Friday, June 20, 2008

Physical Geography of Hurricanes

Named after a Caribbean god of maleficence, the hurricane is an awesome, yet deadly and destructive natural phenomenon of the Earth's occasionally tumultuous atmosphere. Labeled a typhoon in the Pacific Ocean west of the International Dateline and a cyclone over the Bay of Bengal and Arabian Sea, a hurricane is powered by the heat and moisture of the tropics rather than temperature differences across latitudes, as is the case for the more common extratropical cyclone.

A hurricane begins as an area of low air pressure over warm ocean waters (at least 27 degrees Centigrade down to 50 meters below the sea surface). As a result of atmospheric instability, the area of low pressure features numerous showers and thunderstorms that, over several days, organize the winds into a counterclockwise (clockwise in the Southern Hemisphere) swirl. The swirl in turn organizes the existing thunderstorms and helps new thunderstorms develop. The swirl then becomes a tropical storm when the circulating wind speeds, estimated at 10 meters above the ocean, exceed 17 meters per second (averaged over a one-minute time interval).

When the wind speeds reach 33 meters per second or more the tropical storm is called a hurricane. Once formed, the hurricane winds are maintained by the import of heat from the ocean at high temperature and the export of heat at lower temperature in the upper troposphere (near 16 kilometers) similar to the way a steam engine converts thermal energy to mechanical motion.

On average 50 hurricanes occur worldwide each year. Hurricanes develop during the time of the year when the ocean temperatures are hottest. Over the North Atlantic (including the Gulf of Mexico and Caribbean Sea) this includes the months of June through November with a sharp peak from late August through the middle of September when the direct rays of the summer sun have had the largest impact on sea temperature. Worldwide, May is the least active month for hurricanes, while September is the most active.

Hurricanes vary widely in intensity as measured by their fastest moving winds. Hurricane intensities are grouped into five categories (Saffir-Simpson scale) with the weakest category-one winds blowing at most 42 meters per second and the strongest category-five winds exceeding speeds of 69 meters per second. Three category five hurricanes hit the United States during the 20th century including the Florida Keys Hurricane in 1935, Hurricane Camille in 1969, and Hurricane Andrew in 1992.

Hurricanes also vary considerably in size (spatial extent) with the smallest hurricanes measuring only a few hundred kilometers in radius (measured from the eye center to the outermost closed line of constant surface pressure) and the largest exceeding a thousand kilometers or more.

Strong winds are the defining characteristic of a hurricane. Wind is caused by the change in air pressure between two different locations. In the eye of a hurricane the air pressure, which is the weight of a column of air from the surface to the top of the atmosphere, is quite low compared with the air pressure outside the hurricane. This pressure difference causes the air to move from the outside of the hurricane inward toward the center of the hurricane.

By a combination of friction as the air rubs on the ocean below and the spin of the Earth as it rotates on its axis, the air does not move directly inward but rather spirals in a counterclockwise direction toward the region of lowest pressure. The vertical component of the Earth's spin is too weak to support a spiral within about 5 degrees of latitude from the Equator so hurricanes do not develop there.

To a first approximation, the pressure difference between the eye and the surrounding air determines the speed of the wind. Since the pressure outside the hurricane is roughly uniform, a hurricane's central pressure is another measure of a hurricane's intensity. The lower the central pressure the more intense the hurricane. Pressures inside the most intense hurricanes are among the lowest that occur anywhere on the Earth's surface at sea level.

In the largest and most intense hurricanes (like Hurricane Katrina in 2005), the strongest winds are located in the eyewall that surrounds the nearly calm eye. If the hurricane is stationary (spinning, but with no forward motion) the field of winds is shaped like a torus, with a calm center and the fastest winds forming a ring around the center. Concentric rings of incrementally weaker winds are analyzed outward from the core of strongest winds.

The distance from the center of the hurricane to the location of the hurricane's strongest winds is called the radius of maximum winds. In well-developed hurricanes, the radius of maximum winds is found at the inner edge of the eyewall. This distance varies considerably from hurricane to hurricane and, due to cycles of eyewall replacement, even from day to day for a particular storm.

While the wind just above the ocean surface spirals anticlockwise toward the center, the air at high altitudes blows outward in a clockwise spiral. This outward flowing air produces thin cirrus (feathery) clouds that extend great distances (thousands of kilometers) from the center of circulation and the presence of these clouds may be the first sign that a hurricane is approaching.

Hurricanes are steered by large-scale wind streams in the atmosphere above the surface and by the increasing component of the Earth's spin away from the equator. In the deep tropics these forces push a hurricane slightly north of due west (in the Northern Hemisphere). Once north of about 23 degrees of latitude a hurricane tends to take a more northwestward track then eventually northeastward at still higher latitudes. This creates the parabolic shaped track often observed on maps of historical hurricanes. Local fluctuations in the magnitude and direction of steering can result in tracks that deviate significantly from this pattern.

Landfall occurs when the hurricane center crosses a coastline. Because the fastest winds are located in the eyewall it is possible for a hurricane's fastest winds to be over land even if landfall does not occur. Similarly it is possible for a hurricane to make landfall and have its fastest winds remain out at sea. Fortunately, the winds slacken quickly after the hurricane moves over land. Hurricanes made landfall in the United States at an average rate of five every three years during the 20th century.

Winds blowing overland from a hurricane destroy poorly constructed buildings and mobile homes. Debris such as signs, roofing material, and small items left outside become flying projectiles adding to the destructive power of the wind.

Besides the destructive power of the winds, hurricane damage results from two other causes: flooding from torrential rainfall and storm surge. Rainfall is the quantity of water, expressed in millimeters, that falls from the hurricane in a specified area and time interval. Hurricanes derive energy from the ocean by evaporating the water into the air that then gets converted back to liquid water through condensation inside thunderstorm clouds. The water falls from the clouds as rain, and the stronger the hurricane thunderstorms, the greater the amount of rain and thus the greater the potential for flooding.

The amount of rainfall deposited overland from a hurricane depends on many complicated factors including hurricane intensity, forward speed, and the underlying topography. The rainfall of a hurricane can intensify when the strong winds carry the moisture up a mountainside. Antecedent moisture conditions also play a role in whether and to what extent flooding will occur from a hurricane. Freshwater flooding from hurricanes can be a serious danger even hundreds of kilometers from point of landfall.

Bands of showers and thunderstorms that spiral inward toward the hurricane center are the first sensible weather experienced as a hurricane approaches. High wind gusts and heavy downpours occur in the individual rain bands, with relatively calm weather occurring between the bands. Brief tornadoes can form in the rain bands especially as the hurricane crosses the coastline.

Storm surge is ocean water that is pushed toward the shore by the force of the winds moving around the storm. Over the open ocean, the water can flow in all directions (including downward) away from the storm. Strong winds blowing across the ocean surface creates a stress that forces the water levels to increase downwind and decrease upwind. This wind set-up is inversely proportional to ocean depth so over the deep ocean away from land the water level rises are minimal. However, when the hurricane approaches shallow water, there is no room for the water to flow underneath so it rises and gets pushed by the wind as a surge, much like a plow pushes the snow from the roadway.

The advancing surge can increase the water level five meters or more above sea level. In addition, wind-driven waves are superimposed on the storm surge. The total water level can cause severe surge impacts in coastal areas, particularly when the storm surge coincides with the normal high tide.

Slope of the continental shelf also determines the level of surge in a particular area. A shallow slope will allow a greater surge. With a steeper continental shelf coastal areas will not experience as much surge inundation, although large breaking waves can still present major problems. The low pressure in the middle of the storm causes a smaller part of the storm surge.

The pressure in the eye is significantly lower than the surrounding atmosphere, so the atmosphere pressure causes the water in the eye to rise like sucking a drink up a straw. This pressure effect will cause the water level in the open ocean to rise in regions of low pressure and fall in regions of high pressure. In general, for a one-hectopascal drop in surface pressure there is a one-centimeter rise in water level. In short, the cause of storm surge is the combined effect of low air pressure and persistent winds blowing over the water surface.

Hurricanes are tracked with satellites, radar, and specially equipped aircraft reconnaissance flights. Ocean buoys and ships also provide storm information. Successful experiments with remotely piloted drone aircraft (aerosonde) suggest they might be used to help forecast the intensity and track of future hurricanes.

Because of their potential for death and destruction, the U.S. National Hurricane Center issues watches and warnings for hurricanes threatening the United States a few days before a landfall. A hurricane watch means that hurricane conditions in specific coastal areas are possible within 36 hours. A hurricane warning means that hurricane winds associated with a hurricane are expected in a specified coastal area within 24 hours. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane intensity.

Hurricane activity, on time scales of seasons and longer, responds to variations in climate. To a first order, greater ocean warmth and relatively calm winds enhance the potential for hurricanes. The strength and position of the subtropical high-pressure zone determines the steering currents, which is important for predicting landfall probabilities. Many catastrophe models that project future damage losses from hurricanes now include this information.

Increases in ocean temperature will raise a hurricane's potential intensity, all else being equal. However, corresponding increases in atmospheric wind shear--in which winds at different altitudes blow in different directions tear apart the developing hurricane--could counter this tendency by dispersing the hurricane's heat. A recent study based on a set of homogenized satellite-derived wind speeds indicates the strongest hurricanes are getting stronger worldwide. Modeling studies indicate that rainfall from hurricanes may get heavier in the future. However, more research is needed to better understand this important issue.