Natural Disasters

Prof. Stephen A. Nelson

Exceptional Weather (Part 2)

Hurricanes (Tropical Cyclones)

Hurricanes are massive tropical cyclonic storm systems with winds exceeding 119 km/hr (74 miles/hour).  The same phenomena is given different names in different parts of the world.  In the western Pacific they are called typhoons, and in the southern hemisphere they are called cyclones.  But, no matter where they occur they represent the same process.  Hurricanes are dangerous because of their high winds, the storm surge produced as they approach a coast, and the severe thunderstorms associated with them. Although death due to hurricanes has decreased in recent years due to better methods of forecasting and establishment of early warning systems, the economic damage from hurricanes has increased as more and more development takes place along coastlines.  It should be noted that coastal areas are not the only areas subject to hurricane damage.  Although hurricanes loose strength as they move over land, they still carry vast amounts of moisture onto the land causing thunderstorms with associated flash floods and mass-wasting hazards.

Origin of Hurricanes

• When a cold air mass is located above an organized cluster of tropical thunderstorms, an unstable atmosphere results. (This is called a tropical wave). This instability increases the likelihood of convection, which leads to strong updrafts that lift the air and moisture upwards, creating an environment favorable for the development of high, towering clouds. A tropical disturbance is born when this moving mass of thunderstorms maintains its identity for a period of 24 hours or more.  This is  the first stage of a developing hurricane.
   

• Surface convergence (indicated by the small horizontal arrows in the diagram below) causes rising motion around a surface cyclone (labeled as "L"). The air cools as it rises (vertical arrows) and condensation occurs.  The condensation of water vapor to liquid water releases the latent heat of condensation into the atmosphere. This heating causes the air to expand, forcing the air to diverge at the upper levels (horizontal arrows at cloud tops).

• Since pressure is a measure of the weight of the air above an area, removal of air at the upper levels subsequently reduces pressure at the surface. A further reduction in surface pressure leads to increasing convergence (due to an higher pressure gradient), which further intensifies the rising motion, latent heat release, and so on. As long as favorable conditions exist, this process continues to build upon itself.  When cyclonic circulation begins around the central low pressure area, and wind speeds reach 62 km/hr (39 mi/hr) the disturbance is considered a tropical storm and is given a name.  When wind speeds reach 119 km/hr (74 mi/hr) it becomes a hurricane.   Note that all tropical waves, disturbances, or storms do not necessarily develop into hurricanes.

To undergo these steps to form a hurricane, several environmental conditions must first be in place:

1. Warm ocean waters (of at least 26.5°C [80°F]) throughout about the upper 50 m of the tropical ocean must be present.  The heat in these warm waters is necessary to fuel the tropical cyclone.
   
   
2. The atmosphere must cool fast enough with height, such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for tropical cyclone development.
   
   
3. The mid-troposphere (5 km [3 mi]), must contain enough moisture to sustain the thunderstorms. Dry mid levels are not conducive to the continuing development of widespread thunderstorm activity.
   
   
4. The disturbance must occur at a minimum distance of at least 500 km [300 mi] from the equator. For tropical cyclonic storms to occur, there is a requirement that the Coriolis force must be present.  Remember that the Coriolis effect is zero near the equator and increases to the north and south of the equator.  Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
   

5. There must be a pre-existing near-surface disturbance that shows convergence of moist air and is beginning to rotate. Tropical cyclones cannot be generated spontaneously. They require a weakly organized system that begins to spin and has low level inflow of moist air.
   
   
6. There must be low values (less than about 10 m/s [20 mph]) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the rate of change of wind velocity with altitude.  Large values of vertical wind shear disrupt the incipient tropical cyclone by removing the rising moist air too quickly, preventing the development of the tropical cyclone. Or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy it by interfering with the organization around the cyclone center.
   

Note that about 12% of all tropical cyclones develop in the Atlantic Ocean. Those that begin to form near the coast of Africa are often referred to as "Cape Verde" hurricanes, because the area in which they develop is near the Cape Verde Islands.  15% of all tropical cyclones develop in the eastern Pacific Ocean, 30% develop in the western Pacific Ocean, 24% in the Indian Ocean both north and south of the equator, and 12% develop in the southern Pacific Ocean.  It is notable that essentially no tropical cyclones develop south of the Equator in the Atlantic Ocean.

After Abbott (1996)

Since the main source of energy for the storm is the heat contained in the warm tropical and subtropical oceans, if the storm moves over the land, it is cut off from its source of heat and will rapidly dissipate.

The smallest, Cyclone Tracy,  had gale force winds that only extended 50 km in radius when it struck Darwin, Australia, in 1974.  There is very little association between hurricane  intensity (either measured by maximum sustained winds or by central pressure) and size.  Hurricane Andrew is a good example of a very intense tropical cyclone of small size.  It had 922 mb central pressure and 230 km/hr sustained winds at landfall in Florida, but had gale force winds extending out to only about 150 km from the center.

Monitoring and Tracking Hurricanes
In the United States, hurricanes and developing tropical disturbances are monitored very closely.  Such monitoring has drastically reduced the number of deaths from hurricanes in the last 50 years.  Monitoring is conducted using several methods:

• Ships at sea transmit weather reports that help meteorologists locate centers of low pressure that may develop into tropical disturbances. 
  
  
• Images from weather satellites, which are collected every 30 minutes, are then scanned to look for any development or growth of the disturbance.  In particular, the images are examined to detect any rotational development of the storm, an indication that it may be approaching tropical storm strength.
  
  
• If a tropical storm or hurricane is detected and appears to pose a threat to land areas, observation airplanes are sent to examine the storm.  Such planes fly into the storm at an altitude of about 3,000 meters or lower. The plane collects data on wind speed,  air pressure, and moisture content by dropping devices called dropsondes into the storm.  These dropsondes transmit the meteorological data continuously has they fall to the ocean surface, and thus provide information on the vertical structure of the storm.  In addition, radar devices carried on the plane collect data about the intensity of the rainfall and wind velocities.  The planes fly completely through the storm, passing though the eye, sometimes making several passes.  The data collected give meteorologists a 3 dimensional picture of the structure of the storm.
  
  
• Satellite images and radar from land based stations allow scientists to track the position of the storm and report it to all agencies that may be affected if the storm makes landfall.
  

• Hurricane Betsy, a category 3 storm, took a northwestward track from the Caribbean Islands, but then turned abruptly west as it passed north of Puerto Rico.  It then took a  northwest track again, looking like it would hit  along the coast of Georgia or South Carolina.  Suddenly, however, it looped back to the south, passed over the southern tip of Florida, crossed the Gulf of Mexico and hit just east of New Orleans.

In Florida and Louisiana it caused about $7.4 billion (1996 dollars) in damage and killed 76 people along its track.

• Hurricane Elena became a hurricane over the southeastern tip of Cuba in 1985.  It took a northwestward track looking like it would head to New Orleans, but on entering the northern Gulf of Mexico, it turned abruptly toward the east, taking aim on the west coast of Florida. It stalled for about 2 days just south of Pensacola, Florida, then abruptly turned toward the northwest, eventually hitting the coast as a category 3 storm at Biloxi, Mississippi, where it caused $1.7 billion  (1996 dollars) in damages.
  

• Hurricane Donna in 1960 shows the effects of  the land decreasing the intensity of a hurricane.  Donna hit the southern tip of Florida as a category 4 hurricane. It then took a northeastward track across Florida, loosing strength as it crossed the land.  On re-entering the Atlantic Ocean it again increased in intensity due to the warm ocean waters, took a track along the east coast and eventually hit Long Island, New York. This storm caused about $2.1 billion (1996 dollars) in damages and killed over 364 people along its track.
  
  
• In 1992 Hurricane Andrew moved west across southern Florida causing about $20 billion in damages.  It weakened somewhat as it crossed Florida, but on entering the Gulf of Mexico it strengthened, crossed the Gulf and plowed into southwestern Louisiana causing an additional $10 billion in damages, but killed only 26 people along its track.
  
  

• A hurricane that moves along the coast has a coast-parallel hurricane track. From such a track extensive damage would occur along the coastline closest to the storm, with bands of lesser damage extending inland.  Since this track (upper diagram) has the most intense winds offshore (on the right side of the hurricane), the coast would not feel the highest wind velocities.  Thunderstorm activity associated with this track would be most severe on the northern side of the storm, since the spiral rain bands would be feeding off the moist air above the ocean.
  
• A hurricane that approaches the perpendicular to the coast  has a coast-normal track.  Such a storm would produce extreme damage all along the right-hand side of its track, with bands of decreasing damage occurring both to the left and right of the track. Furthermore, as the storm approached the coast areas to right hand side of the storm would receive the heaviest thunderstorm activity, since the rain bands would be feeding off the moist oceanic air.

Reducing Hurricane Damage
There is plenty of historical data on hurricane damage in the United States so that it is not difficult to see ways that damage from hurricanes can be reduced.  In terms of protection of human life, the best possible solution is to evacuate areas before a hurricane and its associated storm surge reaches coastal areas. Other measures can be undertaken to reduce hurricane damage as well.

• Warning and Evacuation - With modern techniques of forecasting and tracking hurricane paths, it is always possible to issue warnings about the probable locations that will be affected by any given hurricane. The National Hurricane Center defines hurricane watches and warnings as follows:
  
  • Hurricane Watch: An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours.
    
    
  • Hurricane Warning: A warning that sustained winds  of 74 mph (119 km/hr) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. 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 force.

  
  The problem, however, is that it may not always be possible to issue such a warning in time for adequate evacuation of these areas.  Because the storm surge and even gale force winds can reach an area many hours before the center of the storm, warnings must be issued long enough before the storm strikes that the surge and winds do not hinder the evacuation process.  The effectiveness of the warning systems also depends on the populace to heed the warning and evacuate the area rather than ride out the storm, and the state of preparedness of local government agencies in terms of evacuation and disaster planning.  New Orleans is a particularly notable example.  Since most of the city is at or below sea level, a storm surge of 6 meters (20 feet) from a category 4 or 5 hurricane would most certainly flood the city and choke all evacuation routes.  Even with 24 hours notice of the approaching surge (which would mean as soon as the storm entered the Gulf of Mexico) it would be difficult to evacuate or convince people to evacuate within that 24 hour period.   A hurricane approaching New Orleans is a disaster waiting to happen.
  

• Maintaining Beach Width and Dune Height - Wide beaches, high dunes, and barrier islands offer natural protection from storm surges.  If beach width and dune height can be maintained or increased by  artificial processes, this could prevent some damage to structures lying inland from the beaches and dunes.
  
  
• Engineering Solutions - In 1900 the city of Galveston Texas was completely covered by the storm surge of an approaching hurricane.  Between 6,000 and 8,000 lives were lost.  Because of this devastation the entire city was elevated by 11 feet and a floodwall 16 feet above low tide was constructed all along the coast.  These engineering steps have protected Galveston from hurricanes ever since, but in modern terms the efforts would be prohibitively expensive.  Another approach, for cities on bays and estuaries is to build a moveable series of walls or gates across the mouth of the bay or estuary to prevent the storm surge from entering.  Such a structure was built across the mouth of Narragansett Bay, Rhode Island to protect the city of Providence after a 1938 hurricane.  Again the structure has proven effective.
  
  
• Construction Codes - Rigorous enforcement of building codes can effectively reduce damage to structures.  In areas where high storm surges might be expected, codes could require construction that is built high enough to not flood and sturdy enough not to be knocked down by the surge or the winds that accompany a hurricane.  Damage to homes can be reduced by requiring aerodynamic architecture, such as rounded rather than angular shapes.  Wood frame home construction should require that each component of a house - foundation, floors, walls, and roof  - be anchored to the others with metal strips (hurricane clips) rather than just nailed together.
  
  
• Zoning/Land Use Practices - As with other natural hazards like flooding, landslides, and coastal erosion, an alternative, although controversial practice of limiting coastal areas to recreational purposes could always reduce the vulnerability of such areas to damage from natural processes, including hurricanes.
  

Other Severe Weather Phenomena

Nor'easters
Nor'easters are extratropical cyclonic storms that originate outside of the tropics.  They are, like hurricanes, centers of low pressure with winds that circulate in a counterclockwise direction around the low pressure. 

El Niņo

One of the dramatic manifestations of the interaction between the oceans and the atmosphere and its effects on both climate and weather is the Southern Oscillation, one of the consequences of which is El Niņo.  The Southern Oscillation is a back and forth variation in atmospheric pressure between a high pressure system normally located off the west coast of South America and a low pressure system normally located in the western Pacific near Indonesia and Australia.  In the U.S. El Niņo conditions  result in  heavy rains, flooding, landslides, and tornadoes in greater than normal amounts because El Niņo conditions drive abnormal amounts of moist warm air across North America. El Niņo causes flooding in Peru, as well as drought and fires in Indonesia and Australia.

The phenomena is manifested by the arrival of warm water off the coast of Peru around Christmas time, and thus is called El Niņo (Spanish for the boy child) because it arrives at this time. An El Niņo event occurs every 2 to 7 years with various degrees of strength.  Some El Niņo events are more intense than others, and the condition lasts from 18 to 24 months.  The following table lists the years of El Niņo events.

The warm water pushed to the west by the trade winds, heats as it flows along the equator, so that on arrival in the western Pacific heat is added to the overlying atmosphere causing it to rise, form clouds, and produce extensive rainfall. The moisture depleted upper atmosphere then circulates back to east where it descends off the coast of South America contributing to the dry conditions. During periods of exceptionally strong trade winds the upwelling of cold water off the South America cools the water even further creating a condition called La Niņa (girl child).

La Niņa conditions have about the opposite effect of El Niņo conditions.   i.e. better fishing harvests off the west coast of South America, drier conditions in North and South America, more hurricanes in the Atlantic, and wetter conditions in Australia and Indonesia.

Over the past 50 years, the oscillation of warm water back and forth across the tropical Pacific Ocean has created El Niņo conditions 31% of the time, La Niņa conditions 23% of the time, and "normal" conditions 46% of the time.