NASA study could improve hurricane strength forecasts

Nov 29, 2012 by Alan Buis
Hurricane Sandy as seen by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra spacecraft on Oct. 28, 2012, when the Category 1 storm was centered off the southeastern U.S. coast. A new NASA-led study finds that analysis of relative humidity levels in the large-scale environment of tropical cyclones may be useful in improving forecasts of their intensity. Credit: NASA GSFC/LANCE MODIS Rapid Response Team

(Phys.org)—Forecasters could soon be better able to predict how intense tropical cyclones like Hurricane Sandy will be by analyzing relative-humidity levels within their large-scale environments, finds a new NASA-led study.

Scientists from NASA's Jet Propulsion Laboratory, Pasadena, Calif., UCLA and the University of Hawaii at Manoa analyzed relative humidity data from the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft for nearly 200 North Atlantic hurricanes between 2002 and 2010. The AIRS data were then compared with various types of post-storm data collected from all available sources by the 's National Hurricane Center, including measured .

The researchers found the hurricanes that rapidly intensified tended to exist within a moister large-scale environment than weaker storms. The rapidly intensifying hurricanes had statistically significant higher relative-humidity levels in their environments than storms whose intensity was weakening or unchanged.

Lead author and former JPL postdoctoral scholar Longtao Wu, now an assistant researcher at the UCLA-JPL Joint Institute for Regional and Engineering, said the study could lead to improvements in hurricane intensity forecasts.

"Our results show relative humidity and its variations within a hurricane's large-scale environment may be useful predictors in improving intensity forecast models," Wu said. "This is the first satellite analysis to quantify this small but statistically significant correlation." Results of the study were published recently in the journal .

Relative humidity levels, in percentage, at 850 hectopascals (millibars) for the large-scale environment of Hurricane Earl on Aug. 31, 2010, as measured by the Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua spacecraft. The red line indicates the hurricane's direction of travel. Earl reached a peak intensity of Category 4 on the Saffir-Simpson scale, with peak maximum sustained winds of 145 miles per hour (126 knots). Credit: NASA/JPL-Caltech

Hurricane forecasters strive to predict where a storm is heading and how strong it will be. Since the early 1990s, they have significantly improved forecasts of hurricane paths in the Atlantic basin, by about two to four percent a year. But forecasts of hurricane intensity have improved much slower—less than one percent a year in the Atlantic basin since the mid-1980s. In other ocean basins, like the eastern and western North Pacific, improvements in tropical cyclone intensity forecasts are nearly 10 times smaller than those of track forecasts.

Changes in are sensitive to numerous factors, both within the storms themselves and also in their surrounding environments. Environmental relative humidity is one factor, and it generally decreases the farther you get from a storm's center. Other factors include sea surface temperature, ocean heat content and vertical wind shear.

Wu and his colleagues sorted the AIRS relative humidity data by storm intensity and intensification rates, and classified them based on their distance from storm center and also by what quadrant of the storm they came from relative to the storm's direction of travel (front right, front left, rear right and rear left). Generally, a hurricane's right side relative to its direction of travel is the most dangerous. This is because a hurricane's wind speed is amplified by the speed of its steering winds. Storm surge is also higher on a hurricane's right side.

The team found substantial differences in relative-humidity levels between storm quadrants. One factor may be the shape of the Atlantic basin. Hurricanes in the Atlantic usually travel to the west or northwest—regions that are drier, climatologically-speaking, than from where the storms originated. This causes the front two quadrants of to be drier than their rear two quadrants.

A unique result the team found is that in their front-right quadrants, rapidly intensifying hurricanes tended to have sharply higher amounts of upper tropospheric moisture near their centers than they did farther from their centers.

"We speculate that decreasing relative humidity levels farther from a storm's center may be an important factor in a cyclone's rapid intensification," said JPL co-author Hui Su. "A drier environment farther from a storm's center limits the development of its outer rain bands and favors the growth of its inner core. Conversely, a wet environment farther from a storm's center can weaken a cyclone by making it easier for rain bands to form outside the storm's core, which compete with the inner core's growth."

"Most scientists have tended to view hurricane intensification as a process that takes place within a cyclone's inner core and depends more on smaller-scale processes than on a storm's large-scale environment," said JPL co-author Bjorn Lambrigtsen. "This study shows a different path, and the usefulness of incorporating large-scale environmental data collected far away from a storm's center."

Su said NASA is exploring collaborations with NOAA forecasters to incorporate AIRS data into NOAA's real-time hurricane prediction system.

Explore further: Magnitude-7.2 earthquake shakes Mexican capital

More information: For more on AIRS, visit: airs.jpl.nasa.gov/ .

Related Stories

Busy Atlantic storm season predicted

Aug 02, 2005

U.S. storm forecasters say they expect 11 to 14 tropical storms with most developing into hurricanes over the remainder of the 2005 Atlantic storm season.

NASA sees wind shear battering Tropical Storm Nadine

Sep 13, 2012

Tropical Storm Nadine is struggling against wind shear and some dry air. Infrared satellite imagery from NASA showed that Nadine's most powerful thunderstorms were being pushed east of the center.

Recommended for you

Magnitude-7.2 earthquake shakes Mexican capital

Apr 18, 2014

A powerful magnitude-7.2 earthquake shook central and southern Mexico on Friday, sending panicked people into the streets. Some walls cracked and fell, but there were no reports of major damage or casualties.

User comments : 0

More news stories

China says massive area of its soil polluted

A huge area of China's soil covering more than twice the size of Spain is estimated to be polluted, the government said Thursday, announcing findings of a survey previously kept secret.

UN weather agency warns of 'El Nino' this year

The UN weather agency Tuesday warned there was a good chance of an "El Nino" climate phenomenon in the Pacific Ocean this year, bringing droughts and heavy rainfall to the rest of the world.

Poll: Big Bang a big question for most Americans

Few Americans question that smoking causes cancer. But they have more skepticism than confidence in global warming, the age of the Earth and evolution and have the most trouble believing a Big Bang created the universe 13.8 ...

Making graphene in your kitchen

Graphene has been touted as a wonder material—the world's thinnest substance, but super-strong. Now scientists say it is so easy to make you could produce some in your kitchen.