Satellite images of Hurricane Katrina indicate the storm experienced several 鈥渉ot tower鈥� 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 鈥� the atmosphere鈥檚 lowest layer. The heat in the 鈥渉ot tower鈥� is generated by water vapour condensing into liquid water.
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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.
鈥淲e think they may form right before storms intensify 鈥� and intensity change is one of the Holy Grail processes,鈥� says Marshall Shepherd, deputy project scientist of the Global Precipitation Measurement Mission (GPMM) and research meteorologist at NASA鈥檚 Goddard Space Flight Center in Green Belt, Maryland, US. 鈥淥ver the years in hurricane forecasting, we鈥檝e gotten fairly good in forecasting where the storm鈥檚 going to track.鈥�
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鈥檚 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 eye鈥檚 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掳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.
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.
