8/31/2005

The GPM Monitor - a publication of Global Precipitation Measurement

Scientists have already successfully utilized more than seven years of data from the spaceborne Tropical Rainfall Measuring Mission (TRMM) to aid in hurricane studies. The GPM era will be particularly exciting for hurricane scientists and forecasters because GPM will provide nearly global precipitation data almost every three hours-a vast improvement over the sporadic TRMM coverage. Scientists expect to use GPM data to achieve corresponding advances in hurricane research. In this article, we take a look at how scientists have used TRMM rainfall data to study hurricanes, and explore how GPM data may further hurricane research.
Monitoring Tropical Cyclones and Their Environment

One of the most serendipitous uses of TRMM data has been in monitoring the "fuel supply" for hurricanes. Hurricanes form and thrive in warm ocean waters exceeding 27 degrees Celsius. The passive microwave radiometer on TRMM (a similar instrument will fly on all of the GPM constellation satellites) provides a capability to measure SST even when clouds are present. The global SST map (Figure 1, below) derived from TRMM and another passive microwave radiometer, illustrates that the Atlantic Ocean and Gulf of Mexico waters were very warm during the summer of 2004-a banner year for hurricane development

Traditional infrared (IR) techniques for measuring Sea Surface Temperature (SST) are only effective under clear sky conditions. In a typical hurricane environment, however, numerous clouds exist. In Figure 2, a cold wake (in blue) from Hurricane Bonnie (1998) is detected by the TRMM microwave imager. This cold wake resulted in the temporary weakening of Hurricane Danielle, which moved into the region a few days later.

TRMM measurements also provide a good assessment of rainfall in the Western Sahel region of Africa. What does this have to do with hurricanes? Many Atlantic storms are born as easterly waves that come from the continent of Africa. Several scientific studies have shown correlations between active hurricane seasons and pre-hurricane season rainfall in western Africa.

Surprisingly, one of the most dangerous elements of tropical cyclones is the inland freshwater flooding that occurs during landfall. Scientists are experimenting with TRMM data in combination with infrared measurement techniques to produce approximate three-hour rainfall totals for areas affected by hurricanes. While this merged technique may contain serious errors, it is a useful "test drive" for the era when GPM will provide more accurate, all microwave-based rainfall estimates over a similar time period. Figure 3 uses this technique to show the accumulated rainfall associated with the Letter Storms A-M for the 2004 Atlantic Hurricane season.

With the capabilities enabled by spaceborne active and passive microwave instruments, it is not surprising that NASA's operational partners like the National Oceanic and Atmospheric Administration (NOAA) and the Department of Defense (DoD) routinely use TRMM data. The DoD's Joint Typhoon Warning Center regularly relocates storm fixes and adjusts intensity assessments based on TRMM imagery. The TRMM passive microwave imagery is particularly suited for detecting early circulations in storms that may be obscured by more traditional visible or infrared techniques (see Figure 4). NOAA's National Hurricane Center also uses TRMM for such purposes. Both organizations enthusiastically await the extended coverage and better sampling time that GPM will offer.




The GPM Monitor - a publication of Global Precipitation Measurement

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