April 7, 2008

Hurricanes (Continued)

n Last time we went through a simplified model for hurricane development that was meant to explain energetically how hurricanes strengthen. However, that model does not explain the actual way these tropical systems organize. This is described below.

n Anatomy of a real hurricane

o As with any cyclone, winds at the surface blow generally counterclockwise (in the Northern Hemisphere), and converge toward the low center (spiral inward).

o Draw a top-down figure of a typical hurricane.

§ Look at figure 11.2 (textbook)

o All hurricanes consist of the following components:

§ Surface low: All tropical cyclones rotate around an area of low atmospheric pressure near the Earth's surface. The pressures recorded at the centers of tropical cyclones are among the lowest that ever occur at sea level.

§ Warm core: Tropical cyclones are characterized and driven by the release of large amounts of latent heat of condensation as moist air is carried upwards and its water vapor condenses. This heat is distributed vertically, around the center of the storm. Thus, at any given altitude (except close to the surface where water temperature dictates air temperature) the environment inside the cyclone is warmer than its outer surroundings.

§ Eye: A strong tropical cyclone will harbor an area of sinking air at the center of circulation. Weather in the eye is normally calm and free of clouds (however, the sea may be extremely violent). The eye is normally circular in shape, and may range in size from 8 km to 200 km (5 miles to 125 miles) in diameter. In weaker cyclones, the clouds may cover the circulation center, resulting in no visible eye.

· The abnormally high pressure found at the top of the storm (above the eye) plays a role in initiating the downward air motion within the eye.

§ Eyewall: The eyewall is a circular band of intense convection and winds immediately surrounding the eye. It has the most severe conditions in a tropical cyclone. Intense cyclones show eyewall replacement cycles, in which outer eye walls form to replace inner ones. The mechanisms that make this occur are still not fully understood. In the eyewall replacement process, the eyewall contracts to a smaller size, and outer rain bands form a new eyewall. This new eyewall weakens the original one, and eventually replaces it completely. During the replacement cycle, the storm weakens, sometimes dramatically, but afterwards the storm will often be stronger than before.

§ Outer or Spiral Rain Bands: Focused areas of low level convergence, rising motion, and heavy rain that rotate counterclockwise around the storm. These may extend hundreds of kilometers from the storm's center. The spiral rain bands are basically aligned with the low level winds which rotate counterclockwise and spiral inward toward the storm's center.

§ Outflow: The upper levels of a tropical cyclone feature winds headed away from the center of the storm with an anticyclonic (clockwise) rotation. Winds at the surface are strongly cyclonic, weaken with height, and eventually reverse themselves. Tropical cyclones owe this unique characteristic to the warm core at the center of the storm.

§ Examine figure 11.3 from the textbook.

n Tropical Storm / Hurricane Occurrence

o Worldwide, 86 tropical cyclones reach tropical storm strength each year on average. Of these, 47 reach hurricane strength with 20 of those being major hurricanes (category 3 or higher).

§ Figure 11.10 shows that tropical storms form in all of the tropical ocean basins, except the south Atlantic and southeastern Pacific Oceans. The reason tropical storms do not form in these areas is ocean temperatures are too cold.

o The table below shows statistics for the 5 most active ocean basins

Basin	Season Start	Season End	Tropical Storms	Hurricanes	Major Hurricanes
Northwest Pacific	Year round	Year round	26.7	16.9	8.5
Northeast Pacific	May	November	16.3	9.0	4.1
Southwest Indian	October	May	13.3	6.7	2.7
North Atlantic	June	November	10.6	5.9	2.0
Southwest Pacific	October	May	10.6	4.8	1.9

n Hurricane Movement

o The most important determinant of hurricane movement is the large-scale environmental air flow in the middle troposphere in the vicinity of the storm. In other words, hurricanes tend to be "steered" around by the winds blowing at about 500 mb pressure. In late summer and early fall the "typical" wind patterns at 500 mb are generally easterly (or east toward west) below or south of 30° latitude and westerly (west toward east) above or north of 30° latitude. The reason for this is that a broad and elongated area of higher pressure is typically found near 30° latitude. Hurricanes to the south 30° latitude will generally be steered from east to west. However, as shown in figure 11.10, hurricanes have a tendency to turn away from the equator (toward the north in the Northern Hemisphere) at some point along their track.

o Overview for Atlantic hurricanes moving into the US coastline.

§ The subtropical high pressure area over the Atlantic Ocean is often found as a high pressure cell. While the position and overall size of this feature changes, its center is often located near the island of Burmuda, hence it is often called the Burmuda High. This is quite important to hurricane movement in the vicinity of the United States as shown in the figure below. In the top panel, tropical cylones moving across the Atlantic will turn northward before reaching Florida, while in the bottom panel, they are steered toward the east coast of Florida. In situations where the Burmuda high extends slightly further west, tropical systems can be steered toward the west either across Florida or south of Florida before turning north to hit the gulf coast. If the Burmuda high extends even further west, tropical storms can move west right across the Gulf of Mexico and hit Texas or Mexico

§ The areas of the United States at risk from hurricanes includes the entire Gulf of Mexico coastline and the entire eastern seaboard, which includes the larger northeastern cities like New York. While it is much less likely that a major hurricane would hit New York City than areas further south (because the Burmuda high would have to be positioned just right and the ocean water termperature decreases on the way), it has happened in the past. The "Great Hurricane of 1938" slammed into Long Island and devastated much of southern New England as a category 3 storm. The last direct hit on New York City was back in 1821. And much of the New York metro area is just above sea level, making the city quite vulnerable to a strong hit.

o Overview for Pacific Hurricanes moving into the US coastline

§ Although the west coast of the United States has never been hit by a hurricane, it is not all that uncommon for the west coast and desert southwest to get heavy rain events from remnants of eastern Pacific tropical storms and hurricanes. As shown in figure 11.10, most eastern Pacific hurricanes move westward out into the Pacific Ocean away from the North American continent. However, if a storm gets "picked up" by the winds associated with a middle latitude 500 mb trough, it can be turned north and east back toward North America. As these storms move northward, they encounter the colder waters of the eastern Pacific and weaken below hurricane strength before reaching the United States. Farther south along Baja California and the Mexican Pacific coast, the water can be warm enough to support a hurricane, and these areas do experience hurricanes from time to time.

n Predicting Hurricane movement and Intensity. Providing public warnings.

o The previous sections of these notes may lead you to conclude that predicting hurricane movement is easy, just look at the 500 mb winds, but this is misleading. The actual path taken by a hurricane can be caotic and difficult to predict. One difficulty lies in the fact that the upper level winds that can steer hurricanes (pressure levels at 500 mb and higher altitudes) are often quite weak in the tropics. When steering winds are weak, the actual path of a hurricane is determined by the structure of the storm and the storm's interaction with the environment, which is difficult to determine. Thus, it is not all that uncommon for hurricanes to take erratic paths and make odd turns that catch weather forecasters by surprise.

§ Take a look at link showing track of all Atlantic hurricanes (1851-2006).

o Hurricane forecasters must predict:

§ Storm movement (future storm positions)

§ Future storm intensity (usually the more difficult of the two)

o The National Hurricane Center in Miami, Florida uses sophisticated computer models to predict hurricane movement. Although there are still many aspects of hurricane evolution that we do not understand, steady progress has been made in improving forecasts of hurricane movement.

§ Show figure linked on the lecture summary page.

§ Notice though that on average there are still significant errors in the prediction of the future position of hurricanes.

o A more challenging problem is to predict the future intensity of hurricanes, that is will they strengthen or weaken and how fast will these changes occur. The problem here is caused by both by our lack of understanding of hurricane evolution which results in poor computer modeling as well as the lack of observations or measurements near the storm's center, which are necessary to understand exactly what is happening.

§ The big problem is pointed out by Max Mayfield, former director of the National Hurricane Center -- "Most major hurricanes become major hurricanes by going through some rapid intensification cycle that we just don't understand ..." This sets up the possibility of what Mayfield calls his "nightmare scenario" -- where for example, a rather weak hurricane just off the coast rapidly intensifies to a major hurricane overnight, and slams into a coastal region where the population is unprepared. "We still haven't had that nightmare scenario where people go to bed prepared for a Category 1 hurricane and wake up to a Category 4," Mayfield says. "That's going to happen one of these days, and it's going to be devastating."

o 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. Although hurricanes can be easily tracked using satellite data, predictions of their future movement and intensity are by no means certain. Based on the estimated uncertainty in future hurricane movement, hurricane watches and warnings are issued for a wide swath as shown in the figure below.

§ The large warning area follows the motto “better safe than sorry”. But it does present a problem. Suppose a large area is evacuated, but the actual hurricane only affects a portion of the warning area. People who evacuated from areas not severely affected by the hurricane may choose to ignore the warning next time … thinking the forecaster have no idea what they are doing.

§ Hurricane Watch: An announcement of specific coastal areas that a hurricane or an incipient hurricane condition (sustained winds > 74 mph) poses a possible threat, generally within 36 hours.

§ Hurricane Warning: Sustained winds of 74 mph or greater associated with a hurricane are expected (have a good probability of happening) in a specified coastal area in 24 hours or less.

n Destruction at Landfall

o Although we usually categorize hurricanes in terms of their wind speed, coastal flooding due to what is called the storm surge often causes the most damage. Most of the spectacular damage done by hurricanes, e.g., buildings, houses, marinas, piers, etc., completely destroyed or even removed from where they were standing is due to the storm surge.

o A storm surge is the rise in sea level along the coast as onshore winds pile up the water. It is more like a big dome of water that can be over 100 km in width, rather than big waves. It is similar to tsunamis in that it is a large mass of water, which is forced onshore. As the moving water encounters the normally shallower water areas near the shore, it is forced to rise upward. Storm surges are measured in feet above normal sea level (see Saffir-Simpson scale). Water is very heavy and damaging, and can “wash away” entire houses and bridges.

§ The damage potential associated with a storm surge depends upon:

1. The strength of the wind … stronger winds, higher surge

2. The slope of the Earth’s surface near the shoreline

o If more gradual, water can move further inland. Thus some areas are more vulnerable than others. Draw simple picture.

3. Timing with respect to normal high and low tides.

o High tides add to the storm surge

o In the Northern Hemisphere, the greatest storm surge and the strongest winds occur to the right of the storm center with respect to the direction that the storm is moving as it makes landfall.

§ A diagram explaining this will be drawn in lecture.

o High winds also cause considerable damage. Hurricane strength winds can damage or destroy vehicles, buildings, bridges, etc. High winds also turn loose debris into flying projectiles, making the outdoor environment even more dangerous. As hurricanes come in many sizes, the area covered by hurricane force winds is a factor in the total amount of damage done. For example, not only was Katrina a very powerful storm, it was also a very large storm.

o Flooding is also caused by the heavy rains associated with hurricanes, especially when a slow moving or nearly stationary hurricane sits just offshore causing a prolonged period of heavy rains over nearby coastal areas. This can be especially problematic if the nearby coast has sharply rising mountains, enhancing the heavy rains by orographic lifting, with the runoff causing landslides.

o Considerable damage may also occur from hurricane-spawned tornadoes that may form as the hurricane interacts with land areas. About one-quarter of them produce significant numbers of tornadoes. Tornadoes are most likely to develop in the right front quadrant of the storm, and are more likely to be associated with a spiral rain band rather than the storm's center. They result from the vertical wind shear present in the lower levels of the hurricane's circulation. Most hurricane-spawned tornadoes are of the weak variety (compared to the monsters that can form over the great plains of the United States), but can still produce significant damage.