As Hurricane Rita entered the Gulf of Mexico, ESA's Envisatsatellite's radar was able to pierce through swirling clouds todirectly show how the storm churns the sea surface. This image has thenbeen used to derive Rita's wind field speeds.
Envisat acquiredthis Advanced Synthetic Aperture Radar (ASAR) image at 0344 UTC on 22September (2345 on 21 September in US Eastern Daylight Saving Time),when Hurricane Rita was passing west of Florida and Cuba. The image wasacquired in Wide Swath Mode with resolution of 150 metres. Envisat'soptical Medium Resolution Imaging Spectrometer (MERIS) is also beingused to observe the storm during daylight, returning details of itscloud structure and pressure.
Notably large waves are seen aroundthe eye of Hurricane Rita in the radar image. ASAR measures thebackscatter, which is a measure of the roughness of the ocean surface.On a basic level, bright areas of the radar image mean higherbackscatter due to surface roughness. This roughness is stronglyinfluenced by the local wind field so that the radar backscatter can beused in turn to measure the wind.
So the Center for SoutheasternTropical Advanced Remote Sensing at the University of Miami used thisASAR image to calculate the speed of Hurricane Rita's surface windfields – showing maximum wind speeds in excess of 200 kilometres perhour.
"The most detailed information about hurricane dynamics andcharacteristics are obtained from dedicated flights by hurricane hunteraircraft," explains Hans Graber of CSTARS. "However these flightmissions cannot always take place. Satellite remote sensing provides acritical alternative approach.
"It is critical for weatherforecasters to obtain reliable characterization of the eye walldimension and the radii of gale- tropical storm- and hurricane-forcewinds in order to provide skilful forecasts and warning. Satellitebased observations will facilitate better understanding of hurricaneevolution and intensification.
"Radar images penetrate throughclouds and can easily detect the eye replacement cycle of hurricaneswhich are precursors to further intensification."
Rita was amaximum Category Five on the Saffir-Simpson Hurricane Scale when theASAR image was acquired. As it continues west through the Gulf ofMexico it has weakened to a still-dangerous Category Four. Rita isexpected to make landfall on the Gulf coast during the morning of 24September.
ERS-2 joins in Rita observations
Thesame day Envisat acquired its ASAR image of Rita, its sister spacecraftERS-2 also made complementary observations of the hurricane'sunderlying wind fields using its radar scatterometer.
Thisinstrument works by firing a trio of high-frequency radar beams down tothe ocean, then analysing the pattern of backscatter reflected upagain. Wind-driven ripples on the ocean surface modify the radarbackscatter, and as the energy in these ripples increases with windvelocity, so backscatter increases as well. Scatterometer resultsenable measurements of not only wind speed but also direction acrossthe water surface.
What makes ERS-2's scatterometer especiallyvaluable is that its C-band radar frequency is almost unaffected byheavy rain, so it can return useful wind data even from the heart ofthe fiercest storms – and is the sole scatterometer of this typecurrently in orbit.
The ERS-2 Scatterometer results for HurricaneRita seen here have been processed by the Royal NetherlandsMeteorological Institute (KNMI). They are also routinely assimilated bythe European Centre for Medium-Range Weather Forecasting (ECMWF) intotheir advanced numerical models used for meteorological predictions.
"Scatterometerdata from the ERS-2 platform provide high-quality wind information inthe vicinity of tropical cyclones," states Hans Hersbach of ECMWF. "Fora Hurricane like Rita, the combination of such observations with[in-situ] dropsonde data enables the analysis system at ECMWF toproduce an improved forecast."
NOAA using Envisat radar altimetry data
AnotherEnvisat instrument called the Radar Altimeter-2 uses radar pulses tomeasure sea surface height (SSH) down to an accuracy of a fewcentimetres.
Near-real time radar altimetry is a powerful toolfor monitoring a hurricane's progress and predicting its potentialimpact. This is because anomalies in SSH can be used to identify warmerocean features such as warm core rings, eddies and currents.
TheUS National Oceanic and Atmospheric Administration (NOAA) is utilisingEnvisat RA-2 results along with those from other space-borne altimetersto chart such regions of 'tropical cyclone heat potential' (TCHP) andimprove the accuracy of Hurricane Rita forecasting.
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 ofthe storm are what ultimately determine the spiralling cloud patternsthat are characteristic of a hurricane.
Additional Envisatinstruments can be used to take the temperature of the warm oceanwaters that power storms during the annual Atlantic hurricane season,along with sea height anomalies related to warm upper ocean features.
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