Lecture 35 - Hurricanes pt. 2

Hurricane Katrina making landfall on Aug. 29, 2005. (source)

On average, hurricanes kill 20 people per year in the United States and cause about $5 billion of damage. As the table below indicates though there are exceptional years (such as 2005) where the death and damage totals greately exceed these average values (data are from www.economics.noaa.gov)

Year	Deaths	Total Damage (billion $ ))
2000	0	< 1
2001	24	6.5 B
2002	51	1.7 B
2003	14	2.3 B
2004	34	22.9 B
2005	1016	107.5 B
2006	0	< 1
2007	1	< 1
2008	11	7.9 B
2009	2	< 1
2010	0	< 1

2005 was, of course, the year hurricane Katrina hit New Orleans. Three of the ten strongest hurricanes ever observed in the N. Atlantic occurred in 2005 (Wilma was the strongest and the new record holder, Rita was 4th and Katrina 6th strongest). The deadliest hurricane in US history is the 1900 Galveston hurricane which caused 6000 - 12,000 deaths. The Great Hurricane of 1780 killed over 20,000 people in the Lesser Antilles. Historic rainfall amounts (75 inches perhaps in some locations) and flooding associated with Hurricane Mitch killed over 19,000 people in Honduras, Guatemala, and Nicaragua in 1998.

The Saffir Simpson Scale is used to rate hurricane intensity (just as the Fujita Scale is used for tornadoes). The scale runs from 1 to 5. Remember that a hurricane must have winds of 74 MPH or above to be considered a hurricane. Category 3,4, and 5 hurricanes are considered "major hurricanes" (in other parts of the world the term super typhoon is used for category 4 or 5 typhoons).

Here's an easy-to-remember version of the scale

Pressure decreases by 20 mb, wind speeds increase by 20 MPH, and the storm surge increases by 5 feet with every change in level on the scale.

The storm surge listed above is a rise in ocean level when a hurricane makes landfall. This causes the most damage and the greatest number of fatalities near a coast.

The converging surface winds associated with a hurricane sweep surface water in toward the center of a hurricane and cause it to pile up. The water sinks and, in deeper water, returns to where it came from. This gets harder and harder to do as the hurricane approaches shore and the ocean gets shallower. So the piled up water gets deeper and the return flow current gets stronger.

The National Weather Service has developed the SLOSH computer model that tries to predict the height and extant of a hurricane storm surge (SLOSH stands for Sea, Lake, and Overland Surges from Hurricanes). You can see some animations of SLOSH predictions run for hurricanes of historical interest (including the Galveston 1900) hurricane at a National Hurricane Center website (http://www.nhc.noaa.gov/surge)

If you watch closely you'll notice the highest surge is not where the center of the hurricane makes landfall. We can use the figure below to understand why this is true.

A hurricane is approaching a north-south oriented coast from the east at 15 MPH. The winds are spinning in a counterclockwise direction at 100 MPH around the center of the hurricane. Will the fastest winds be on the north, south, east or west side of the hurricane?

The fastest winds will be on the north side, because the direction of motion and the direction of the winds are both in the same direction. They add, the winds are blowing straight toward the coast at 115 MPH. On the south side the winds are pointing opposite the direction of motion. Now you subtract the speed of motion from the wind speed. The winds are 85 MPH and are blowing away from the coast on the south side of the hurricane.

Once a hurricane moves onshore the winds weaken rapidly. The greatest threat now becomes flooding from the tremendous amounts of rain that a hurricane can produce. Tornadoes are also a danger.

One of the reasons the 1900 Galveston hurricane was so deadly was that without radar and weather satellites it wasn't possible to provide much advance warning of its approach. Meteorologists depended on surface observations of pressure, winds, and ocean waves. Now, of course, the National Hurricane Center can monitor development, direction of motion, and strengthening as tropical storms move across the Atlantic and can issue watches and warnings as needed.

	Tropical Storm	Hurricane	Comments
Watch	Tropical storm conditions are possible within the specified coastal area within 48 hours	Hurricane conditions are possible within 48 hours. Issued 48 hours before the expected start of tropical storm strength winds	People in the watch area should obtain supplies, secure their homes and be prepared to evacuate
Warning	Tropical storm conditions are expected within 36 hours or less.	Hurricane conditions are expected within the specified coastal area. Issued 36 hours before the expected onset of tropical storm strength winds.

Watch and warning leadtimes of 36 and 24 hours were used prior to the 2010 hurricane season.

Predicting the future path of a hurricane is difficult because hurricane movement is affected by nearby weather systems and the ocean.

The figure above shows that there has been considerable reduction in the forecast errors. Predicting changes in the intensity of a hurricane remains a problem however.

Some researchers also try to forecast how active the upcoming hurricane season will be. Probably the best known forecas of this type is issued by Dr. William Gray from Colorado State University. You can look at the latest predictions here.

We have mentioned that 2005 was a record breaking year, at least as far as N. Atlantic hurricanes were concerned. There were 28 named storms (tropical storms or hurricanes) which easily beat the old record of 21 named storms in a year. As a matter of fact the pool of available names was exhausted and the last few hurricanes were named using Greek characters.

The most intense N. Atlantic hurricane ever (Wilma) occurred in 2005; hurricane Katrina was the 3rd most intense Atlantic hurricane to hit the US mainland. The 1935 Labor Day storm and Camille are still #1 and #2, Andrew is #4. Katrina became (easily) the most costly natural disaster in US history.

There is a tendency to blame an unusual year like this on global warming. We have seen that there has been a small increase in global average surface temperatures over the past 150 years. The world's oceans also appear to have warmed. We know that hurricanes form over warm ocean water so it is reasonable to expect that warmer oceans might produce more frequent and more intense hurricanes.

In the Atlantic there does seem to be a pretty good correlation between the Atlantic Multidecadal Oscillation (AMO) Index, which is a measure of sea surface temperature, and the frequency of major hurricanes. Greater than average numbers of major hurricanes tend to occur during warm periods of the AMO Index and vice versa. The correlation isn't quite as apparent when the total number of hurricanes is plotted. We only have reliable hurricane frequency data for a limited period of time, for the period of time when satellites have been able to monitor hurricane development over the oceans. This is probably not enough time to be able to say whether global warming has had an effect or not.

Globally there are about 90 tropical cyclones per year and this number doesn't seem to have changed over the past 40 to 50 years either.

At one point scientists thought that there might have been a recent increase in the intensity of hurricanes that could be tied to global warming. But more recently scientists have come to question that conclusion also. One recent article suggests that climate change might cause the number of hurricanes that occur globally to decrease. Hurricanes that do form however would most likely be more intense and produce more rain. It seems clear that we will need several more decades of hurricane data before we can determine whether global warming has an effect on hurricane numbers or strength.