Testing the Waters Page 4
Another way in which the TRMM satellite may aid in future intensity forecasts is in its ability to measure the temperature of the sea through cloud cover. Before a jump in intensity can occur in a hurricane, the hurricane must first pass over an area where the water is especially warm. Traditionally, scientists have used satellite sensors' measurements of thermal infrared radiation—aboard satellites such as GOES and AVHRR—to retrieve sea surface temperatures. In general, the warmer the water the more infrared radiation it emits. But again, the problem with testing the waters under a hurricane is that infrared radiation cannot penetrate the thick cloud cover. Most forecasting models now use sea surface temperature data gathered up to a week before a hurricane moves into an area.
 
 
sea surface temperature following Hurricane Bonnie

Frank Wentz, a physicist and president of Remote Sensing Systems in California, explains that knowing sea surface temperatures directly underneath a hurricane is crucial to predicting its intensity. All types of unexpected changes can occur. In Opal's case, some researchers believe an undetected warm eddy floating through the Gulf wandered into the hurricane's path and thus provided it with an unexpected supply of warm water. In other instances, warm waters in the ocean will run deeper than scientists expect. The hurricane will intensify because it has a much greater supply of undetected fuel.

For the past three years, Wentz's company has been using data from the TRMM TMI instrument to create daily global data sets of sea surface temperatures below both clear skies and cloud cover. (In particular, they use TMI's 10.7 GHz channel.) Wentz explains that when the sun heats up the water, the water emits microwave radiation as well as infrared radiation. "Unlike infrared radiation, nearly 97 percent of the microwave radiation that comes from the Earth's surface goes through the clouds. Using the other channels on TMI, we can penetrate the clouds to see the water vapor down a column of atmosphere, liquid water within a cloud, and rain rates." Wentz uses these values to account for the remaining 3 percent of the 10.7 GHz signal that is lost as it travels through the atmosphere. In short, Wentz and his associates observe the lower frequencies of the microwave spectrum that the instrument detects-the more radiation it measures, the warmer the water is. Already they have uncovered some interesting findings by analyzing TRMM data of hurricanes Bonnie and Danielle. In August of 1998, hurricane Bonnie struck the coast of North Carolina with Category 3 winds, causing some moderate flood damage. Danielle followed on Bonnie's heels and was expected to brush up against the North Carolina coast before heading north out into the Atlantic. Yet, when Danielle hit Bonnie's wake, it dropped steeply in intensity from a high Category 2 storm to a low Category 1. While the forecasters told North Carolina residents to expect another torrential rain, all they got were some good surfing conditions and late summer showers.

At the time, scientists suspected that Bonnie's wake somehow altered the sea surface temperatures, but no one knew exactly how. Upon reviewing the TRMM data of sea surface temperatures under both hurricanes, Wentz and his team discovered that Bonnie stole all of Danielle's fuel! "Bonnie gathered much of the heat energy from the ocean's surface. The friction from Bonnie's winds also churned up the warmer surface water and brought up cool water from deep in the ocean," says Wentz. When Danielle hit this patch of cold water, it immediately dropped in intensity. The TRMM reading told the whole story and provided evidence that one hurricane can slow down another that follows in its wake.

At this point, Wentz says he and his team are carefully reviewing the data to see what they can find on a case-to-case basis. By analyzing previous model forecasts and comparing them to actual data throughout a given hurricane's lifetime, his team can correlate errors in the input data with errors in the model predictions. This process, over time, will help them refine their model.

"Eventually what we'd like to see are numerical models that predict both the track and intensity of a hurricane from sea surface temperatures. One way this could happen is if we start using microwave measurements of sea surface temperature," says Wentz. It is not simply enough to plug these new data into existing hurricane models that use sea surface temperatures taken by infrared remote sensors. Instead, agencies like NOAA need to develop new forecasting models that are tuned to these new data and incorporate them operationally.

Halverson explains that his work and Wentz's could be used in conjunction to help the forecasting community predict a storm's intensity. With a model that uses continuous sea surface temperature data, forecasters could see how a hurricane is developing at all times. If warm water turns up, then the model could be employed to monitor the hurricane as it passes over and look for sudden flare-ups in intensity. "The idea is that all this information will converge and we will get a complete picture of the mechanisms that drive a hurricane," says Halverson. "Then we may be able to better predict disasters such as Hurricane Opal."

References

Kidder, S. Q., M. D. Goldberg, R. M. Zehr, M. DeMaria, J. F. W. Purdom, C. S. Velden, N. C. Grody, and S. J. Kusselson, 2000: Satellite analysis of tropical cyclones using the Advanced Microwave Sounding Unit (AMSU). Bulletin of the American Meteorological Society, Volume 81, pp. 1241-1259.

Henson, R., 1998: The Intensity Problem, Weatherwise, September/October, pp. 1-7.

NASA TRMM Website, 1996: Nasa Facts: TRMM Instruments, Greenbelt, MD. (http://trmm.gsfc.nasa.gov/)

National Hurricane Center Public Affairs, 1996: Hurricane Tracking Models: Helping to Forecast Severe Storms, Miami, FL.

NESDIS Public Affairs, 1995: NOAA's Geostationary and Polar-Orbiting Environmental Satellites, Suitland, MD.

University of Illinois, 1998: WW2010, Urbana-Champaign, IL. (http://ww2010.atmos.uiuc.edu/)

Wentz, Frank J., Chelle Gentemann, Deborah Smith, Dudley Chelton, 2000:"Satellite Measurements of Sea-Surface Temperature Through Clouds." Science, Volume 288, Number 5467, pp. 847 - 850.

back TRMMing Off the Tops of Clouds

  One of the major stumbling blocks for forecasters has been the precise measurement of sea surface temperatures under a storm as it forms and evolves over time. In this scene, clouds (acquired by GOES) have been made translucent to allow an unobstructed view of the surface. Notice Hurricane Bonnie approaching the Carolina Coast (upper left) and Hurricane Danielle following roughly in its path (lower right). The ocean surface has been falsely colored to show a map of water temperature, measured by the TRMM Microwave Imager (TMI). Dark blues are around 75°F, light blues are about 80°F, greens are about 85°F, and yellows are roughly 90°F. In the animation, notice that as Hurricane Danielle followed in Bonnie's path, the wind speed of the second storm dropped markedly, as available energy to fuel the storm dropped off because the surface waters in Bonnie's wake were cooler. But when Danielle left Bonnie's wake, wind speeds increased due to temperature increases in surface water around the storm. view animation (6MB)

Image courtesy TRMM Project, Remote Sensing Systems, and Scientific Visualization Studio, NASA Goddard Space Flight Center

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