Tue., Dec. 4 notes

Tuesday Dec. 4, 2007

This first section of material was actually covered in class on Tuesday Nov. 27.

There will be 5 basic questions about hurricanes on the Final Exam. Here is a detailed summary or review of the material that we will be covering.

You'll find the following sketch comparing a middle latitude storm and a hurricane on p. 141 in the photocopied notes.

The table below compares middle latitude storms and hurricanes. Items in red weren't mentioned in class.

Differences	Similarities	Differences
generally larger than hurricanes		usually smaller than middle latitude storms
Found at middle latitudes Can form over land or water	Both storms have low pressure centers. (the low pressure becomes high pressure at the top of a hurricane) The term cyclone refers to winds spinning around low pressure. Winds spin CCW in NH, CW in SH	Found in the tropics (5 to 20 latitude) Only form over warm ocean water
Movement is from west to east	Upper level divergence can lower the surface pressure and cause both types of storms to intensity	Movement is from east to west
Fronts separate warm and cold air masses brought together by converging winds.		Warm moist air mass only
Storm winds intensify with altitude		Storm winds weaken with altitude
Strongest storms winter to early spring		Strongest storms late summer to fall
Produce rain, snow, sleet, freezing rain		Mostly just heavy rain

The figure above shows the relative frequency of tropical cyclone development in different parts of the world. The name hurricane, cyclone, and typhoon all refer to the same type of storm (tropical cyclone is a general name that can be used anywhere). In most years the ocean off the coast of SE Asia is the world's most active hurricane zone. Hurricanes are very rare off the east and west coasts of South America.

Hurricanes form over warm ocean water. The warm layer of water must be fairly deep to contain enough energy to fuel a hurricane and in order that mixing doesn't bring cold water up to the ocean surface. The atmosphere must be unstable so that thunderstorms can develop. Hurricanes will only form when there is very little or no vertical wind shear (changing wind direction or speed with altitude). Hurricanes don't form at the equator because there is no Coriolis force there.

Note that more tropical cyclones form off the west coast of the US than off the east coast. The west coast hurricanes don't generally get the same attention, because they move away from the coast and usually don't present a threat to the US (except occasionally to the state of Hawaii). The moisture from these storms will sometimes be pulled up into the southwestern US where it can lead to heavy rain and flooding.

We'll try to cover most of the following material in class on Tuesday.

This figure shows when hurricanes are most common in the Atlantic. Hurricane season in the Atlantic officially runs from June 1 through to November 30. The peak of hurricane season is in September. In 2005, an unusually active hurricane season in the Atlantic, hurricanes continued through December and even into January 2006. Hurricane season in the Pacific begins two weeks earlier on May 15 and runs through Nov. 30.

Some kind of meteorological process that produces low level convergence is needed to initiate a hurricane. One possibility, and the one that fuels most of the strong N. Atlantic hurricanes, is an "easterly wave." This is just a "wiggle" in the wind flow pattern. Easterly waves often form over Africa or just off the African coast and then travel toward the west across the N. Atlantic. Winds converge as they approach the wave and then diverge once they are past it . The convergence will cause air to rise and thunderstorms to begin to develop.

In some ways winds blowing through an easterly wave resembles traffic on a multi-lane highway. Traffic will back up as it approaches a section of the highway with a closed lane. Once through the "bottleneck" traffic will begin to flow more freely.

In an average year, in the N. Atlantic, there will be 10 named storms (tropical storms or hurricanes) that develop during hurricane season. 2005 was, if you remember, a very unusual year. There were 28 named storms in the N. Atlantic in 2005. That beat the previous record of 21 names storms that had been set in 1933. Of the 28 named storms, 15 developed into hurricanes.

Winds blowing over mountains on the west coast of Mexico will sometimes form a surface low on the downwind side of the mountains. Note there are generally a few more tropical storms and hurricanes in the E. Pacific than in the N. Atlantic. They generally move away from the US coast, though the Hawaiian Islands are sometimes affected.

Once a cluster of thunderstorms to begin to form, if all the right conditions exist, the next figure shows what can happen next:

1. The converging winds pick up heat and moisture from the ocean. These are the two mains sources of energy for the hurricane.

2.   Rising air cools and thunderstorm clouds form. The release of latent heat during condensation warms the atmosphere. The core of a hurricane is warm.

3.   Pressure decreases more slowly with increasing altitude in the warm core of the hurricane. The result is that pressure at the top center of the hurricane is higher than the pressure further out from the hurricane (pressure at the top center is still lower than the pressure at the bottom center of the hurricane). Upper levels winds diverge and spiral outward from the top center of the hurricane.

4.   The upper level divergence causes the surface pressure to decrease. The speed of the converging surface winds increases and the storm intensifies. The converging winds pick up additional heat and moisture which warms the core of the hurricane even more. The upper level high pressure and the upper level divergence increase. The increased divergence lowers the surface pressure even more.

Stages of storm development. The developing storm receives a name when it reaches tropical storm strength. To be considered a hurricane the winds must be 74 MPH or greater (75 MPH might be easier to remember). Note the weather map symbols for northern hemisphere tropical storms and hurricanes (counterclockwise rotation)

A crossectional view of a mature hurricane and a picture like you might see on a satellite photograph.

Sinking air in the very center of a hurricane produces the clear skies of the eye, a hurricane's most distinctive feature. The eye is typically a few 10s of miles across, though it may only be a few miles across in the strongest hurricanes.

A ring of strong thunderstorms, the eye wall, surrounds the eye. This is where the hurricane's strongest winds found.

Additional concentric rings of thunderstorms are found as you move outward from the center of the hurricane. These are called rain bands. These usually aren't visible until you get to the outer edge of the hurricane because they are covered by high altitude clouds.

Out at sea, the converging surface winds create surface currents in the ocean that transport water toward the center of the hurricane. The rise in ocean level is probably only a few feet, though the waves are much larger. A return flow develops underwater that carries the water back to where it came from.

As the hurricane approaches shore, the ocean becomes shallower. The return flow must pass through a more restricted space. A rise in ocean level will increase the underwater pressure and the return flow will speed up. More pressure and an even faster return flow is needed as the hurricane gets near the coast.

Here is a link to the storm surge website (from the Hurricane Research Division of the Atlantic Oceanographic and Meteorological Labororatory). It has an interesting animation showing output from the SLOSH model used to predict hurricane storm surges.

Hurricanes can, of course, be very destructive. Out at sea the main hazards are the strong winds and the large waves. The Perfect Storm by Sebastian Junger describes the sinking of the Andrea Gail in a strong hurricane like storm in October 1991. The exact fate of the fishing ship is not known but it may have been turned end over end by a large wave (pitch poled). Large waves can also flood a ship and begin to fill it with water.

Along a coast the greatest threat is from the hurricane winds and the storm surge. Large waves are superimposed on the storm surge.

The hurricanes winds slow quickly as it moves onshore, though tornadoes may form. The biggest threat is from flooding. Hurricanes can easily drop a foot or more of rain on an area as they pass through.