Hurricanes are one of those forces of nature that can only fully be captured by satellite imagery. For Hurricane Frances, which recently thundered toward the United States coast, the European Space Agency's (ESA) Envisat is going one better, peering through the hurricane from top to bottom, even helping to "see" under the waves to map hidden forces powering the storm.
Frances punched the Bahamas with 235-km-per-hour winds, so to have waited and watched for Frances might have been suicidal for human beings. But space-based observers such as Envisat observed its passage without danger. "Because of Envisat's multi-sensor capability, we can slice right through the hurricane with just a single satellite," explained Jose Achache, ESA Director of Earth Observation Programs. Effectively Frances is taken apart for meteorologists to study. The data returned by Envisat includes cloud structure and height at the top of the hurricane, wind and wave fields at the bottom, sea surface temperature and even sea height anomalies indicative of upper ocean thermal conditions that influence its intensity."
Important processes occur at a range of altitudes and locations throughout a hurricane--basically a large powerful storm centered around a zone of extreme low pressure. Strong low-level surface winds and bands of intense precipitation combine with strong updrafts and outflows of moist air at higher altitudes, with energy released as rainy thunderstorms. Until now, the only reliable source of such high- resolution measurements at different altitudes was from aircraft flown directly through the hurricane.
Envisat carries both optical and radar instruments, enabling researchers to observe high-atmosphere cloud structure and pressure in the visible and infrared spectrum, while at around the same time using radar backscatter to measure roughness of the sea surface and so derive the wind fields just over it. Those winds converging on the low-pressure eye of the storm are what ultimately determine the spiraling cloud patterns that are characteristic of a hurricane.
Florida-based scientists have begun to take advantage of this unique single-spacecraft combination of instruments--the Medium Resolution Imaging Spectrometer (MERIS) and Advanced Synthetic Aperture Radar (ASAR)--as hurricane season gets into full swing.
The University of Miami's Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) ground station has an agreement to acquire ASAR and MERIS data direct from Envisat, with ERS-2 wind scatterometer data set to follow in the near future.
Their access to Envisat data has come just as the second hurricane in less than a month is heading towards the Florida coast. "With MERIS and ASAR, Envisat can image both the ocean and atmosphere pretty much simultaneously, which is a very useful capability during hurricane season," said Hans Graber, Professor of Applied Marine Physics at the University of Miami and Co-Director of CSTARS.
While MERIS returns detail on the swirling clouds at the top of the hurricane, ASAR pierces right through the clouds to show the wind- wracked face of the sea beneath the storm.
"Specifically in terms of Frances, the eye of the hurricane seems to be rolling a lot right now from the top of the clouds, looking quite unstable, the information from an ASAR image should help localize its size and position on the ocean," Graber said. "And wind fields around the eye wall can be derived from ASAR data--right now all we have to go on are measurements from the hurricane hunter planes that fly right through the storm."
Simultaneous MERIS and ASAR acquisitions are planned for Friday by CSTARS, even as the storm comes closer to predicted landfall the following morning. "Our current activity is along the lines of a shakedown - we're investigating how this can be used," added Graber. "Our final goal is to get this working on an operational basis during hurricane season. …