HURRICANES Responding to timely warnings holds the key to tempering their impact
Boston — AMERICANS who endured Hurricane Gloria, Hurricane Juan, or prepared for Hurricane Nele in the Pacific might spare a thought for Beniot Vies. The Cuban Roman Catholic priest organized one of the first hurricane warning services a century ago. Hundreds of observers around the Cuban coastline kept a weather eye out for ominous sea swells or streaming cirrus clouds that foretold a storm. When danger threatened, the observers sent warnings by pony express.
With today's high-techology surveillance, no hurricane sneaks in undetected. Even the uncertain forecasts of Gloria's track represent a quantum leap in alertness beyond the pony riders. Yet for recipients of the warnings, the message forecasters want to get across is the same as that of Vies -- it is better to prepare for a potential threat that fails to materialize than to act as though there were no threat at all.
National Weather Service (NWS) meteorologist James I. Campbell makes no apology for the Gloria forecasts. He calls the many watches and warnings ``timely for this storm.''
Neil Frank, director of the National Oceanographic and Atmospheric Administration's National Hurricane Center in Miami, repeatedly stresses the inherent uncertainty in hurricane forecasts. The NOAA center found errors for the period 1970-79 of 434 miles, 281 miles, and 125 miles respectively in its 72, 48, and 24 hour forecasts of hurricane tracks. Dr. Frank says he does not ``expect a major improvement in the next decade.'' Under these circumstances, overwarning is part of the price of hurricane safety .
Meteorologists such as Frank urge people not to let what may seem to be false alarms make them complacent. Decades of research have given atmospheric scientists a healthy respect for a hurricane's power. Named for Huracan
Named for Huracan, an ancient Central American evil deity, hurricanes are a product of atmosphere and ocean. They develop amid torrential thunderstorms spawned by disturbances over warm tropical seas. Water vapor, flowing in from distant regions, feeds their growth and drives their fury. For most storms, the strongest sustained winds blow at about 112 miles per hour. Yet sustained winds of nearly 200 m.p.h. have been recorded; gusts have peaked as high as 224 m.p.h. Hurricanes have earned their ti tle of the fiercest storms on Earth.
Although the role they play in the atmosphere's general circulation remains obscure, decades of research have shown that hurricanes strongly influence their regional environment. Low-level winds spiraling toward the storm from distances of several hundred to a thousand miles or more move in such a way that air throughout the troposphere -- the layer of atmosphere that ranges from the earth's surface to between 10,000 and 20,000 feet in altitude -- slowly sinks. This subsidence suppresses cloud formation
so that unusually clear skies in the normally cloud studded tropics often signal the approaching storm.
However, within about 250 miles of the storm center, inflowing air begins to converge strongly. Tropospheric air begins to lift and the towering convective cumulus clouds, which form the main body of the storm appear. This transition between clear skies and storm clouds -- the so-called ``bar'' of the storm -- can be sharp, although high-level winds may carry a veil of cirrus clouds well beyond it. Indeed, the appearance of cirrus together with unusual ocean swell is another sign of a hurricane's approa ch. Columbus read this portent during his fourth voyage (1502-04) and warned the governor of Hispanola of the danger. But the governor ignored the forecast, sent his treasure fleet to sea, and lost it.
Wind speeds pick up as the spiraling air nears the storm center. While still approximately 10 to 60 miles out, the air flow turns sharply upward in the ring of massive cumulus clouds called the eye wall. Here is where the heaviest rains fall and the strongest winds blow. Inside that wall lies the calm of the hurricane eye. Sea level air pressure here is 5 to 10 percent or more lower than it is outside the storm. The sea surface domes up. This mound of water, which moves with the storm, adds to water hea ped up by the powerful winds to create the storm surge that sweeps ashore like a monstrous tide when a hurricane makes landfall. Atmospheric powerhouse
A mature hurricane is an atmospheric powerhouse. Typically, the vigorous circulation -- in at the bottom, up through the clouds, out at the top -- moves about 2 million metric tons of air per second to heights of 40,000 to 60,000 feet. The power required to lift air at that rate is several hundred times the 675,000 megawatts of installed electrical generating capacity of the United States. Additional power to maintain the horizontal winds is puny by comparison -- about 15 megawatts, on average, wh ich is the capacity of a moderate-size power plant.
A hurricane's prodigious energy appetite is fed by the heat released as something like 16 billion to 20 billion metric tons of water are condensed from the storm's daily diet of moist air. That's comparable to the annual runoff of the Colorado River. It's little wonder that rainfall rates in the eye wall can reach 50 centimeters (some 19.7 inches) a day and higher. The rate at which so-called latent heat is released as the water vapor condenses and water droplets subsequently freeze is 600 to 800 times the US electrical generating capacity. So there's plenty of energy to run the storm.
Outside the eye wall, hurricane clouds tend to be organized into rain bands that form a spiral around the eye. This is why areas over which a hurricane passes receive unequal amounts of rain. Although meteorologists do not fully understand how such bands form and persist, some of them appear to be the result of atmospheric waves within the storm.
In fact, the interference of such waves with one another may underlie one of the oddest features of hurricanes -- the striking geometric patterns they sometimes exhibit. Radar and satellite images have shown eye walls shaped as hexagons, squares, triangles, and other polygons. They have revealed similar sharply polygonal structures in rain bands.
Such modern observing tools as radar, weather satellites, and instrument-laden aircraft, plus the ability to model storms on computers, help scientists learn many details of hurricane structure. The storms, however, also have to be studied as part of the larger circulation of the global atmosphere.
Hurricanes are an intense form of tropical cyclone that occur in the North Atlantic, eastern and western North Pacific (where they are called typhoons), and in the western South Pacific and Indian Ocean (where they are simply called cyclones). They are unknown in the eastern South Pacific and South Atlantic Oceans.
For reasons unknown, tropical cyclones can't get organized when the sea surface is cooler than 80 degrees Fahrenheit. Also, they never form within 4 or 5 degrees latitude of the equator where the influence of Earth's rotation is too weak to help spin up their winds. Only about 13 percent of them form beyond 22 degrees. Most arise between latitudes 10 and 20 degrees.
Some 50 to 75 percent of the 80 or so tropical cyclones that form each year reach the 74 m.p.h. minimum wind speed to be classed as a typhoon or hurricane. The Atlantic average is six hurricanes out of nine annual tropical cyclones. Numbers vary from season to season. Also, the vast majority of Atlantic storms occur within the official hurricane season, which starts June 1st and runs through the rest of the year. Most storms arise during the intense part of the season from August through October.
Scientists are only beginning to understand global factors that influence hurricane formation. William M. Gray of Colorado State University leads a research group which has found some unexpected connections. During El Nio years, when unusually warm water appears on the South American tropical west coast and in the equatorial central Pacific, Atlantic hurricane formation is inhibited. Dr. Gray suggests that El Nio warming enhances atmospheric convection over the eastern Pacific. This, in turn, strengthen s westerly wind patterns in the upper troposphere over the Caribbean basin and western Atlantic, creating an environment unfavorable to hurricanes. The westerly winds flowing above the normal easterly trade winds tend to pull incipient hurricanes apart, explains Gray's associate Robert Merrill.
His group has also linked Atlantic hurricane formation to a roughly two-year reversal of east-west winds high in the equatorial stratosphers called the Quasi-Biennial Oscillation. Westerly winds at this level are associated with slightly enhanced hurricane formation. When these winds shift easterly, hurricane activity is inhibited.
This unexpected apparent linkage has yet to be explained. ``It's a wide-open area for study because no one thought it mattered,'' Dr. Merrill says.
Althought they still have much to learn about hurricanes, some scientists have wondered if it may be possible at least to temper the winds. This is an elusive goal.
Attempts to modify hurricanes by seeding clouds have been frustrating disappointments. Apparent success in several experiments now is known to have been illusory. Key assumptions in the underlying theory turned out to be unrealistic. (See accompanying box.)
Thus, with little immediate prospect for improving forecasts of hurricanes or for taming their fury, the onus for mitigating hurricane destruction is where Neil Frank has long said it is -- people.
``If there is going to be any increase in the effectiveness of the hurricane-warning system, it's got to come from the people,'' Frank maintains.
This means more than heeding warnings, securing property, and evacuating when asked to do so. For hurricane-prone areas, it also means having a sophisticated understanding, community by community, of the potential for damage from hurricanse of various intensities, arrival times, and directions of approach. This would help communities develop response plans for each contingency. This can help minimize the cost of effective preparedness -- an expense that is part of the hurricane's economic impact.
The National Hurricane Center's Robert C. Sheets estimates that preparedness costs for a typical warned area for an average hurricane now run to nearly $50 million. It is inevitable, given present forecasting inaccuracies, that much of the area will not be touched. Responding to potential threats that don't materialize is part of the cost of minimizing the long-term impact of hurricanes. But, as with other types of insurance premiums, it should be kept within reasonable bounds, Dr. Sheets notes.
Not every warning calls for the most drastic action, such as evacuation of coastal residents. A storm surge is less devastating at low tide than when it comes at high tide. Not all storms that strike a community will hit at full power. Some will deal only a glancing blow. And perhaps 50 to 60 percent of the warned area will not be touched at all. Yet officials often have to decide what, if any, action to take many hours in advance.
To help community officials plan for different levels of action and decide when to act, the NWS puts probabilities on its hurricane forecasts up to 72 hours in advance. These are a measure of the estimated forecast accuracy. Probabilities of a storm hitting any one community are low at first. They increase for the most threatened areas as the storm approaches and the forecast lead time shortens.
Sheets explains that the probabilities do not indicate the conditions a storm is likely to generate. They only indicate the likelihood that the storm will move over a specific area. Value of probability forecasts
Yet when coupled with knowledge of what is likely to happen locally when hurricanes of different intensities move in from different directions, local officials have a guide to help them delay the most costly actions as long as possible while watching how the storm threat develops.
Also, Sheets observes, probability forecasts help officials explain why they took a costly action that hindsight may show to have been unnecessary.
``If the probability were 20 percent when the action was taken, the official could state that there was a 1-in-5 chance that X number of lives could be lost and that the official was not willing to take that chance with those lives,'' he explains.
Many such decisions were taken as Gloria moved along the US Atlantic coast. Hindsight has shown some community actions were unnecessary. Others turned out to be timely and prudent. But perhaps the greatest lesson of Gloria was to show that many communities do have such planning and are prepared to use Weather Service information intelligently.
Coupled with hurricane-wise land use, such community planning can cut hurricane losses. Concerned meteorologists, however, see indiscriminate coastal development as a potential man-made disaster.
NOAA spokesmen, such as Neil Frank, have repeatedly warned that millions of people along the US Atlantic and Gulf Coasts have moved into areas that are vulnerable to storm surges and hard to evacuate. Condominiums and other structures are being built with no protection from the sea.
As Frank has observed, he and other meteorologists aren't objecting to coastal development in itself. They object to building what they term death traps. They urge that coastal communities ensure safe building practices and adequate evacuation routes.
Because of an editing error, an article Friday on hurricanes incorrectly gave the altitude range of the troposphere as 10,000 to 20,000 feet above Earth. The range is roughly 30,000 to 50,000 feet, depending on latitude and season.