Hurricane Camille was a ferocious Category 5 hurricane when it hit Mississippi on August 17, 1969. Camille's 190 mph sustained winds at landfall were the highest winds ever recorded for a U.S. landfalling hurricane. Camille drove a record storm surge of 22.6 feet to Pass Christian, Mississippi. On August 29, 2005, Hurricane Katrina hit the same region of Mississippi coast, as a Category 3 hurricane with 130 mph sustained winds. Yet Katrina's storm surge exceeded Camille's at all locations, topping out at 27.8 feet at Pass Christian (Fritz et al., 2008). How could a Category 3 hurricane's storm surge exceed that of a Category 5?
Figure 1. A comparison of the maximum storm surge computed by NOAA's SLOSH model from Category 3 Hurricane Katrina at landfall (left) and Category 5 Hurricane Camille (right). The surge is shown as the height above each grid cell. Katrina's surge was higher and covered a much larger area than Camille's, due to the large size of Katrina.
Let's consider the three parts of the storm surge. Firstly, the suction effect of having a large low pressure area over the ocean pulls up the surface of the water about 1 cm for each millibar of pressure drop. Hurricane Camille of 1969 had a central pressure of 905 mb near landfall, good for about 3.5 feet of storm surge. Hurricane Katrina had a central pressure between 918 and 927 mb at landfall, creating about 3 feet of storm surge. No major difference there (ironically, the barometer that recorded the 909 mb low pressure reading in Camille was destroyed during Hurricane Katrina).
Secondly, as a hurricane's winds blow onshore, the waves created pile water up onto the shore faster than the rip currents that form can drain the water away. This effect, called wave setup, is only good for about 1 - 3 feet of storm surge where there is large area of shallow water (the coastal shelf) of more than 50 km expanse. Again, there is no major difference between Katrina and Camille's surges due to this effect, since we are only talking about 1 - 3 feet of storm surge. Wave setup is much more important near coasts where the deep water is less than ten miles offshore, like Miami.
|Figure 2. Storm surge damage from Hurricane Camille at Biloxi, Mississippi. Image credit: NOAA Photo Library.||Figure 3. Storm surge damage from Hurricane Katrina at Empire, Louisiana.. Image credit: NOAA.|
The third factor, which causes the majority of the storm surge, is due to the rotational motion of the ocean waters near the hurricane's center imparted by the circular winds blowing in the storm's eyewall. The spiraling winds of a hurricane push ocean water to the center, where it sinks down and is carried away by deep ocean currents. So, in the deep ocean, the only storm surge is that of the suction effect; in Katrina's case, three feet. But while the hurricane's winds blow over the ocean, they create a circular vortex motion in a large region of water over the area spanned by the storm's radius of maximum winds (where the eyewall is). This rotating area of water takes the shape of a lopsided donut, with a thicker dimension on the strong side of the hurricane (the right side in the Northern Hemisphere), and tapering off in thickness as one moves away from the radius of maximum winds. The depth of moving water typically extends to 100 meters (328 feet) on the strong side of the storm. Camille was a small storm with a very tight eye, and hurricane-force winds extended out only 60 miles to the east of the center at landfall. Camille had an 11 mi diameter eye and a radius of maximum winds of approximately 15 miles. Katrina, though, had a large 37 mile diameter eye, and hurricane-force winds extended out 120 miles to the east of the center. Katrina's radius of maximum winds was about 30 miles, double Camille's. Since the volume of the lopsided donut of rotating waters is proportional to the square of the radius of the area of maximum winds, Katrina set in motion a volume of water about four times greater than Camille did.
Figure 4. A comparison of the wind field from Category 3 Hurricane Katrina at landfall (right) and Category 5 Hurricane Camille (left). The diameter of hurricane forces winds (yellow, orange, red, and pink colors) was roughly twice as great for Katrina. Image credit: NOAA Hurricane Research Division
Once a hurricane's lopsided donut of rotating waters hits the shallow waters of the Continental Shelf, deep water currents can no longer carry away the water piled up near the center. In order to obey the law of physics that requires a object to maintain the amount of spin it has (conservation of angular momentum), the lopsided donut of rotating waters tries to spread out horizontally. However, friction with the ocean bottom limits the amount the water can spread out horizontally, so much of the rotating lopsided donut is forced to move upwards against the force of gravity, in order to conserve angular momentum. This creates a high mound of water--the majority of the storm surge. Since Katrina set in motion four times as much water as Camille, Katrina was able to push a higher storm surge to the coast that covered a much longer stretch of coast.
Storm Surge Increase with Storm Size
In late 2004, NHC ran SLOSH simulations of Category 4 Hurricane Charley to study the effect of storm size on surge. Earlier work by Jelesnianski and Chen showed that there was a strong relationship between the two. According to Dr. Stephen Baig, the retired head of the National Hurricane Center's storm surge team, the simulations used the same track and the same storm central pressure (which is how SLOSH defines storm strength and generates its wind field), but a varying Radius of Maxiumum Winds (Figure 5). Charley was an extremely intense but compact Category 4 hurricane with 140 mph winds, and had an observed Radius of Maxiumum Winds (RMW) at landfall of 6 miles. The simulations done with a 6-mile RMW generated a surge of 6 - 7 feet--very close to what was observed. However, when the size of storm was increased to create a RMW of 10 miles, the maximum surge increased to 10 feet. The surge kept increasing in height as the RMW was increased, and reached a maximum of 18 feet when the RMW reached 30 - 40 miles--about what Katrina's RMW was. Note from Figure 5 that as the size of the storm was increased (while keeping Charley's central pressure the same), the maximum sustained winds decreased. This occurred because of conservation of angular momentum. If one spreads the maximum winds out so that they cover a larger area, the maximum winds must decrease so that the total momentum of the winds remains unchanged. As a result, Charley would have generated a much more devastating surge had it been a strong Category 2 hurricane with 110 mph winds and a Radius of Maximum Winds of 40 miles.
Recall the unexpectedly high surge--up to 17 feet--from Category 2 Hurricane Ike of 2008? Ike had a RMW of 40 - 50 miles a day prior to landfall, and was able to generate a huge surge despite being "only" a Category 2 hurricane with 110 mph winds. Ike's central pressure at landfall was 950 mb, a pressure more typical of a strong Category 3 storm with 130 mph winds, and was able to drive a storm surge more typical of a strong Category 3 hurricane to shore.
Figure 5. A NOAA SLOSH model simulation of Hurricane Chaley of 2004. The storm surge (pink squares) increases and the maximum winds (dark blue squares) decrease as the simulated Radius of Maximum Winds (RMW) is increased. The central pressure is held constant. Image credit: Dr. Stephen Baig, NOAA/NHC.
Fitzpatrick, P., et al., 2006, Storm Surge Issues of Hurricane Katrina (PDF File), 60th Interdepartmental Hurricane Conference.
Fritz et al., 2008, "Hurricane Katrina Storm Surge Reconnaissance", Journal of Geotechnical and Geoenvironmental Engineering, May 2008, pp 644-656.
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