ESA's multi-sensor Envisat satellite has gathered a uniqueview of Hurricane Katrina in the Gulf of Mexico. While an optical imageshows characteristic spiralling cloud patterns, a simultaneous radarobservation pierces through the clouds to show how Katrina's250-kilometre-an-hour winds scour the sea surface.
Envisatsimultaneously acquired these images at 1550 UTC (1150 US EasternDaylight Saving Time) on 28 August, with its Medium Resolution ImagingSpectrometer (MERIS) and Advanced Synthetic Aperture Radar (ASAR).
Atthe time of acquisition Hurricane Katrina was a maximum Category Fiveon the Saffir-Simpson scale. Further ESA acquisitions are beingprioritised because the International Charter on Space and MajorDisasters has been activated.
The eye of the hurricane is seenclearly in both images, with the wall of the eye clearly visible in theMERIS image, the area of the hurricane where the fastest winds andhighest rainfall is found.
The ASAR instrument works by recordingsignal backscatter based on surface roughness: the wind-free stormcentre – seen enclosed within a dark ring feature measuringapproximately 60 kilometres across - shows up darker because the watershere are hardly rippled by comparison to the sea around it.
HurricaneKatrina, which formed in the Bahamas in mid-August, struck SouthFlorida on 25 August, killing nine people and leaving a million morewithout electricity. It is poised to strike coastal Louisiana and NewOrleans. Hurricane-strength winds of at least 119 kilometres per hourcurrently extend 136 kilometres from its centre.
Katrina hastoday dropped from a maximum Category Five to a Category Four storm onthe Saffir-Simpson scale, but forecasters warn it could again grow inintensity having made landfall.
With serious flooding anticipatedin the hurricane's wake as well as direct storm damage, the USGeological Service (USGS) activated the International Charter on Spaceand Major Disasters on 27 August.
ESA is a founding member of theCharter, which represents a joint effort by global space agencies toput resources at the service of rescue authorities responding to majornatural or man-made disasters. To date the Charter has been activatedmore than 80 times.
Ahurricane is basically a large, powerful storm centred around a zone ofextreme low pressure. Strong low-level surface winds and bands ofintense precipitation combine strong updrafts and outflows of moist airat higher altitudes, with energy released as rainy thunderstorms.
Envisatcarries both optical and radar instruments, enabling researchers toobserve high-atmosphere cloud structure and pressure in the visible andinfrared spectrum, while at the same time using radar backscatter tomeasure the roughness of the sea surface and so derive the wind fieldsjust above it.
Those winds converging on the low-pressure eye of thestorm are what ultimately determine the spiralling cloud patterns thatare characteristic of a hurricane.
Additionally Envisatinstruments can potentially be used to take the temperature of the warmocean waters that power storms during the annual Atlantic hurricaneseason, or detect sea height anomalies related to warm upper oceanfeatures known as 'loop currents'.
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