|Figure 1: GOES-8, 0815 UT, Sep 12, 2002||Figure 2: GOES-8, 0815 UT, Sep 28, 2002|
|Figure 3: AMSU-A channel 2, Sep 12, 2002||Figure 4: AMSU-A channel 2, Sep 28, 2002|
Tropical Storm Isidore was born in mid-September north of Venezuela. It subsequently hit Mexico's Yucatan Peninsula as a Category 3 hurricane and came ashore near New Orleans on September 26th packing winds just below hurricane strength. Around the time of September 27, the storm was downgraded to a tropical depression as the system moved into Tennessee.
At the time the Aqua spacecraft first passed over Isidore, it was classified as a Category 3 (possibly 4) hurricane, with minimum pressure of 934 mbar, maximum sustained wind speeds of 110 knots (gusting to 135) and an eye diameter of 20 nautical miles. Isidore was later downgraded to a Tropical Storm and then a Tropical Depression as it lost energy.
Figures 1 and 2, two images from the National Oceanic and Atmospheric Administration's Geostationary Operational Environmental Satellites show no significant weather systems over the southeastern United States on September 12 and September 28 (16 days apart). However, the microwave component of the Atmospheric Infrared Sounder Experiment on NASA's Aqua spacecraft shows a striking difference. The difference in the two microwave images (figures 3 and 4) from the AIRS Advanced Microwave Sounding Unit is primarily due to flooding after Tropical Storm Isidore. Water has a very low surface emissivity at this frequency, and that causes surface water to appear very cold (even though it is not). Land appears relatively warm (well above freezing - 273 K, even at night as seen is these images), but if there is standing water, the apparent temperature drops precipitously. Figure 4, taken just about a day after the remnants of Isidore passed over the southeast, shows heavy flooding along the Mississippi, especially in the states of Mississippi and Tennessee, but other states are also affected. The spatial resolution of the AMSU-A instrument is relatively large (each measurement spot is about 25 miles in diameter at the center of the swath), but the enormous thermal contrast in the microwave between land and water makes even small flooded areas stand out.
Figure 5: Difference image, 9/12 and 9/28)
The Aqua spacecraft has an exact 16-day repeat cycle, that is why the pre-Isidore image is 16 days prior to the post-Isidore image. They have exactly the same coverage, which makes it possible to obtain a difference image (figure 5). The difference image is the difference between the September 28 and September 12 images shown. In the difference image, white indicates no difference at all, green is very little difference, blue/purple indicates primarily heavy flooding. Red indicates warming likely due to warmer weather. (The straight lines on the right and left edges of the difference image are caused by slight differences between the two repeat passes of Aqua).
The Atmospheric Infrared Sounder, AIRS, in conjunction with the Advanced Microwave Sounding Unit, AMSU, senses emitted infrared and microwave radiation from Earth to provide a three-dimensional look at Earth's weather and climate. Working in tandem, the two instruments make simultaneous observations all the way down to Earth's surface, even in the presence of heavy clouds. With more than 2,000 channels sensing different regions of the atmosphere, the system creates a global, three-dimensional map of atmospheric temperature and humidity, cloud amounts and heights, greenhouse gas concentrations, and many other atmospheric phenomena. Launched into Earth orbit in 2002, the AIRS and AMSU instruments fly onboard NASA's Aqua spacecraft and are managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif., under contract to NASA. JPL is a division of the California Institute of Technology in Pasadena.