Colorado State Researchers to Model Historical Instances of Extreme Precipitation in Order to Better Understand High-Altitude Rainfall

Colorado State University scientists are figuring out why and when extremely heavy rainfall that can cause severe flooding occurs at high elevations in Colorado.

Funded by the Colorado Department of Natural Resources, researchers William R. Cotton and Thomas B. McKee, both professors of atmospheric science, are using a weather modeling system to describe how extreme precipitation can occur.

The project, which began this spring and will continue through June 2001, will examine some 20 historical floods in the state that have occurred since 1957. By comparing records of each incident with the model’s predictions, the researchers and their associates hope to supply the Department of Natural Resources with information about designing spillways for dams that may have to withstand extreme precipitation events. The work may help the researchers learn what variables can predict flooding across the state.

The Extreme Precipitation Estimation Project will use Colorado State’s Regional Atmospheric Modeling System (RAMS) to simulate extremely heavy rainfall in areas above 7,500 feet. RAMS, which Cotton helped develop and has applied to other state weather events, is a three-dimensional storm model that performs detailed numerical simulations of precipitation processes and other weather phenomena in any area of Colorado.

There has been relatively little meteorological research on how extreme precipitation occurs across the state. An earlier study found it varies in its patterns in each of six areas of the state, according to Cotton, from eastern plains to southwest mountains.

Most heavy rainfall occurs below 7,500 feet, he said, and may be channeled by canyons and other elements of Colorado’s topography to create severe, life-threatening floods.

Above that level, precipitation is more likely to fall as snow and melt slowly, and moisture in air moving upslope tends to condense and precipitate before reaching that altitude. But, Cotton said, so little is known about the relationship between altitude and extreme precipitation events that some weather experts believe amounts of precipitation may be vastly overestimated.

The 20 test events, chosen from some 300 historical floods, all resulted from extreme precipitation events such as the Big Thompson Canyon Flood of 1976.

"We will, in effect, be simulating rare cases that occur perhaps once over very long-term periods at a given location, in which numerous environmental factors have added up to produce extreme precipitation events," Cotton said.

Once the model is working to Cotton’s and McKee’s satisfaction, they’ll develop a graphical user interface, a type of software that will enable the user to select an arbitrary Colorado location on a map. The user then can direct the RAMS model to determine estimates of extreme precipitation for that site and the associated drainage basin.

In the final six months of the project, Cotton and McKee will select a series of test sites in different regions of the state, run simulations and determine how well the model works. In addition, the effort should produce estimates of the extreme precipitation possible at each site.

The project, under consideration for some time by the Department of Natural Resources, won’t produce preliminary results until the end of the project in spring of 2001.

"This is a research project that is literally breaking new ground, and as such the results of our demonstration calculations should be used with caution," Cotton said. "This is a first step toward a larger understanding of extreme precipitation in Colorado."

At the request of Rep. Joel Hefley (R-Colo.), Cotton and McKee have agreed to add the Cherry Creek drainage basin in Douglas County as one of the test sites to be examined at project’s end.