Cooling Rain May Weaken Hurricanes, Increase Tornado Chances

That same cooling rain, however, might also cause shallow warm and cold "fronts" within a hurricane system, making it more likely to spin off tornadoes as the storm weakens.

"It's almost a case of pick your poison," said Dr. Kevin Knupp, a professor of atmospheric science and director of the severe weather research team in UAH's Earth System Science Center. Knupp's team of scientists and students has put itself and a suite of mobile weather instruments and sensors in the paths of seven hurricanes and tropical storms since 1998.

The "cooling rain" observations were made as tropical storm Gabriellecame ashore near Venice, Fla., (directly over the UAH team and a mobile Doppler radar from the University of Oklahoma) on Sept. 14, 2001. Results of this research were published in the January 2006 edition of the "Journal of Atmospheric Sciences."

As the storm moved over the UAH/OU team, instruments showed that the "off shore" flow in front of the storm was stable and cooling. Temperatures dropped to as low as 70 degrees Fahrenheit (21 Celsius). At the same time, wind blowing from out at sea onto the shore stayed warm and became turbulent. The boundary layer separating winds at the surface from the fast-moving higher altitude "jet-like flow" dropped from 600 meters in front of the storm down to 300 meters over the water.

Knupp and his research collaborators theorize that the cooling was caused by downdrafts and evaporative cooling from extensive "stratiform" rain that fell on shore in front of the storm.

This effect might be "forecastable," said Knupp. "With current models, which provide a fairly good forecast of wind shear, you could use this theory to provide intensity change forecasts one or two days before hurricane landfall."

Some hurricanes and tropical cyclones, such as hurricane Ivan, have stratiform rain in front of them when they come ashore, Knupp said. Other hurricanes, including hurricane Katrina, might also have stratiform rain without the pronounced cooling. Additional observations are needed to establish a relationship between pre existing atmospheric conditions and strong cooling in stratiform rain.

"That might be one of the reasons Katrina remained relatively strong well into Mississippi," he said. "Hurricanes assume many forms, so we need additional mobile measurements from several hurricanes to test this hypothesis. At the same time, now that we know what to look for we can go back and look at existing data from past storms to substantiate our findings from Gabrielle."