Homework 8: Hurricane Damage and Hurricane Forecasts
(adapted from Pipkin et al.)
Tropical cyclones have a significant impact on coastal areas of the world. In the Atlantic and Eastern Pacific Oceans they are called hurricanes(from Hurican, the Carib god of evil), whereas similar storms in the Pacific Ocean are called typhoons. They are a significant part of global weather systems because they transfer large quantities of water and warm, moist air from equatorial regions to higher latitudes. In the Caribbean Sea, the Gulf of Mexico, and the southeastern United States, hurricanes have shaped low-lying coastal areas and, at times, have caused dramatic losses of life and property. Even with the use of orbiting weather satellites, hurricane prediction and tracking remains a major challenge for meteorologists.
The hurricane season extends from June 1 to October 1 in the North Atlantic Ocean. This is because hurricanes get most of their energy from the equatorial oceans in the summer and early fall, when seawater temperatures are warmest, usually above 25°C (about 800F). Hurricanes begin as tropical waves, areas of organized clouds 200 - 500 kilometers in diameter (Figure 8-1), in the eastern equatorial Atlantic. As hot air rises over the warm ocean water, the atmospheric pressure drops, and cyclonic (counterclockwise) circulation begins, with strong winds rushing toward the low pressure center. At this point the wave becomes a tropical depression. It becomes a tropical stormwhen winds 65 -118 kilometers / hour (39 -74 miles/hour) develop. If winds exceed 118 kilometers/hour, the storm is called a hurricane (in the Atlantic and eastern Pacific oceans) or a typhoon (in the western Pacific Ocean).
Figure 8-1 Atmospheric Waves West of Africa
The airflow off of the western Africa in the tropics are unstable during the NH summer creating atmospheric depressions and sometimes hurricanes that propagate westward.
Figure 8-2 Hurricane Structure – Satellite View
Tropical storms and hurricanes have cyclonic or counterclockwise circulation in the Northern Hemisphere because the Coriolis effect deflects the inward air flow toward to the right. In the Southern Hemisphere, the Coriolis deflection is to the left so that cyclones in the southern tropics have clockwise rotation.
Prior to 1953, hurricanes were named for the year or month of their occurrence, for the writer who described their passage, or for places sustaining severe damage, like the Galveston Hurricane of 1900 (Figure 8-3). During World War II, U.S. Navy and Army Air Corps meteorologists forecasting tropical cyclones in the Pacific Ocean began to informally give typhoons women's names, probably after their wives or girlfriends. Other naming schemes were women's names in alphabetical order, and by the international phonetic alphabet-Alpha, Bravo, Charlie, Delta, Echo, Foxtrot, and so on. In 1953 the U.S. Weather Bureau began to name Atlantic hurricanes with an alphabetical sequence of female names. The current hurricane-naming method is one of alternating male and female names, such as the recent names Alberto, Beryl, Chris, Debby, and so forth. Many names have been retired to the hurricane "Hall of Fame" because their storms were so devastating-Andrew, Gilbert, and Camille, and Katrina, for example.
Figure 8-3 Galveston Damage - 1900
The 1900 hurricane that hit Galveston Texas completely destroyed the town and killed many of the inhabitants.
The strength of hurricanes is measured by the Saffir-Simpson scale (Table 8-1 and Figure 8-4), which uses minimum sustained wind speeds to categorize storms from Category 1, a minimal hurricane, to Category 5, the most intense hurricane. Hurricane Mitch, which devastated Central America, and Hurricane Georges, which struck the coastal United States in 1998, were Category 5 events.
Because hurricanes have diameters of hundreds of kilometers and may have dozens of convective storms embedded in them, they may affect coastal areas far from the main storm center. As they approach the shoreline, NOAA's Hurricane Warning Center in Miami Beach, Florida, may is-sue a hurricane watch, a 36-hour advance notice of high winds and storm surge, or a hurricane warning, which means that hurricane winds and surge are expected within 24 hours.
Hurricane damage is caused by high winds, flooding, and storm surge. Surface winds can cause an abnormal rise in sea level, reaching from 1 to 6 meters above normal sea level (Figure 8-6). Along the Gulf Coast, the estimated 330 kilometer/hour winds of Hurricane Camille in 1969 caused a maximum storm surge of 8 meters, with 3-meter-high wind-driven waves atop the surge. The end result was a l0-meter-high wall of water approaching the Mississippi coastline. Failure to evacuate caused significant loss of life. One survivor of a "hurricane party" was found floating on a sofa 8 kilometers further inland from the coastal apartment in which the party was held.
Figure 8-6 Storm Surge.
(top) A sea level record on Mustang Island, Texas is partitioned into the (middle) contribution of storm surge (atmospheric wind and pressure) and (bottom) astronomical (sun and moon). Note the extreme, storm surge-related high water around February 1.
Not all tropical storms develop into hurricanes, and not all hurricanes develop damaging storm surge. Hurricanes tend to be cyclical; with periods during which relatively weak tropical storms impact coastlines followed by periods of stronger hurricanes. This is a dangerous sequence, because coastal residents tend to remember only the most recent tropical storm events. Hurricane development and strength may be cyclical because El Nino/Southern Oscillation events (see HW 10) may suppress tropical storms in the Atlantic Ocean, while permitting stronger storms to approach the west coast of North America.
The relative strength of hurricanes can be assessed from the atmospheric pressure of each storm, measured in millibars (mb) or millimeters of mercury (mm Hg). Inches of mercury is used in aviation and in some weather reporting formats (see Table 8-2 for conversions). Minimum surface pressure, however, is not necessarily a measure of the destructive potential of a hurricane. The lowest atmospheric pressure ever measured was during Hurricane Gilbert in 1998 in the Caribbean Sea: 888 mb, which is a record low. However, Gilbert was not as destructive as some other less intense hurricanes because its landfall was onto rising terrain and into less densely populated areas of the United States and Mexico. Table 8-2 shows the 20 most intense North American hurricanes since 1900 based on atmospheric pressure in the storm cell. Table 8-3 lists the most expensive American hurricanes between 1900 and 1996. For each storm, the damage estimates have been adjusted to 1996 dollars.
Movement of Hurricanes
The advance of a tropical storm or hurricane is controlled by the prevailing surface currents of the equatorial Atlantic Ocean. Tropical storms are moved westward by the North Equatorial Current, but there is extreme variability in the latitudinal (north-south) location of the storm track. Tropical cyclones may move through the Caribbean Sea between South America and the east-west chain of islands east of Cuba and Hispaniola, or they may be deflected north of Cuba into the Bahamas. Storms approaching the Atlantic coast of North America may be deflected northward by the Gulf Stream, or their advance may be sufficiently rapid to move them onshore into Florida or Georgia. Tropical cyclones developing in the Caribbean may move westward to Central America, or as did Hurricane Mitch in 1998, they may be forced northward across the Gulf of Mexico by the Loop Current. Since 1949, seven Atlantic hurricanes have crossed into the eastern Pacific Ocean, becoming Northeast Pacific hurricanes or tropical storms.
Another factor that may influence the advance of tropical storms in the Atlantic Ocean is the presence of warm or cold weather fronts or cells on the North American continent during the approach of the storm. Strong clockwise (anti-cyclonic) winds of high-pressure weather cells may slow the approach of tropical storms, or even stop their approach entirely. Hurricanes "stalled" in the Gulf of Mexico or the Atlantic Ocean may intensify, degenerate, or be deflected to the northeast without making any landfall on the North American continent. The unpredictable nature of hurricane movement is perhaps best shown by the "zig-zag" approach of Hurricane Elena in September 1985 (Figure 9-7).
Figure 9-7Hurricane Elena Trajectory
The track of Hurricane Elena in the Gulf of Mexico during September, 1995. Elena strengthened to hurricane status in the northern Caribbean Sea between Cuba and Hispaniola, then moved westward until it encountered the Loop Current in the Gulf of Mexico. The Loop Current deflected the storm to the northeast, where it "stalled" off Florida, then made a "retrograde" motion nearly due westward across the coastal areas of the Florida Panhandle and Alabama-before making landfall on the Mississippi coast.
Forecasting of Hurricanes
Because it is so critical to assess the strength of tropical cyclonic storms, the National Oceanic and Atmospheric Administration (NOAA) operates the National Hurricane Center in Miami, Florida. The National Hurricane Center distributes tropical weather, hurricane, and severe storm information to the public by television, radio, and the Internet. Its web site, at , contains the latest forecasts for tropical storm activity, reconnaissance data, and historical storm data for the North Atlantic, Eastern Pacific, Central Pacific, Western Pacific, and other regions. Another «official" source of information is the NOAA National Climatic Data Center, which contains information on hurricanes, El Nino, global climate, and other weather data. The NCDC has images and movies of hurricanes, tropical storms, typhoons, and other severe storms at satellite/oIimages.html>. Other web sites providing hurricane information and graphics are listed in the Bibliography.
As a hurricane is expected to make landfall, the weather service will issue a Hurricane Watch and then a Warning. Definitions of these are as follows:
WATCH: Hurricane conditions are possible in the specified area of the WATCH, usually within 36 hours.
WARNING: Hurricane conditions are expected in the specified area of the WARNING, usually within 24 hours.
Homework 8 - Hurricane Damage and Forecasts Name _____________________ EXERCISE 8-1 HURRICANE DAMAGE 1. Compare hurricane strength (storm category and atmospheric pressure) to hurricane damage using the information in Tables 8-2 and 8-3. Answer the questions on the next page.
(a) Is there a relationship between hurricane strength and damage? ___________
If so, describe it. If not, why not?
(b) Does a Category 5 storm always produce more damage than a Category 4 storm?______
Explain your answer with specific examples.
(c) Is there a predictable relationship between storm damage and meteorologically measurable data (wind speed, storm category, atmospheric pressure)?________________
If so, describe it. If not, why not?
(d) What factors could produce more damage from a "weaker" hurricane?
EXERCISE 8-2 HURRICANE FORECASTING 2. The hurricane tracking chart provided at the end of this exercise is a modified version of one available from the National Oceanic and Atmospheric Administration (NOAA). You are strongly urged to print some extras from the website because they are more clear than the attached map. (http://www.nhc.noaa.gov/AT_Track_chart2.pdf). However, you will have to set the transparent color on the picture toolbar in order to get rid of the black background.
Plot the location of the eye of hurricane Zeke from July 12 to 14 1998 using the information in Table 8-4 only.
(a) If the radius of maximum wind from the eye of the hurricane is 150 nautical miles, and the radius of all storm winds is 300 nautical miles, predict the location and estimated time of landfall for Hurricane Zeke. Recall that one degree of latitude equals 60 nautical miles.
(b) At what date and time would you issue a hurricane watch for the Miami region?
(c) When would you issue a hurricane warning for the Miami region?
3. Continue to plot the location of the eye of Hurricane Zeke during July 15 and 16 1998 on the same map as in #2, using the data in Table 8-5
(a) Compare your estimated landfall from Question 2 to the true landfall. How accurate (day and time) was your estimate? Be specific.
(b) Go back to the positions you plotted for Hurricane Zeke when the Hurricane Watch and Hurricane Warnings were posted, according to #2 (b) and (c). How accurate were they, now that you know the true landfall?
(c) If Hurricane Zeke did not make landfall at your predicted location, why not? (What atmospheric or oceanographic factors might have affected the storm's path?). Give at least TWO reasons.