In 2005 weather forecasters watched as a tropical depression deepened into a tropical storm. They could predict its path with confidence, but not its strength. Katrina rose to hurricane force over Florida, in the US, then dropped back to tropical storm levels. But it finished by accelerating to a category 5 hurricane before striking Louisiana.
Katrina underlined the need for better ways of forecasting hurricane strength. The challenge is to find out what is happening at sea level, where heat from the water drives the winds, in an area completely obscured by the mass of rotating cloud.
The eight satellites of the Cyclone Global Navigation Satellite System (CYGNSS) developed by Nasa and the University of Michigan, will use an unusual form of radar to spy on hurricanes. Higher wind speed makes the water rougher, and this affects radar returns, just as the smoothness or roughness of a surface affects light.
The CYGNSS satellites, weighing 30kg each, are too small to carry their own transmitters and instead use the omnipresent radio signals broadcast by the GPS satnav network. Sensitive detectors on CYGNSS will pick up GPS reflections from the sea, so the sea state and wind speed can be deduced in a process known as scatterometry. The mapping instrument originates with the British company Surrey Satellite Technology.
CYGNSS will give researchers a view of conditions in the inner core of hurricanes as they strengthen. This should lead to improved understanding of the intensification process, helping to produce better predictions of when storms will become hurricanes.
