Mon., Apr. 21 notes

Monday Apr. 21

Some Cuban music seemed appropriate for the few minutes before class started today since we would be spending the day covering hurricanes. A selection titled "El Cuarto de Tula" from the 1997 Buena Vista Social Club CD was played. The song tells an interesting story, you'll find the words in Spanish and English here. Wikipedia has some background information about the album.

The Experiment #3 revised reports were collected today.

The Experiment #4 reports have all been graded and can be picked up. Revised reports are due on or before Wed., April 30. Please return your original report with your revised report.

The Topic 6 (Rain Shadow Effect) 1S1P reports have been graded and were returned today together with the in class optional assignment from last Friday. Here are page 1 and page 2 of the answers to that assignment.

Here is a little more detailed listing of the similarities and differences between middle latitude storms and hurricanes.

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

Most of the similarities and differences are pretty readily understood. The role that upper level divergence plays in the intensification of a hurricane will be discussed later.

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 between 5 and 20 degrees latitude, over warm ocean water, north and south of the equator. 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 (the Coriolis force is what gives hurricanes their spin and it causes hurricanes to spin in opposite directions in the northern and southern hemispheres.

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 much 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.

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 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.

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.

Another possibility is a lee side low.

Winds blowing over mountains on the west coast of Mexico will sometimes form a surface low on the downwind side of the mountains. Surface winds will spiral inward toward the center of the low. 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.

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.

The increase in intensity is shown in the following figure (not shown in class)

The lower the surface pressure, the stronger the storm.

A 20 minute segment from a NOVA program (PBS network) on hurricanes was shown at the end of class. A film crew was on board a NOAA reconnaissance plane as it flew into the narrow eye of hurricane GILBERT. Gilbert set the record low sea level pressure reading for the Atlantic ocean (888 mb). That record stood until the 2005 hurricane season when WILMA set a new record of 882 mb. The world record low sea level pressure, 870 mb, was set in a SE Asian typhoon in 1979.

Here are some of the comments written down during the video. We will review the Saffir Simpson scale in class on Wednesday and look at the 3-dimensional structure of hurricanes in more detail.