Natural Disasters

Prof. Stephen A. Nelson

Tropical Cyclones

We have already discussed the factors involved in the circulation of the atmosphere and oceans. Knowledge of the general atmospheric circulation patterns, the Coriolis effect, and circulation around high and low atmospheric pressure areas, and the principles concerning the importance of water in transferring heat between the atmosphere and oceans from the previous lecture is essential for understanding the material in this section.

Hurricanes (Tropical Cyclones)

Tropical Cyclones  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 Atlantic Ocean and eastern Pacific ocean they are called hurricanes.  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.  Tropical cyclones 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 Tropical Cyclones (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 tropical cyclone, 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 and used for tropical cyclone development.
   
3. The mid-troposphere (5 km [3 mi] altitude), 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, although one occurred off the coast of Brazil in March of 2004.

After Abbott (1996)
              

Tropical Cyclone Structure

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 begin to dissipate.

Winds spiraling counterclockwise (in the northern hemisphere) into the eye of the hurricane achieve high velocities as they approach the low pressure of the eye. The velocity of these winds is called the hurricane-wind velocity. 

The central low pressure center of the eye also moves across the surface of the Earth as it is pushed by regional winds.  The velocity at which the eye moves across the surface is called the storm center velocity. 

Thus, when we consider the velocity of winds around the hurricane we must take into account both the wind velocity and the storm center velocity. Depending on the side of the hurricane, these velocities can either add or subtract.  In the example below, the hurricane is traveling north with a storm center velocity of 30 km/hr, and a hurricane-wind velocity of 150 km/hr.

On the right hand side of the storm both velocities are to the north so the total wind velocity is 180 km/hr (30 + 150). On the left hand side of the storm, however, the wind is blowing to the south. Thus, since the storm is moving in the opposite direction to the winds, the velocities subtract and the total wind velocity is 120 km/hr (150 - 30).

This is an important point.  Winds are always stronger on the right side of a moving hurricane in the northern hemisphere.  (The opposite is true in the southern hemisphere, since winds circulate in a clockwise direction, the winds are stronger on the left-hand side of the storm in the southern hemisphere).

Tropical Cyclone Size
Since winds spiral inward toward the central low pressure area in the eye of a hurricane, hurricane-wind velocity increases toward the eye.  The distance outward from the eye to which hurricane strength winds occur determines the size of the hurricane.  Winds in the eye wall itself have the highest velocity and this zone can extend outward from the center to distances of 16 to 40 km. Hurricane force winds (winds with velocities greater than 119 km/hr) can extend out to 120 km from the center of the storm.  The largest tropical cyclone recorded, Typhoon Tip, had tropical storm force winds (39 mph/62 km/hr) which extended out for 1100 km in radius in the Northwest Pacific in 1979. Hurricane Katrina, in 2005, was a large hurricane with tropical storm force winds extending outward from the eye about 320 km. 

The smallest, Tropical Storm Marco,  had tropical storm force winds that only extended 19 km in radius when it struck Mexico's Gulf coast in 2008.  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 tropical storm force winds extending out to only about 150 km from the center.

Hurricane Intensity and Frequency
Once a hurricane develops, the Saffir-Simpson Scale is used to classify a hurricane's intensity and damage potential.

There are five possible categories. Category 1 storms are more common than category 5 storms.  In a typical year, there may be many category 1 storms, but category 5 storms occur very infrequently.

In 2010 the Saffir-Simpson scale was modified and simplified. Many years worth of data collected by the National Hurricane Center provided much evidence that many other factors control the height of the storm surge besides wind speed.  Such factors as storm size (radius), shape of the coastline, and topography of the sea-floor are sometimes more important.  Furthermore, the data did not show a good correlation between wind speed and central pressure.   Thus the current version of the Saffir-Simpson Scale is based solely on wind speed and the central pressure and storm surge information is not part of the scale.

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 west of New Orleans. In Florida and Louisiana it caused about $10.8 billion (2004 dollars) in damage and killed 76 people.
  

• 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 $2.6 billion  (2004 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 $3 billion (2004 dollars) in damages and killed over 364 people.
  
  
• In 1992 Hurricane Andrew moved west across southern Florida causing about $30 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 $16 billion in damages, but killed only 26 people along its track.
  
  
  
  
  
     
  

Angle of Hurricane Approach to Coast
The amount of damage that occurs when a hurricane approaches a coast depends on the angle of approach.   Two extreme examples illustrate this point.

• 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.

Hurricanes with a coast-parallel track  have the additional danger that small shifts in course make landfall forecasts difficult.  Because of such problems, forecast maps from the National Hurricane Center no longer show the line in the center of the forecast storm track.

Storm Surge

Heavy winds produced by hurricanes push the ocean in front of them. As this water gets pushed into the shallow zones along the coastline sea level rises.  Since the storm surge is driven by the winds, the height of the rise in sea level is related to the velocity of the wind.  For a moving storm the greater winds occur on the right side of the storm (in the northern hemisphere).  Sea level also rises beneath the eye of the storm due to the low pressure in the eye.  But, the surge generated by this low pressure is usually much less than the wind-driven surge.  The height of the storm surge depends on wind speed, the shape of the coastline, and variations in the water depth along the coast line.  Height also depends on tidal cycles.  If a storm approaches the coast during high tide, the storm surge will be higher than if it approaches during low tide.  Category 5 tropical cyclones can produce storm surges in excess of  6m (20 feet).  The highest storm surge measured, 12.8 m (42 feet) occurred in 1899 in Australia.

Because the storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the hurricane makes landfall.  This is an important point to remember because flooding caused by the surge can destroy roads and bridges making evacuation before the storm impossible.

Since thunderstorms accompany hurricanes, and these storms can strike inland areas long before the hurricane arrives, water draining from the land in streams and estuaries may be impeded by the storm surge that has pushed water up the streams and estuaries. 

It is also important to remember that water that is pushed onto the land by the approaching storm (the flood surge) will have to drain off after the storm has passed.   Furthermore after passage of the storm the winds typically change direction and push the water in the opposite direction.  Damage can also be caused by the retreating surge, called the ebb surge.

Along coastal areas with barrier islands offshore, the surge may first destroy any bridges leading to the islands, and then cause water to overflow the islands.  Barrier islands are not very safe places to be during an approaching hurricane!

Hurricane Damage
Hurricanes cause damage as a result of the high winds, the storm surge, heavy rain, and tornadoes that are often generated from the thunderstorms as they cross land areas.  Strong winds can cause damage to structures, vegetation, and crops, as described in the Saffir-Simpson scale discussed previously.  The collapse of structures can cause death. The storm surge and associated flooding, however, is what is most responsible for casualties. Extreme cases of storm surge casualties have occurred as recently as 1970 and 1990 in Bangladesh and 2008 in Myanmar.  Bangladesh is an area with high population density and with over 30% of the land surface less than 6 m above sea level.  In 1970 a cyclone struck Bangladesh during the highest high tides (full moon).  The storm surge was 7 m (23 ft.) high and resulted in about 400,000 deaths.  Another cyclone in 1990 created a storm surge 6 m high and resulted in 148,000 deaths. The May 2008 cyclone in Myanmar is estimated to have killed 138,000.

The amount of damage caused by a tropical cyclone is directly related to the intensity of the storm, the duration of the storm (related to its storm-center velocity, as discussed above), the angle at which it approaches the land, and the population density along the coastline. The table below shows how damages are expected to increase with increasing tropical storm category.  Like the Richter scale for earthquakes, damage does not increase linearly with increasing hurricane category.

Prediction of hurricane intensity (wind speed) is more problematic as too many factors are involved.  Hurricanes are continually changing their intensity as they evolve and move into different environments. Without the ability to know which environmental factors are going to change, it is very difficult to expect improvement on intensity forecasting.

Hurricane Katrina was expected to loose intensity as moved out of the warmer waters of the Gulf of Mexico. But, it showed a more rapid drop in intensity just before landfall because a mass of cooler dry air was pulled in from the northwest.

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 was a disaster waiting to happen as we can all testify.

• 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.
   
• In Louisiana, restoring coastal marshes and land lost to recent erosion could reduce the storm surge that reaches populated parts of the region (like New Orleans).
  
  
• 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.
  
  After Hurricane Katrina in 2005 work was immediately undertaken to repair breached levees and floodwalls and to build gates across the entrance to the 3 drainage canals at Lake Pontchartrain. A major plan was then developed to build a barrier walls and navigation gates to prevent storm surge from entering the Industrial Canal from Lake Pontchartrain, Lake Borgne (along the MR-GO) and the Intracoastal Waterway. These projects are supposed to protect the New Orleans area form surge events with a 1% chance of occurring each year (the 100-year event). The project has been substantially completed by 2014..
  
  
• 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.
  

Examples of questions on this material that could be asked on an exam

1. What conditions are necessary to form a tropical cyclone?
   
   
2. From what source do tropical cyclones derive their energy?
   
   
3. What is the difference between a tropical cyclone, a hurricane, and a typhoon?
   
   
4. What are the stages of development of a storm before becoming a tropical cyclone?
   
   
5. How are tropical cyclones classified in terms of their intensity?
   
   
6. In the northern hemisphere, why would one expect more damage and higher storm surge on the right-hand side of the storm and more rainfall in the northeast quadrant of the storm?
   
   
7. Describe the frequency distribution of hurricanes in the Atlantic and Gulf of Mexico.   When does the peak of the hurricane season occur?
   
   
8. What is the storm surge and what factors control its height?
   
   
9. How are tropical cyclones monitored and their paths predicted?
   
   
10. What can be done to reduce the risk from hurricanes to human lives and property?

References
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