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News  Hurricane Dean tracked from space
ESA satellites are tracking the path of Hurricane Dean as it rips across the Caribbean Sea carrying winds as high as 260 km/h. The hurricane, which has already claimed eight lives, is forecast to slam into Mexico’s Yucatan Peninsula on Tuesday morning. Dean was upgraded early Tuesday to a Category 5 – the highest on the Saffir-Simpson scale – before pummelling the peninsula. Knowing the strength and path of hurricanes is critical for issuing timely warnings; satellites are the best means of providing data on the forces that power the storm, such as cloud structure, wind and wave fields, sea surface temperature and sea surface height.
Instruments aboard ESA’s Envisat and ERS-2 satellites allow them to peer through hurricanes. Envisat carries both optical and radar instruments, enabling researchers to observe high-atmosphere cloud structure and pressure in the visible and infrared spectrum.
ERS-2 uses its radar scatterometer to observe the hurricane's underlying wind fields. The scatterometer instrument works by firing a trio of high-frequency radar beams down to the ocean, then analysing the pattern of backscatter reflected up again. Wind-driven ripples on the ocean surface modify the radar backscatter, and as the energy in these ripples increases with wind velocity, backscatter increases as well. Scatterometer results enable measurements not only of wind speed but also of direction across the water surface.
What makes ERS-2's scatterometer especially valuable is that its C-band radar frequency is almost unaffected by heavy rain, so it can return useful wind data even from the heart of the fiercest storms.
"The wind field derived from the ESA ERS-2 scatterometer measurements are distributed via a EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites) project to a registered database of a few hundred users, originating from all over the world, includng the Americas, Australia, Asia and Europe. Scatterometer winds are used directly by shift meteorologists in forecast rooms and to initialise Numerical Weather Prediction models aiding the forecasting of hurricanes 5 days ahead."
Water temperatures are the main underlying energy reservoir that power hurricanes; together with the correct atmospheric conditions, temperatures need to exceed 26ºC in order to form and maintain a tropical cyclone. Because warm water expands, scientists can locate warm underwater ocean features by detecting bulges in the ocean surface height, as detected by RA-2.
The US National Oceanic and Atmospheric Administration (NOAA) is using Envisat RA-2 results along with those from other space-borne altimeters to chart TCHP and improve the accuracy of hurricane forecasting.
AATSR information can be correlated with MERIS data cloud height and development to gain a good estimate of the hurricane's precipitation potential, and improve understanding of how this relates to its overall intensity. Condensation of water vapour releases latent heat, which warms the vicinity of the hurricane eye. This in turn evaporates more surface water and feeds the heat engine powering the hurricane.
The International Charter 'Space and Major Disasters' has been activated to provide Earth Observation satellite data for assessing the damage of Hurricane Dean in Belize.
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