by Mary Caperton Morton Friday, August 2, 2013
Hurricanes are one of the few natural disasters that strike with some advance notice. Forecasts can be made hours or even days ahead of landfall, giving communities time to prepare and evacuate. Nevertheless, the forecasts are not exact, and improving their accuracy — in terms of timing, location of landfall and wind intensity — poses an ongoing challenge. Two recent studies detailing the latest advances in data collection and assimilation may help improve forecasting as early as this year.
In the U.S., hurricane forecasts are handled by the National Hurricane Center, a division of NOAA based at the National Centers for Environmental Prediction, located at Florida International University in Miami. However, the latest advances have come from independent projects known as the Hurricane Forecast Improvement Project (HFIP), NASA’s 2010 Genesis and Rapid Intensification Processes (GRIP) Field Experiment and its replacement, the Hurricane and Severe Storm Sentinel (HS3) project.
HFIP was started in 2009. “After Katrina and Rita, it was clear we needed to ramp up our ability to forecast hurricanes and make faster progress than we had been,” says Robert Gall, a meteorologist with HFIP and lead author of the project’s new report, one of the two published by the American Meteorological Society (AMS). “The idea was to lasso the research community, open a high level of communication and provide funding for different groups working on hurricane forecast research.”
HFIP set out to reduce the average errors in hurricane track and intensity forecasts by 20 percent within five years and by 50 percent in 10 years, while extending the average forecast period out to seven days ahead of landfall. Four years into the project, Gall says, “there’s no doubt that we have achieved the 20 percent goal and are well on our way to the 50 percent goal for tracking.” Forecasting storm intensity, however, is proving much more difficult.
“Intensity forecasting is very different from track forecasting,” says James Franklin, chief of hurricane forecast operations at the National Hurricane Center who was not involved in either of the new reports. “To forecast track successfully you can get away without knowing too much about what’s going on in the core [of the hurricane] because the track is primarily being driven by the environment around the storm,” he says. Successful intensity forecasting, however, depends on what is happening inside the storm itself, data that can often only be gleaned by flying instruments directly into the maw of a hurricane, a costly and dangerous endeavor.
Enter NASA’s unmanned Global Hawk flights, operated last year by GRIP and this year by HS3. The Global Hawk can fly for up to 28 hours at a time inside and above even the biggest storms, says Scott Braun, a meteorologist with the GRIP and HS3 projects and lead author of one of the new AMS reports. “With the Global Hawks, we’re now able to see a bigger window in the life cycle of a storm and collect data on aspects of hurricanes that we were previously unable to study,” Braun says.
Last year GRIP flew one Global Hawk, and this year the fleet has doubled for the HS3 project and relocated from California to Virginia. One plane is equipped with instruments that probe the inner structure of the storm, including Doppler radar and microwave instruments to measure surface wind speed and rainfall. The other is designed to study how the wider environment surrounding the storm influences the storm itself, primarily using lidar, an imaging tool that captures reflections of light from objects below it.
While the flights continue to provide an unprecedented look into how hurricanes evolve over time, assimilating the data into the forecast models at the National Hurricane Center in real time has proved challenging, Franklin says.
“You can’t just start feeding data into the supercomputer and expect it to make sense,” Franklin says. “You have to know how the various data types are going to interact and combine them in an intelligent way. Data assimilation is a challenge all to itself.”
So far, the only datasets that have been successfully assimilated into the National Hurricane Center models are those produced from dropsondes: small GPS-equipped capsules dropped from aircraft that measure parameters such as pressure, temperature and humidity within the storm system.
The lack of application doesn’t mean the GRIP project failed, Braun says. “Our objective is to maximize the amount of scientific data collection,” he says. “We’re focused more on understanding than application, but I think these kinds of flights can and will benefit forecasting more and more.”
So far, the improvements in precision made by HFIP have proved more useful to the forecasters at the National Hurricane Center, Franklin says. “We have a lot of interactions with HFIP. That project is a bit older and our ties are stronger,” he says. For example, HFIP plays a role in vetting new computer models before they are used by the National Hurricane Center forecasters to track storms in a program known as Stream 1.5.
This season’s collaborative goal between the National Hurricane Center and HFIP will be to incorporate Doppler radar data collected by NOAA’s manned “Hurricane Hunter” aircraft into the working forecast models in real time in hopes of improving intensity forecasts. “This will be the first time that Doppler radar data will be appearing in an operational forecast model,” Franklin says. “It’s very exciting.”
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