On Tuesday I sat down with a small group of journalists and Kerry Emanuel, Professor of Atmospheric Science at MIT and author of Divine Wind: The History and Science of Hurricanes who walked us through the current state of the science of predicting hurricanes. He also talked about climate change and prospects for a change in the incidence and intensity of hurricanes over the next century.
Hurricanes and “Hybrid Storms”
Emanuel introduced us to the many terms for “tropical cyclone” such as “hurricane” or “God of Evil”, “Furicane” a term Shakespeare used, and Japanese “Typhoon” or “Divine Wind”. But the name of a hurricane is not just an etymological curiosity: insurance companies will not pay hurricane insurance unless a storm is categorized as a hurricane by the National Weather Service. Sandy was a hurricane coming up the East coast over open water but when it began to come ashore it changed to a “hybrid storm”, much wider than a hurricane, less powerful, but more destructive due to its ability to produce water surges. Tell that to home owners on the Jersey Shore who lost their homes to Sandy but can’t get an insurance payout due to how a storm is defined!
What Produces a Hurricane?
In order for a hurricane to develop the following conditions must be present:
- small vertical shear of horizontal wind
- interaction of radiation with clouds and/or water vapor
- feedback of convective downdraft surface winds on surface fluxes
- sufficiently high surface temperature
Emaneul set us straight about the incidence of hurricanes around the globe. Only 12% of the world’s hurricanes occur in the Atlantic but they get 90% of the press. He pulled up a real time map of the world which showed two hurricanes occurring at that very moment in the South Indian Ocean and near Australia. The following diagram shows the parts of a hurricane.
The Carnot Cycle
Emanuel likened the energy production of a hurricane to the steam engine perfected by James Watt in Scotland in 1765. The Scots came up with the invention but the thermodynamic theory, called the Carnot Cycle, was conceived of by the French. The Carnot Cycle is the most efficient cycle for converting a given amount of thermal energy into wind; a hurricane is a perfect Carnot cycle.
When ocean temperatures are warm one third of the heat contained in them can be converted to wind very quickly. It is similar to water evaporating from the skin when you step out of a shower. As the water evaporates you become cold and towel off quickly; the water has evaporated from your skin and is now in the air.
One of the forecasting devices Emanuel monitors is the Tropical Cyclone Maximum Potential Intensity Forecast (TCMPI) which shows areas of the world that are more likely to see the most intense storms in the future. Emanuel is involved in the Intergovernmental Panel on Climate Change (IPCC) five year report. Although the Atlantic Ocean has become more active recently, the TCMPI shows the most active area of the world in the coming decades will be the Pacific Ocean.
Emanuel is also involved in the Risk Prediction Initiative (RPI) seeking to understand the risks involved in hurricanes. According to Emanuel Sandy was a one in 700 year event but it is not yet clear whether global warming is changing things so that Sandy events will happen much more frequently, say, once in 100 or once in 70 years. Sitting on the board of insurance companies Emanuel has seen how unpredictable weather can hit the bottom line of insurers.
The Genesis Puzzle
Emanuel has been working with software that is becoming increasingly effective in cracking the Genesis Puzzle, the holy grail of hurricane scientists, which seeks to understand and model the origin and development of hurricanes. Emanuel described the formation of a hurricane as the coming together of many different cloud types into a white center with clear skies all around. Within a matter of hours the white mass begins to spin and an eye forms. By focusing on this phenomenon with computer modeling Emanuel is making great progress in determining how hurricanes may form; the image below show just such a cloud.
Emanuel has ridden into the eye of many a hurricane and hopes to one day lead tourists on scientific expeditions to have the experience that he describes as awesome. Two-dimensional pictures, he says, do not do justice to the beauty of the eye walls of a hurricane.