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Contact: Jana Goldman
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The season's second tropical cyclone to reach hurricane strength gave the Commerce Department's National Oceanic and Atmospheric Administration's (NOAA) hurricane hunters the opportunity to monitor and measure a tropical storm undergoing rapid intensification before making landfall in the Yucatan peninsula. Isidore, now moving north from the southern Gulf of Mexico, is poised to hit the Louisiana and Mississippi coast by Thursday morning. “The key ingredient that NOAA scientists and colleagues are watching is the warm ocean temperature profile below the sea surface, that provides the primary source of energy that fuels storms during the height of hurricane season from August through October,” said Frank Marks, acting director of NOAA’s Hurricane Research Division at the Atlantic Oceanographic and Meteorological Laboratory in Miami.
Since 1998, Peter Black of NOAA’s Hurricane Research Division and Lynn K. Shay of the University of Miami have studied how areas of the ocean that have a deep layer of warm water near the surface play a significant role in allowing hurricanes to rapidly intensify, one of the most difficult situations to forecast and one of the most dangerous to residents along the coast. Their previous work focused on specific sources of deep warm water found in the Loop Current and large-scale eddies that spin off from this source and traverse the Gulf of Mexico.
On Sept. 19, the scientists dropped a series of ocean probes known as AXBTs and AXCPs (airborne expendable bathyothermographs and current probes, respectively) from the two NOAA WP-3D hurricane hunter aircraft. The probes were released in the projected path of Isidore in the southern Gulf of Mexico. The probes measured the ocean temperature and currents down to 200 meters (about 600 feet), the depth at which hurricane winds usually churn up colder water and cool the overall temperature of water below the storm.
“The significance of Isidore being over the Gulf Stream loop current is that the warm water below extends to great depths, not allowing Isidore’s winds to cool the surface temperature thereby keeping the heat reservoir intact and allowing further intensification of the hurricane,” said Shay.
During the second phase of the study ocean probes were deployed both as Isidore passed over the same region, and then immediately afterwards. Black and Shay will measure the difference in the ocean temperature structure during all three of these phases. During the time Isidore passed over the array of ocean probes dropped the day before, NOAA and University of Miami scientists collected data that describes both the complete structure of Isidore and the ocean beneath the storm.
“This data will allow for a much more analytical analysis of the ocean's role in intensification, the eventual goal being a greater ability to predict when and how much a storm may intensify, a challenge considered to be the next key step in hurricane forecasting,” said Black and Shay.
Marks notes that the ocean is not the only factor in hurricane intensity change. NOAA hurricane researchers also consider environmental data gathered around the storm by NOAA's Gulfstream-IV jet (G-IV), a surveillance aircraft operated by NOAA's Aircraft Operations Center. Though the G-IV missions are designed to improved track forecasts for landfalling hurricanes, an added benefit is measurement of the wind shear and environmental moisture and stability that can affect intensity. Wind shear is the difference in wind velocity at upper and low levels in the atmosphere. High wind shear, dry air, and low stability are associated with weakening of tropical storms. Direct measurements of these variables are unavailable over the Gulf of Mexico without use of the Gulfstream jet.
“We hope to discover the relative importance of the loop current and environmental wind shear in determining hurricane intensity change,” Black says.
Black and Shay hope that data gathered during this study will enhance knowledge and predictability of major hurricanes, which translates into improved intensity forecast, increased warning time, and better preparedness in coastal regions.
The mission of NOAA’s Hurricane Research Division is to advance the understanding and prediction of hurricanes and other tropical weather. Their research is based on a combination of models, theories, and observations, with particular emphasis on data obtained with the NOAA research aircraft. A key goal of this research is to advance the prediction of tropical cyclone intensity change by improving understanding of the processes that modulate internal storm dynamics and interactions of a storm with the atmosphere and ocean.
The Commerce Department’s National Oceanic and Atmospheric Administration (NOAA) is dedicated to enhancing economic security and national safety through the prediction and research of weather and climate-related events and providing environmental stewardship of our nation’s coastal and marine resources. To learn more about NOAA, please visit http://www.noaa.gov.
To learn more about hurricane research, visit: http://www.aoml.noaa.gov/hrd.
To learn about NOAA’s Aircraft Operation Center, visit: http://www.nc.noaa.gov/aoc.
To learn about the University of Miami’s Rosenstiel School for Marine and Atmospheric Science, visit: http://www.rsmas.miami.edu.