NASA Africa Mission Investigates Hurricane Origin and Development

Aug 3, 2006
Author: Cheryl Pellerin
Washington File Staff Writer

Washington – Scientists from NASA, the National Oceanic and Atmospheric Administration (NOAA), universities and international agencies are planning a field campaign to study how African winds and dust conditions influence the birth of hurricanes in the Atlantic Ocean.

The campaign, called NASA African Monsoon Multidisciplinary Analyses (NAMMA) 2006, will run from 15 August to mid-September in the Cape Verde Islands, 563 kilometers off the coast of Senegal in West Africa. The campaign is a part of a broader international project, the African Monsoon Multidisciplinary Analyses, aimed at improving knowledge and understanding of the West African Monsoon and its variability.

“We are primarily interested in the African easterly waves,” said Ramesh Kakar, weather focus area leader at NASA headquarters in Washington, during a 26 July teleconference briefing.

These lower-tropospheric (from the ocean surface to about 5 kilometers above it), low-pressure disturbances often serve as “seedling” circulations for a large proportion of tropical cyclones over the North Atlantic Ocean.

“More studies are needed to better understand how variations in these waves impact hurricanes,” he added.

STUDYING TROPICAL CYCLONES

Researchers will use satellite data, weather station information, computer models and aircraft to give scientists better insight into all the conditions that enhance the development of tropical cyclones, the general name given to tropical depressions, storms and hurricanes. The research is expected to help hurricane forecasters better understand the behavior of these deadly storms.

“Understanding the complete genesis or birth process of hurricanes is one of the great unsolved mysteries of atmospheric science,” said Jeffrey Halverson, a meteorology professor and a hurricane scientist at Halverson Scientific Consulting in Maryland.

“The tropical Atlantic experiences 60 to 70 tropical waves in a given season, but on average only 10 percent of these become named storms,” Halverson added. Storms are given names when they become tropical storms with winds exceeding 63 kilometers per hour. “The overarching goal of the NAMMA experiment is to understand why so few of these tropical waves develop into full-fledged hurricanes.”

For hurricanes to develop, specific environmental conditions must be present -- warm ocean waters, high humidity and favorable atmospheric and upward-spiraling wind patterns off the ocean surface.

Atlantic hurricanes usually start as weak tropical disturbances off the coast of West Africa and intensify into rotating storms with weak winds, called tropical depressions. An intensifying depression may evolve into a tropical storm, and then be declared a hurricane when its winds exceed 117 kilometers per hour.

To study these environmental conditions, researchers will use NASA's DC-8 research aircraft as a platform for advanced atmospheric research instruments.

Remote and onsite sensing devices will let scientists target specific areas in developing storms. Sensors aboard the aircraft will measure cloud and particle sizes and shapes, wind speed and direction, rainfall rates, atmospheric temperature, pressure and relative humidity.

LOOKING AT STORMS FROM SPACE


Satellite map shows thunderstorm activity over the Atlantic Ocean off the African continent. (Eumetsat via NASA photo)

The campaign will use extensive data from NASA's fleet of Earth-observing satellites, including the Tropical Rainfall Measurement Mission, QuikSCAT, Aqua, and the recently launched Cloudsat and CALIPSO, which can map precisely the three-dimensional structure of aerosol particles, including dust plumes.

These advanced satellites will give unprecedented views into the vertical structures of the tropical systems, scientists say. Field observations will help validate data from the new satellites.

To better understand the physics of hurricanes, researchers are seeking answers to questions about hurricane development, air currents and the effects of dust on clouds.

During the campaign, scientists hope to get a better understanding of the role of the Saharan air layer and how its dryness, strong embedded winds and dust influences cyclone development. The layer forms over the Sahara Desert during the Northern Hemisphere’s late spring, summer and early fall, and usually moves out over the tropical Atlantic Ocean. As part of looking at the Saharan air layer, scientists want to better understand dust's effect on clouds.

“Dust blown off the Sahara and Sahel can interfere with the birth process of hurricanes,” said Jason Dunion, director of the NOAA Hurricane Research Division 2006 Hurricane Field Program in Miami.

“Occasionally,” he added, “great elevated plumes of hot dusty air interact with African waves as they move across the Atlantic. When the extremely dry air within the dust layer is ingested by a wave disturbance, it evaporates the deep clouds and thus limits the potential of a wave to develop into a storm.”

Cloud models must account for any such effect, so scientists need to measure cloud droplet concentrations and size in clean ocean air and dusty air from the Sahara.

UNDERSTANDING HURRICANE FORMATION

Researchers also will look at what happens to air currents as they move from land to ocean waters. Information on clouds and moisture, heat, air movement and precipitation in an unstable atmosphere will be collected, analyzed and simulated in computer models.

Understanding hurricane formation requires measurements from very small to very large scales – from microscopic dust and raindrops to cloud formations and air currents spanning hundreds of kilometers.

The NAMMA scientists will collect and analyze an enormous amount of scientific data, Dunion said, and each mission scientist will make preliminary images and datasets available to the science team.

“The great share of the analysis will take longer to unfold in the months after the mission because the data must be carefully inspected for quality,” Dunion added. “But many key findings will emerge when computer simulations are run on this data. Some of the very first simulations will probably yield new discoveries within this current hurricane season.”

Additional information about NASA hurricane research and missions is available on the NASA Web site.

(The Washington File is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://usinfo.state.gov)