Probing the Eye of the Storm: Hurricane Hunting Satellites

Never has a hurricane been monitored more closely from space than Hurricane Charley as it gathered strength in the Caribbean, smashed through Cuba, and devastated the Florida coastline. Like many hurricanes, Charlie defied easy prediction; thus, correct satellite analysis was essential for correct estimates of strength and position. An infrared view of Charlie (Figure 1) from the Geosynchronous Orbiting Environmental Satellite (GOES-East) shows the central core over Cuba as a circular white mass. The problem with this traditional type of weather satellite image is that it only sees the top canopy of cirrus clouds. It often cannot see the fierce, devastating eye wall hidden well below the canopy, nor can it always see the eye, that area of remarkable calm at the center of the vortex.

Thus, we turn to an image (Figure 2) provided by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) at the same time as Figure 1. Seeing through the various cloud layers that obscured our view in Figure 1, TRMM TMI sees the red ring that marks the whirling center of the storm. Only about 50 miles across, the ring represents a punishing zone of torrential precipitation and damaging winds. Near this time the wind gusted to 124 miles per hour just west of Havana. The tiny eye, a zone of no wind, can be seen within.

Figure 1: GOES-East infrared image of Hurricane Charlie over western Cuba. Hurricane forecasters look at these kinds of images to locate the storm center, but no telltale eye appears here. Click on image to see enlarged.

A ground-based weather radar would give a view similar the satellite image in Figure 2, though the technology employed on the satellite is different from ground radar in a key respect. A weather radar is an "active" microwave system: using a power source, it continuously emits pulses of microwave energy and then calibrates the response from rain bands within the storm. However, the microwave radiometer aboard the TRMM spacecraft is a "passive" system, receiving microwave information naturally emitted by precipitation. Since there is no need for a cumbersome onboard power source, passive systems are relatively inexpensive, allowing the sensors to monitor huge regions of the earth. With its high spatial resolution and tropical coverage, TRMM TMI has been warmly welcomed by hurricane forecasters, despite the fact that it was originally launched to study climate!