Energy release from hurricanes
Image of Hurricane Katrina in the Gulf of Mexico taken on August 28, 2005, where it powered up to a Category 5 storm on the Saffir-Simpson hurricane scale, packing winds estimated at 175 mph. (Source: NOAA) 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 kilometers/8 miles 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.
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 centimeters/day (0.6 inches/day) of rain inside a circle of radius 665 kilometers (km) (360 nautical miles). (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 gram. 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 wind speed cubed.
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 wind speed for the inner core of the hurricane. Doing the latter and using 40 meter/second (90 mph) winds on a scale of radius 60 km (40 n.mi.), one gets a wind dissipation rate (wind generation rate) of 1.5 x 1012 Watts. 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 spiraling winds is a huge ratio of 400 to 1.
Further Reading
