TRACKING HURRICANES

North American meteorologists from the National Oceanic and Atmospheric Administration (NOAA)'s Hurricane Research Division have recently improved the success rate in their forecasting of where hurricanes are likely to hit land by an estimated 15 to 30%. This increase in accuracy is due to the use of instruments called GPS-dropwindsondes, which can probe the atmosphere surrounding a hurricane while it is still out at sea. The atmospheric characteristics of hurricanes over land are well understood because investigation is possible with weather balloons containing sophisticated meteorological instruments. When hurricanes are out of reach of balloons, gathering information is decidedly more difficult. Little is known of the weather conditions that guide hurricanes towards land.

An accurate estimation of where a hurricane will strike is essential in order to reduce loss of life and property. Hurricane Andrew, the most costly hurricane in U.S. history, killed 15 people and caused damage of $35 billion in today's dollars in 1902. However, the unnamed category 4 hurricane which struck southeast Florida in 1926 and killed 243 people would have caused an estimated $77 billion if it had struck today. The reason for this is the explosion in population growth and development along the south-east coast of the U.S. during the last half century.

Hurricanes occur in cycles every few decades, the last intense period in the U.S. being from 1940 to 1969. 'Camille', a Category 5 hurricane of such catastrophic force that it caused over a billion and a half dollars worth of damage at the time and killed 256 people, struck the coast of the Gulf of Mexico in 1969 with winds over 320 km/h. Yet, for the last quarter century, hurricane activity has been relatively mild. Scientists do not know the precise reason for the cycles of hurricane activity, but they could be caused by a phenomenon called the 'Atlantic Conveyor'. This is the name given to the gigantic current of water that flows cold from the top of the globe slowly along the Atlantic ocean floor to Antarctica and resurfaces decades later before flowing back north, absorbing heat as it crosses the equator. Since hurricanes derive their energy from the heat of warm water, it is thought that an increase in the speed of the' Conveyor', as it pulls warm water to the north, is an indicator of intensifying hurricane activity.

The use of GPS-dropwindsondes began in 1997. Small sensing devices dropped from planes at very high altitudes and over a wide area, they are far more revealing than previously used sensors. Because they weigh only 0.4 kilograms, they are able to stay aloft for longer periods and broadcast more data to the ground. Each sonde carries its own global positioning satellite receiver. The GPS signals received are used to calculate the direction and speed of wind, and data on temperature, humidity, and barometric pressure at half second intervals all the way down to the ocean surface.

Dropwindsonde information is fed into a special meteorological computer in Maryland which generates a global computer model of wind patterns. Data analysts have discovered a greater variability in the winds at sea level than previously believed, but many forecasting problems are beyond a solution, at least for the time being. For instance, it is not yet known why hurricanes can suddenly change in intensity; current computer models often fail to predict whether a hurricane will reach land or else cannot pinpoint where a strike will take place.

One surprising result of a recent computer simulation was the destruction of a large part of downtown New York. Hurricane researchers believe that the city is more likely than Miami to suffer a direct hit in the near future. Also, certain geographical features of the coastline near New York make it conceivable that a wall of water called a storm surge pushed ashore by hurricane winds would cause a devastating flooding of Manhattan. A storm surge was responsible for the more than 8000 deaths caused by the hurricane that destroyed the city of Galveston in 1900.

Date: 2016-01-14; view: 689