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published on 7 January 2006 in air

In the eye of the cyclone

A year of studies
Recent studies confirm that the warming of the atmosphere causes an increase in the temperature of surface water and that these warmer waters are “fuel” for hurricane formation. Data analysis of the past 50 years confirmed that everytime that there was an increase in the average global temperature during the hurricane season, the consequent rise in water temperature of the tropical Atlantic would increase storm formation.
Hurricane winds stir the ocean waters bringing to the surface the colder water from down below which then reduces the intensity of the storm. When a hurricane passes over a layer of hot water which is over 100 meters deep, the winds will continue to bring up “fuel” which powers the hurricane; at the tropics the ocean is characterized by these “bags” of hot and deep water. In the Gulf of Mexico the Loop Current is a warm water current which is responsible for the sudden upgrade to stage 5 of Katrina and Rita last year.
The waves, instead can weaken storm intensity: by reaching heights over 30 meters they offer resistance to the very same winds that caused them.

Difficulty in forecasting
To improve forecasts it is necessary to understand how a hurricane works and this is why it is necessary to have detailed information from the heart of storms. In 2005, the most intense hurricane season ever registered, the experts studied storms with satellites, airplanes, and specially equipped probes. Yet we musn’t be surprised by the sudden and unexpected evolutions in the behavior of a hurricane.
Available data and models used for analysis aren’t detailed enough to forecast the sudden changes in the behavior of hurricanes. Compared to the early nineties, when errors could be in the range of up to 37 kilometers per hour, the three-day forecast of wind speeds has greatly improved, but a hurricane can undergo sudden changes and a stage 5 storm can decrease to stage 3 or viceversa within just few hours. For this reason new computerized models which are able to simulate in detail the interaction between atmosphere, waves and ocean are being studied.
The data gathered by radio controlled probes is analysed through the use of computerized models that simulate a storm and its possible evolution in order to hone the forecast of its path. In the seventies, the three day forecast of a storm path could be off by 770 kilometers, in 2005 this error range has been reduced to 290 kilometers.

Inside the hurricane
But how is the data gathered? The top part of a hurricane is registered by satellites at an altitude ranging from 35.000 to 800 kilometers. Weather satellites are very useful to follow the path of a hurricane but they only show us an image of the top part of the clouds; to trace shape and position, to locate the eye and the stronger rising currents it is necessary to use satellites equipped with infrared sensors. Thanks to satellites equipped with radar and microwave sensors it is possible to trace precipitation maps as well.
“Hurricane hunters” are equipped with instruments to detect only high altitude condition, above the heaviest turbolence, flying to the heart of the hurricane. The G-IV jet of the NOAA (National Oceanic & Atmospheric Administration) flies over and around budding hurricanes at a hight of 13.000 meters, while the P-3  hurricane hunter flies at an altitude ranging from 4000 to 2500 meters and it is equipped with a high resolution Doppler radar. Three P-3 airplanes crossed for the first time simultaneously the downpours of a hurricane, gathering data which showed how these turbulent rings interact with the wall of clouds surrounding  the cyclone eye causing an intensification or weakening of the storm.
In flight the airplanes launch radioprobes with  parachutes that transmit data on pressure, speed and wind direction, humidity and temperature. Last year a group of scientists sent , a remote controlled experimental aircraf,  across the middle of tropical storm Ophelia. This aircraft (called Aerosonde)  stayed in the vortex for 10 hours  and descending to  an altitude of 400 meters, monitored winds and the influx of heat and humidity from the ocean to the storm.
The Aerosonde, that weighs just 13 kilograms, has faced winds up to 125 kilometers per hour transmitting data every half second!
Given the importance of ocean water temperatures, airplanes drop ocean probes that register conditions as they sink down to over 900 meters deep.

To improve the forecast
Greater precision in foreseeing the direction and intensity of a storm is imperative for local authorities that must act on emergency plans (warning and evacuation of high risk areas).
Computerized models apply a grid to the hurricane and through sophisticated methods they can calculate variations in wind speed, temperature, humidity and clouds. The thicker the grid, the greater the resolution and forecast precision because it will be possible to gather important information which couldn’t be done with other models in the past.
A new high resolution model is being developed and will become operative in 2007: NOAA’s Hurricane Research and Forecasting (HWRF). We must bear in mind that the current model’s resolution is 12 kilometers whereas the HWRF’s will be 9 kilometers! Furthermore, the HWRF will be able to analize data on waves, ocean and coasts immediately as they are gathered.

Written by Elisabetta Monistier

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
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