Hurricane Florence made landfall on Friday and has since battered the Carolinas, dumping record breaking amounts of rain. The death toll has risen to 23, including 17 in North Carolina. AFP photo.
The amount of energy released in a hurricane that goes to maintaining spiralling winds is a ratio of 400 to 1
By Chris Landsea (National Hurricane Center)
This article was published by the National Hurricane Center.
Hurricanes can be thought of, to a first approximation, as a heat engine; obtaining its heat input from the warm, humid air over the tropical ocean, and releasing this heat through the condensation of water vapor into water droplets in deep thunderstorms of the eyewall and rainbands, then giving off a cold exhaust in the upper levels of the troposphere (~12 km/8 mi up).
One can look at the energetics of a hurricane in two ways:
- the total amount of energy released by the condensation of water droplets or …
- the amount of kinetic energy generated to maintain the strong swirling winds of the hurricane (Emanuel 1999).
It turns out that the vast majority of the heat released in the condensation process is used to cause rising motions in the thunderstorms and only a small portion drives the storm’s horizontal winds.
- Method 1) – Total energy released through cloud/rain formation:An average hurricane produces 1.5 cm/day (0.6 inches/day) of rain inside a circle of radius 665 km (360 n.mi) (Gray 1981). More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands. Converting this to a volume of rain gives 2.1 x 1016 cm3/day. A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives
5.2 x 1019 Joules/day or
6.0 x 1014 Watts.
This is equivalent to 200 times the world-wide electrical generating capacity – an incredible amount of energy produced!
- Method 2) – Total kinetic energy (wind energy) generated:For a mature hurricane, the amount of kinetic energy generated is equal to that being dissipated due to friction. The dissipation rate per unit area is air density times the drag coefficient times the windspeed cubed (See Emanuel 1999 for details). One could either integrate a typical wind profile over a range of radii from the hurricane’s center to the outer radius encompassing the storm, or assume an average windspeed for the inner core of the hurricane. Doing the latter and using 40 m/s (90 mph) winds on a scale of radius 60 km (40 n.mi.), one gets a wind dissipation rate (wind generation rate) of
1.3 x 1017 Joules/day or
1.5 x 1012Watts.
This is equivalent to about half the world-wide electrical generating capacity – also an amazing amount of energy being produced!
Either method is an enormous amount energy being generated by hurricanes. However, one can see that the amount of energy released in a hurricane (by creating clouds/rain) that actually goes to maintaining the hurricane’s spiralling winds is a huge ratio of 400 to 1.
About the author: Christopher W. Landsea is the branch chief of the Tropical Analysis and Forecast Branch (TAFB) at NOAA’s National Hurricane Center (NHC) in Miami. The branch generates wind and wave forecasts for the Caribbean Sea, Gulf of Mexico, tropical North Atlantic Ocean, and tropical northeastern Pacific Ocean. The TAFB supports the Hurricane Specialist Unit at NHC by providing tropical cyclone position and intensity estimates based on the Dvorak technique.