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2005 June — Vol. XIV, No. 4 |
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Current Issues |
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EOM June 2005 > Features
Probing the Eye of the Storm: Hurricane Hunting Satellites
Thomas F. Lee
Jeffrey D. Hawkins
F. Joseph Turk
Kim Richardson
Charles Sampson
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!
Figure 2: TRMM TMI 85 GHz image of Charlie at the same time as Fig. 1. Microwave energy, on which this image is based, penetrates through clouds to see tiny eye and surrounding eye wall (red ring over western Cuba). The colored portion of the figure is from the TRMM TMI satellite sensor. Surrounding black and white is from the GOES infrared sensor and fills in gaps outside the covered area. Click on image to see enlarged.
A day and a half later the TRMM TMI captures the storm again on its destructive path through Florida (Figure 3). Having strengthened in the Gulf of Mexico, it now engulfs a large portion of the southern part of the state. Although its spiral shape signifies an intense circulation, the storm is starting to lose definition as it moves over land.
Figure 3: TRMM TMI 85 GHz image of Charlie over Florida. Notice the increased size of the central circulation (red and yellow) compared to Figure 2. Insured losses from Charlie in Florida are estimated at $7.4 billion. Click on image to see enlarged.
Passive microwave images are most vital over open water where other observations are sparse. The images are particularly valuable to the Joint Typhoon Warning Center (JTWC) in Pearl Harbor Hawaii, a forecasting facility that must monitor storms over huge regions of the Pacific and Indian Oceans. Let's look at a passive microwave image of Typhoon Rananim from the Advanced Microwave Scanning Radiometer (AMSR-E) aboard the NASA Aqua satellite (Figure 4). Composed of concentric rainbands, the typhoon is destined for a destructive swath through Taiwan and mainland China. There are no hurricane hunter (or "reconnaissance") flights into storms in the Pacific, and these satellite images are essential to gauge the strength and to estimate the center position of storms. However, passive microwave imagers like AMSR-E provide at most two images per twenty-four hour period over a given storm. Fortunately, other microwave imagers like those aboard the Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) operational satellites often fill in the coverage gaps.
Figure 4: AMSR-E image of Typhoon Rananim approaching Taiwan and the Chinese mainland. Notice the huge eye (blue dot in the storm center) compared to Hurricane Charlie in Figure 2. Curved bands of red, yellow, and green indicate high winds and heavy precipitation at the surface. Click on image to see enlarged.
Public use of images like Figures 1- 4 is growing as websites distribute the information in near-real-time. The United States Navy maintains two such websites from the Naval Research Laboratory and the Fleet Numerical Meteorology and Oceanography Center. Satellite images from around the world are shown along with tropical cyclone track prediction maps, forecasting the location of storm landfall and expected winds. One key limitation is latency, the data relay time between the overpass of a satellite and the appearance of images on the web applications. Prolonged latencies of several hours can deny forecasters crucial information, impacting evacuation orders and other warnings.
Fortunately, the National Polar-Orbiting Operational Environmental Satellite System (NPOESS), first scheduled for launch in 2010, will dramatically improve data latency. Hurricane and typhoon images will be available everywhere within 15 minutes (average elapsed time) of satellite overpass time, accelerating the integration of accurate information into emergency forecasts. The microwave imager aboard NPOESS, the Conical Microwave Imager Sounder (CMIS), draws upon the lessons learned from current instruments. It will have many more sensing channels than most of the current sensors, as well as improved information about vertical storm structure and precipitation. Significantly, it will produce maps of ocean surface wind speed and direction. The full NPOESS constellation of satellites, which will come into fruition during the decade starting in 2010, will expand hurricane hunting from space to an amazing new level. 
Acknowledgements
The authors greatfully acknowledge the support of the research sponsor, the National Polar-orbiting Operational Environmental Satellite System's (NPOESS) Integrated Program Office (IPO) located in Silver Spring, Maryland.
About the Authors
The authors are members of the Satellite Tropical Cyclone team at the Naval Research Laboratory in Monterey California. They maintain a World Wide Web application that makes timely satellite images of tropical cyclones available to the public and forecasting agencies around the world. The lead author can be reached at [email protected].
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