water-repellent), which increases the risk of rapid runoff and flooding once the rains return.
The stations will provide real-time data that will help the UDFCD and NWS monitor for flash floods. The instruments at the stations are better at discriminating between rain and hail particles than most weather radars and traditional rain gauges. “This is important for flash floods because, while a hailstone is heavy and contains water, it’s in a frozen form and thus doesn’t cause immediate runoff response,” Dave says.
Dave will use the data for research purposes as well, as part of ongoing work in RAL that uses the Front Range as a testbed for studying rainfall and radar in complex terrain, flash floods, hydrological modeling, and more.
The USGS’s role in the partnership is to study erosion in the burn area. A severe fire such as Fourmile can, in some places, burn all vegetation from the ground and tree canopies, while also altering soil structure and lessening its cohesion, making it highly erodible. The new instruments will zero in on rainfall intensity and the size distribution of raindrops, information that is critical for gauging erosion since the amount of energy transferred onto the ground disturbs the soil and drives the process.
Matt Kelsch, a hydrometeorological specialist in COMET and the official cooperative climate observer for Boulder, also has his eye on the Fourmile burn area. This spring, Matt met several times with local police, firefighters, and search-and-rescue personnel to fill them in on the potential for the burn area to send a flash flood into Boulder during a big summer thunderstorm. Matt looked at data from the USGS, UDFCD, and several Boulder-based engineering firms.
“The potential for a serious flood or debris flow is still small, but it’s certainly elevated and is much greater than it was a year ago before the fire,” Matt says.
A major thunderstorm could threaten Boulder Creek, into which Fourmile Creek flows, as well as Fourmile Canyon Creek to the northeast, which flows into the Wagon Wheel neighborhood and then into north Boulder. Based on local climate, the peak risk of a Fourmile flood will occur from late July into early August, when monsoon moisture typically moves into Colorado from Arizona and New Mexico. This year’s monsoon arrived in early July, leading to localized flooding and debris flows in Fourmile Canyon.
Millions at risk
RAL’s idea is to monitor satellite measurements of changes in river width and use the information to project flows downstream. They’re applying the technique to Bangladesh’s Ganges and Brahmaputra rivers, known for their devastating floods, and have a paper in the works.
“We’re hopeful that this can be a useful tool for flood forecasting,” Tom says.
The new research draws on existing data from Tom’s prior experience with forecasting for densely populated Bangladesh, where he helped develop a flood forecasting system launched in 2003 that uses a combination of weather forecast models, satellite observations, river gauges, and hydrologic modeling techniques. It delivers 1- to 10-day forecasts directly to more than 100,000 people living on floodplains of the Brahmaputra and Ganges. When flood risk rises, the system alerts a network of volunteers in Bangladesh who notify residents at risk. Since its launch, the system has been transferred to Bangladeshi engineers who are running the system operationally in their home country.
The human dimension
In NESL/MMM, Rebecca Morss looks at the interplay between floods and their hydrometeorological and societal contexts. In research published in Natural Hazards Review in 2010, she compared three flood case studies to examine the interactions among predictions, decisions, and flood-related outcomes, finding that a useful function of flood predictions can be to supplement experiential knowledge and reduce surprise, benefiting critical decisions to reduce flood-related harm. Another paper slated for publication later this year looks at societal aspects of extreme weather events in conjunction with climate change.
“The idea is to look at the warning process and how different groups view flash flood risk, to see if there are gaps in the way that warnings and risk are communicated and understood,” Rebecca says. “We hope to use the information to provide recommendations on improving the warning process and creating better flash flood communications.”
In COMET, scientists and educators are working to train the next generation of hydrometeorological forecasters. Matt is coordinating the NWS’s Advanced Hydrologic Sciences course this summer, which trains forecasters in the science behind newer models, remote sensing, data assimilation techniques, and more. COMET will host the two-part course onsite while students in 24 forecast offices participate using virtual technology. The course will be delivered during the weeks of July 25 and August 15.
In addition, COMET offers a number of courses through its MetEd program that examine flooding, ranging from meteorological inputs such as rainfall detection and forecasting to the hydrologic response of watersheds. In the past year, MetEd hydrology and hydrometeorology courses have drawn more than 51,000 users, with about 30,000 of them logging in from abroad. The courses are freely available on MetEd’s recently upgraded website.
The latest, “Flash Flood Processes: International Edition,” was released in February and is part of the Basic Hydrologic Sciences online course. Designed for those making forecasts without formal training in hydrology, it covers all aspects of the hydrologic cycle from rainfall and snowmelt to runoff and the response of stream channels. Special cases such as flash flooding and river ice flooding are also covered. The course is available in both English and Spanish.