Aircraft Captures Windy Details In Hurricane's Ups And Downs

While meteorologists have made considerable strides in forecasting a hurricane's track, intensity predictions have remained a more elusive challenge. Part of the difficulty is that the many factors that control intensity, particularly the speed, direction and spin of air throughout the atmosphere, are constantly changing and tricky to measure. Aircraft are able to gather detailed, precise measurements of winds in a hurricane that can help researchers understand what is going on inside the storm, allowing better forecasts to be made.

In July 2005, Hurricane Dennis experienced several periods of rapid intensity fluctuations, providing for several excellent opportunities to learn about tropical cyclone behavior. Dennis reached hurricane strength on July 7, 2005, in the eastern Caribbean Sea, and rapidly strengthened into a category 4 storm before making landfall in Cuba on July 8. After weakening considerably as the storm moved over Cuba, Dennis attained category 4 hurricane status again with a pressure drop of 11 millibars in under two hours, indicative of rapid intensification. A typical low-pressure system in the United States might intensify that much over the course of an entire day.

Flying over Hurricane Dennis with NASA's ER-2 aircraft and the National Oceanic and Atmospheric Administration's (NOAA) P-3 aircraft, scientists gathered data on the storm's internal structure, including the distribution of winds, rainfall, temperature and moisture. The aircraft information has provided insight into the evolution of a hurricane's warm inner core; one of the many factors that impact storm development.

The research flights were conducted as part of the Tropical Cloud Systems and Processes (TCSP) mission in Costa Rica, a NASA field experiment with cooperative participation from NOAA and several universities. This experiment was aimed at studying the birthing conditions for tropical storms and hurricanes and identifying the factors that cause them to strengthen or weaken.

"This campaign was particularly unique because two types of aircraft provided measurements on different atmospheric variables," said Joe Turk of the Naval Research Laboratory, Monterey, Calif. "The information is also being used to determine how accurately satellites capture storm details."

The aircraft data provide high resolution measurements with a level of detail far superior to current weather satellites. During the mission, the NASA ER-2 aircraft flew over Dennis at 65,000 feet while taking scientific measurements that probed downward through the cloud layers. At times, the NOAA P-3 flew identical and coordinated patterns, but from an altitude of 12,000 feet, probing the storm from the inside.

As the hurricane fluctuated in intensity, flights into the storm continued, taking critical measurements of wind, temperature, and moisture. "The erratic nature of the storm and the timing of the research mission allowed scientists to pierce through the core of the hurricane at many stages of its life cycle and for the first time map a hurricane's entire evolution," said Steve Guimond of Florida State University, Tallahassee, Fla.

NASA's ER-2 Doppler radar measured wind speed along the track of the aircraft including measurements indicative of the size and concentration of raindrops and ice particles, while another ER-2 instrument, the Advanced Microwave Precipitation Radiometer, gathered microwave imagery of the internal structure of rain clouds. By analyzing when and where strong winds are occurring, researchers can better determine when intensity changes may occur. Data on the storm's vertical temperature structure - indirectly related to wind speed and rainfall - was also examined from overpasses of NASA and NOAA satellites.

These key aircraft observations not only assist in understanding the rapid intensification of hurricanes, they can also help scientists recreate storms on computer models that are used in forecasting. Just small changes in wind speed and direction patterns can significantly rearrange a storm's rain and wind structure, altering the evolution of its predicted track and intensity.

Previous research has suggested that rapid hurricane intensification, like that seen in Dennis, is associated with "hot towers." These are columns of rapidly rising air that reach and in some cases overshoot the top of the troposphere - the lowest layer of the atmosphere - about nine miles high in the tropics. They are called "hot" because of the large amount of heat they release through condensation of water vapor, providing fuel for strong winds and heavy rainfall.

"With Dennis, it appears the hot towers played a major role in the rapid intensification of the storm, giving clues on how energy is concentrated and winds evolve at various stages of development," said Guimond. "The observations also helped place the storm's behavior in greater context and matched well with computer model simulations, suggesting that we are making progress in replicating hurricane development."

"Improved knowledge of how both the heating and rotation or 'spin' of air parcels associated with these hot towers interacts with the greater organized system is thought to be another key ingredient to improving hurricane intensity forecasts," said Steve Miller of the Naval Research Laboratory. "While our preliminary findings based on satellite views of Dennis support the idea that such physical links may in fact exist, additional insight requires the kind of three-dimensional detailed perspective on internal storm structure that is only available in a field experiment, such as the TCSP mission."

As researchers identify other factors most critical in hurricane development, those elements can be targeted for increased observation in future field missions to obtain the big pieces of the puzzle needed to solve the mysteries of hurricane behavior.