Text: Satellites spot Œhot towers¹ in Hurricane Katrina 13:49 31 August 2005 NewScientist.com news service Kelly Young This image from the AMSR-E instrument on NASA's Aqua satellite shows the average sea surface temperatures from August 25 to 27. Areas in yellow, orange or red represents 28°C (82°F) or above ­ enough to enable a hurricane to strengthen (Image: NASA/SVS) Enlarge image A particularly tall hot tower was spotted by the TRMM satellite above Hurricane Bonnie in August 1998 (Image: NASA) Satellite images of Hurricane Katrina indicate the storm experienced several "hot tower" clouds during its development, say NASA. Thunderstorms surround the eye of hurricanes and hot towers are tall rain clouds that reach far above the rest of the hurricane near the wall of the eye. They stretch at least to the ceiling of the troposphere ­ the atmosphere's lowest layer. The heat in the "hot tower" is generated by water vapour condensing into liquid water. The hot towers, also known as convective bursts, are significant because scientists think they could be a precursor to a hurricane intensifying, a process that is still not well understood. "We think they may form right before storms intensify ­ and intensity change is one of the Holy Grail processes,² says Marshall Shepherd, deputy project scientist of the Global Precipitation Measurement Mission (GPMM) and research meteorologist at NASA's Goddard Space Flight Center in Green Belt, Maryland, US. ³Over the years in hurricane forecasting, we've gotten fairly good in forecasting where the storm's going to track." But predicting hurricane intensity is not as easy. If people in coastal cities could get a better warning of when hurricane winds speeds are about to increase, they might be able to prepare their communities for a stronger storm. Supercharged pistons The vigorous convection of the hot towers releases a lot of energy in a short amount of time into the centre of a hurricane, acting like supercharged pistons in the hurricane's engine. This can result in a lower surface pressure, which causes the wind speed to increase. This is what appears to have happened with Katrina. As well as the GPMM images, the Tropical Rainfall Measuring Mission (TRMM) satellite, run by NASA and the Japanese space agency, also saw at least two hot towers. The tower closest to the eye¹s wall measured 16 kilometres high. Soon after the TRMM image was taken, Katrina intensified to a Category 4 storm on the Saffir-Simpson scale. TRMM was launched in 1997 and was the first satellite to spot a hot tower. With its radar, TRMM measures rainfall intensity in a storm. Hurricane Bonnie in 1998 produced several hot towers as high as 18 kilometres. Another NASA satellite, Aqua, measures the surface water temperature in the Atlantic Ocean. In 2005, its Advanced Microwave Scanning Radiometer instrument has observed that water in the Atlantic basin is an average of 2-4°C warmer than in previous years. Warmer waters make it easier for hurricanes to form and could be one of the reasons there have been more hurricanes than usual this season. View a NASA movie (mpg format) of sea surface temperatures and clouds from June 9 to August 29, showing the different tracks of Hurricanes Dennis, Emily, and at the end Katrina.

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