How much, how long and how often does it rain in Alaska?
These are the questions researchers from both the National Oceanic and Atmospheric Administration (NOAA) and the University of Alaska Fairbanks (UAF) sought to answer in a collaborative research project meant to update the Alaska section of NOAA’s Precipitation Frequency Atlas of the U.S. Results from the three-year project were published electronically on NOAA’s National Weather Service website in early February 2012.
The importance of determining precipitation frequency goes far beyond planning for proper raingear, said project researcher Sveta Stuefer, assistant professor with the Water and Environmental Research Center (WERC).
“This information is fundamental in planning the design of roads, culverts, bridges, and other infrastructure,” Stuefer said.
Photo courtesy Jayashree Narayanan
SNOTEL precipitation gauge in the Brooks Range, Alaska, 999.
Updating the atlas
The Hydrometeorological Design Studies Center (HDSC), a center within NOAA’S National Weather Service (NWS), updates precipitation frequency atlases periodically for all of the U.S. Due in part to challenges inherent to gathering weather data in Alaska (think remote locations, extreme climate and the vast amount of topography to be covered), HDSC had not updated the Alaska atlas since 1963. Douglas Kane, professor emeritus with WERC and the university’s Principal Investigator on this project, noted that because that last map only included data up to around 1960, approximately 50 years of additional precipitation data is now available for analyses and prediction.
“For years, users of this precipitation frequency estimation had been talking about the need for updating the original 1963 report because there was more data available,” Kane explained.
Kane himself had been working to organize a study that would collect data on precipitation frequency in the road corridors of Alaska to assist in construction-related plans and projects; he proposed this reduced study to the Alaska University Transportation Center (AUTC) and the Alaska Department of Transportation & Public Facilities (ADOT&PF). It didn’t take long for UAF WERC and NOAA HDSC researchers to realize that collaboration could be mutually beneficial.
“They were planning to do the entire state, not just the road corridors,” Kane explained. “They got wind of what we were doing, we all got together to establish a plan and we’ve ended up working jointly.”
All other HDSC state frequency maps, like the one recently completed for California, were completed solely by NOAA’S HDSC, Stuefer said. Alaska is the only such atlas to involve researchers from outside HDSC. Kane and Stuefer worked with Dr. Amy Tidwell (now with ExxonMobil), research assistant Elizabeth Lilly and graduate students Erica Betts and Jayashree Narayanan on the project, with funding from AUTC and ADOT&PF and assistance from a variety of institutions. HDSC’s team of 14 was directed by Dr. Sanja Perica with their funding through NOAA. Conference calls were held twice per month between HDSC and WERC researchers to coordinate activities.
The challenges of Alaska
The largest state in the U.S. at more than 1,518,000 square kilometers, Alaska is considerable in size, sparsely populated, and has a wide range of contrasting climatic zones. Researchers had to address poor quality precipitation data, especially in windy environments for solid precipitation such as snow, sleet and hail. Limited access to both remote and sparsely populated areas, including rough and complex terrain resulted in large areas of no or minimal precipitation data. For example, the Arctic Slope of Alaska (one-sixth of the area of Alaska north of the Brooks Range) is one of the least-understood climatic regions of the country. That area, which is more than 230,000 square kilometers, has only a handful of long-term precipitation gauges with a majority of the existing gauges are unattended and usually only measuring liquid precipitation.
Collaboration was key to gathering a significant amount data in such a challenging environment. The research team collected data from USDA Natural Resources Conservation Service, NOAA NWS, USDI USGS, Environment Canada and others, combining it with data from UAF stations throughout the state. Data from more than 1,200 meteorological stations were eventually evaluated for use in the project.
Compile and contrast
Compiling data from so many stations involved more than crunching numbers. Stuefer and Kane explained that stations used various methods to measure and record precipitation. Most logged data on the amount of rainfall per day (24 hours), while a few others measured hourly or 15-minute interval rates. The team had to use all of the data available to develop frequency and intensity rates for 15-minute, hourly, and daily durations in order to develop meaningful frequency estimates.
This is where the “how much, how long and how often” concept comes in to play. As a design tool, the precipitation frequency atlas needs to answer all three questions. Kane explained that the combination of those factors influence engineering plans for things like culverts at stream crossings and storm drains for parking lots and street, plus numerous other applications.
“Runoff response is fast in places like parking lots, and it inundates the storm drains. Some places might get 10 inches of rain in a day, but when you look at the amount and intensity that falls for shorter durations, that number decreases,” he said, noting that the decrease affects the magnitude of the peak flow that will inundate a particular storm drain.
Photo courtesy Jayashree Narayanan
Summary of the stations with daily precipitation collected and used for the precipitation frequency analysis.
Compounding the problem in parking lots is the loss of vegetation and the addition of asphalt, Stuefer noted, which causes the magnitude of runoff to increase. Having accurate information regarding precipitation intensity (how much), duration (how long) and frequency (how often) can help engineers adjust for all factors involved and plan the construction of storm drains. The information is equally important in plans for stream crossings where culverts, bridges and other road related structures are required.
The team used advanced statistical methods and modeling techniques (methods developed over the years by HDSC while performing parallel studies in other regions in the U. S.) to analyze the entire data set including the original data collected prior to the 1963 atlas and the new data. Early in the study, team members began compiling all precipitation data (plus other supportive data), initially collecting up to 2008 when the project was initiated. Kane said the team decided about six months from the end of the project to add easily attainable precipitation data sets from 2008-2010, so the new atlas would include a majority of available precipitation data during a time period spanning more than 100 years.
“The oldest data we had dated back to the late 1800s, but many of those stations do not exist today,” Kane said.
The process involved “a lot of hunting,” Kane said, in part because so many agencies were involved in collecting the data and because of the sparse data network we wanted to use all good quality data we could find. Stuefer noted there was also “numerous quality control issues.”
“The bottom line is the quality of the precipitation data available could be a lot better,” Kane indicated.
The team also looked into the issue runoff generation; rain can runoff immediately after falling while solid precipitation cannot runoff until it melts. Kane said many stations only measured rainfall, but NWS stations measured all precipitation. Based on air temperature, the team could estimate what events represented rain and what represented solid precipitation. Heavier, more intense periods of precipitation occurred in warmer periods and consisted of rain, Stuefer said.
There are several significant differences in the 1960s study and the one completed here. In addition to having larger and longer data set for analyses, the analyses were carried out on computers (in contrast to hand calculations) allowing for more complex analyses. Also, the results are presented in interactive electronic form, making it much easier for the user to determine intensities for a wide range of durations and frequencies. These analyses are published in Atlas 14 (Volume 7) of the NOAA Precipitation Atlas of the U.S., in a spatially-distributed electronic format.