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Satellites Unlock Hurricane Lili's Sudden Death
01.30.06
 
Using a fleet of NASA and other satellites as well as aircraft and other observations, scientists were able to unlock the secret of Hurricane Lili's unexpected, rapid weakening as she moved toward a Louisiana landfall in 2002. The data from several satellites helped researchers see dry air just above the ocean’s surface move into the lower levels of the storm. The dry air helped Hurricane Lili weaken quickly because a storm strengthens from warm, moist air, and weakens from dry air.

Hurricane Lili in 2002. Image to right: Hurricane Lili was a Category 1 hurricane, and was centered over Louisiana on Oct. 3, 2002. This image was taken by the Moderate Imaging Spectroradiometer (MODIS) instrument, aboard NASA's Terra satellite. At this time, Lili had sustained winds of 92 mph near the center. On October 4, Lili was absorbed by an extratropical low while moving northeastward near the Tennessee/Arkansas border. Click image to enlarge. Credit NASA/GSFC/ MODIS Rapid Response

Over a 13-hour period on Oct. 3, 2002, Hurricane Lili weakened from a category 4 to a category 1 storm, its strongest winds falling by almost 52 miles per hour before it made landfall in Louisiana. Computer forecast models failed to predict this rapid weakening, which is not well-understood.

The key to solving this puzzle was having many satellite eyes in the sky. "Because a polar-orbiting satellite can only obtain regional observations once per day, the ability to combine observations from multiple satellites over the data-sparse ocean is a key to understanding tropical cyclone intensity change," says Pat Fitzpatrick, the principal investigator from Mississippi State University.

Scientists turned to data from NASA's Terra, Aqua, QuikSCAT and Tropical Rainfall Measuring Mission (TRMM) satellites, as well as data from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer (AVHRR) onboard the Geostationary Operational Environmental Satellite (GOES). They also looked at data from instruments called "dropsondes" that were dropped from hurricane hunter airplanes while flying over Hurricane Lili. Those dropsondes provided temperature, humidity and wind data.

Track of Hurricane Lili in 2002 Image to left: Hurricane Lili's Storm Track September 21 through October 4, 2002. Click image to enlarge. Credit: NOAA

The satellites provided data that looked at different parts of the hurricane. QuikSCAT provided surface winds; Aqua provided temperature and moisture information; and GOES-8 supplied upper-level winds. Sea surface temperature data was also measured from Aqua, Terra, TRMM and AVHRR. Standard weather observations were also incorporated, including ocean surface data from the National Data Buoy Center.

All of this information was fed into a computer model that re-creates how the atmosphere and the ocean was at the time of Lili. The computer model was able to simulate the conditions when Hurricane Lili weakened, so scientists could better understand the cause of the drop in strength. The model showed that drier air closer to the ocean’s surface moved into the west side of Lili, on Oct. 2, at 8:00 p.m. ET, partially explaining the storm’s weakening.

That dry air helped break up the powerful thunderstorms that circle the open air center (eye) of the hurricane, which weakened the storm quickly.

This poster entitled, Studies on the Initialization and Simulation of Hurricane Lili's
Rapid Weakening, will be presented at the American Meteorological Society's Annual meeting, the week of January 29, 2006. Image to right: AMS Poster: Hurricane Lili -- This poster, "Studies on the Initialization and Simulation of Hurricane Lili's (2002) Rapid Weakening" will be presented at the American Meteorological Society's Annual meeting, the week of January 29, 2006. Click on image to enlarge. Credit: NCAR/MSU

The computer model also showed that wind data from the GOES and QuikSCAT satellites can improve the forecast of the hurricane's path. When wind data from those two satellites was added, the computer model was able to better re-create Hurricane Lili's actual route and landfall.

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Rob Gutro
Goddard Space Flight Center