It's official: the 2005 hurricane season will go on record as the most active in history! There have been so many storms this year that for the first time, scientists exhausted the predefined annual list of hurricane names, and have resorted to naming subsequent storms after letters of the Greek alphabet (Alpha, etc.). Researchers at NASA and other organizations continue to marvel at and study these large systems using current satellite systems such as the Tropical Rainfall Measuring Mission (TRMM) and Aqua. The development of the Global Precipitation Measurement (GPM) mission will be a valuable tool for the next decade of hurricane research. During the 2005 hurricane season, GPM's predecessor TRMM has provided some amazing and insightful perspectives on storms such as Hurricane Rita and Katrina.

For example, NASA's TRMM spacecraft allowed us to look under Hurricane Rita's clouds to see the rain structure on September 19, 2005 (see Figure 1). Spikes in the rain structure known as 'hot towers' indicate storm intensity. The term 'Hot Towers' refers to tall cumulonimbus clouds which are one of the mechanisms by which the intensity of a tropical cyclone is maintained. Because of the size (1-20 km) and short duration (30 minutes to 2 hours) of these hot towers, studies of these events previously were limited to descriptive studies from aircraft observations. Before TRMM, no data set existed that could show globally and definitively the presence of these hot towers in cyclone systems.

Figure 1: NASA's TRMM spacecraft allows us to look under Hurricane Rita's clouds to see the rain structure on September 19, 2005.

With a ground resolution of 5 km, the TRMM Precipitation Radar provides the needed data set for examining the predictive value of hot towers in cyclone intensification. At the time this data was taken, Rita was classified as a Tropical Storm with winds of 55 knots and a pressure of 994mb. The existence of these 18 km towers in the eye wall could have been a sign of intensification. Within 48 hours of this dataset, the storm was a very strong category 4 hurricane. GPM's advanced dual-frequency precipitation radar or DPR will allow us to continue to see the hurricane "engine" parts with similar or even better fidelity.

In tropical systems like hurricanes, one of the most dangerous and underrated hazards is the inland freshwater flooding associated with rainfall and surge. The image in Figure 2 is the accumulated rainfall measured by TRMM during the lifespan of Hurricane Katrina. Heaviest rains (well over a foot) fell as the storm was a category 1—during and after landfall over south Florida. There is also significant accumulation of rainfall near the Louisiana landfall, and the rain swath widened dramatically as the storm intensified to category 5. The data also indicates that the rapid forward speed of the storm limited inland rain accumulations. GPM's constellation of passive microwave-based satellites will provide more frequent, accurate measurement of rainfall associated with hurricanes and may possibly supply the science community with data at resolutions and frequencies useful for flood assessment and prediction.

Figure 2: The accumulated rainfall measured by TRMM during the lifespan of Hurricane Katrina

Space-based observations can be utilized to monitor hurricane development, as well. Passive microwave radiometers like the TRMM microwave imager (TMI) and the forthcoming GPM Microwave Imager (GMI) are valuable tools for measuring sea surface temperatures (SSTs). Warm SSTs (i.e., greater than 82° F) are an essential ingredient for hurricanes to develop and intensify. Typically, the immediate hurricane environment is not cloud-free; therefore, traditional infrared (IR) satellite techniques for measuring SSTs are ineffective. Passive microwave techniques can measure SSTs in clear or cloudy environments.

Figure 3 below is a composite of SSTs in the Gulf of Mexico as measured by the Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite. The large hurricane, indicated by the NOAA GOES satellite, is Hurricane Rita. The red and orange colors indicate that the Gulf of Mexico and Western Caribbean Sea were very warm and provided "high octane" fuel for these massive natural heat engines. GPM's constellation of passive microwave instruments will enable the capability to monitor SSTs in clear and cloudy environments for years to come.

Figure 3: The SST from September 17 to September 21, 2005. Areas in yellow, orange or red represent 82 degrees F or above. Temperature data is from the AMSR-E instrument on the Aqua satellite; the cloud images of Hurricane Rita were taken by the Imager on the GOES-12 satellite.

In summary, the 2005 Hurricane Season will be studied and debated by scientists for many years, and much of the data will come from satellites like TRMM. Even with its limitations (e.g., one satellite, single frequency radar, tropical inclimation orbit), TRMM has been a valuable asset for hurricane researchers and forecasters alike. GPM will provide a more comprehensive observing constellation and more advanced instrumentation so that researchers and operational meteorologist can continue to study (and be baffled by!) these amazing forces of nature that we call hurricanes.

By Dr. J. Marshall Shepherd (GPM Deputy Project Scientist) and Dr. Jeff Halverson (TRMM Outreach Scientist). For further information, please contact one of the authors at Marshall.Shepherd@nasa.gov or Jeffrey.B.Halverson.1@gsfc.nasa.gov.

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