Why do cyclones begin to dissipate

In the Norwegian cyclone model, the cold front overtakes the warm front and the cyclone becomes occluded. Cloud cover and precipitation cover a wide area and the storm is usually most intense at this stage. The point where the cold front, warm front, and occluded front intersect is called the triple-point. Occasionally, a secondary low may form at this triple point, move eastward, and intensify into another cyclone. Eventually, as occlusion advances, the low pressure center will begin to dissipate, because cold air exists on both sides of the occluded front.

The sector of warm, rising air is removed from the center of the storm, so the storm gets cut off from its primary energy supply. Eventually the old storm dies out and gradually disappears. This sequence of a developing mid-latitude cyclone is similar to a whirling, spinning eddy in a river that forms behind a stick or log, moves along with the river, and quickly disappears further downstream.

This entire life cycle can last from several days to a little more than a week. As mentioned before, some cyclones form from dying previous cyclones and become a part of the succession.

This polar front model of development for a mid-latitude cyclone is rather simplified and, in fact, very few storms follow this model exactly. However, it is a good foundation for understanding storm structure. The development of a mid-latitude cyclone is a process called cyclogenesis.

Certain regions in North America are more favorable for cyclogenesis, including the eastern slopes of mountain ranges like the Rockies and Sierra Nevada, the Atlantic Ocean off the Carolina Coast, and the Gulf of Mexico. When air flows westward across a north-south extending mountain range, the air on the leeward downwind side tends to have cyclonic curvature, which adds to the development of a cyclone.

These storms can bring heavy rain or snow and high winds to areas along the East Coast. Typical cyclone storm tracks are named after the region in which they form, like the Hatteras low , Alberta Clipper , or Colorado low.

Alberta clippers and Colorado lows form or re-develop on the lee-side of the Rockies. Mid-latitude cyclones always move toward the east due to the prevailing westerlies. What influences the strength of a mid-latitude cyclone, and determines how long it will persist? There are some surface conditions that influence cyclogenesis, but the real key to mid-latitude cyclone development lies in the winds aloft. How are mid-latitude cyclones influenced by upper-level flow? Developing surface lows are usually more intense with height and appear on upper-level charts as a trough or a closed low.

However, the low in the upper-levels usually exists to the west of the surface low again, in the Northern Hemisphere. This is a necessary condition for a low pressure system to continue to develop and intensify. If the upper-level low were directly over the surface low, the surface low would quickly dissipate. This is because winds converge inward toward the low, but only at the surface. The increase in air mass causes surface pressures to rise, and the low fills in and dissipates.

How then do cyclones intensify and develop? The vertical structure of the atmosphere must allow for air to rise out of a surface low pressure. The following figure shows an idealized model of the vertical structure of a cyclone and anticyclone in the Northern Hemisphere. A surface low and a surface high are accompanied by an upper level trough and ridge respectively. On the right hand side is a Northern Hemisphere frontal cyclone with a warm and cold front. The cold air behind the cold front at the surface also extends upward aloft.

Recall that the layer between two pressure surfaces is thinner when the air temperature of the layer is cold more dense , and thicker when the layer is warm less dense. The planes fly completely through the storm, passing though the eye, sometimes making several passes. The data collected give meteorologists a 3 dimensional picture of the structure of the storm.

Satellite images and radar from land based stations allow scientists to track the position of the storm and report it to all agencies that may be affected if the storm makes landfall. Changes in Hurricane Tracks and Intensities Because hurricanes are influenced by large-scale air masses, they sometimes move along rather erratic paths. Hurricanes are especially influenced by the strength and direction of upper level winds. As noted above, strong upper level winds create a vertical wind shear that cause the top of the hurricane to be sheared off and result in the loss of strength of the storm.

The erratic nature of a hurricane's path often makes it difficult to predict where and when it will make landfall prior to several hours before it actually does make landfall.

This increase in storm center velocity usually results from the interaction of the storm with other air masses. Off the eastern coast of the United States there is an area of semi-permanent high pressure, known as the Bermuda High.

Other high pressure centers are continually moving eastward off of North America. If the hurricane encounters a low pressure trough between two high pressure centers, it is steered into the trough and follows it along a northeastward trend, increasing its velocity as it does so.

Interaction with the land and other air masses are most responsible for changes in hurricane tracks and intensities. Some examples are shown on the map below. Two of the most erratic hurricane paths recorded are shown by Hurricane Betsy, in and Hurricane Elena in Hurricane Betsy, a category 3 storm, took a northwestward track from the Caribbean Islands, but then turned abruptly west as it passed north of Puerto Rico.

It then took a northwest track again, looking like it would hit along the coast of Georgia or South Carolina. Suddenly, however, it looped back to the south, passed over the southern tip of Florida, crossed the Gulf of Mexico and hit just east of New Orleans.

Because the storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the hurricane makes landfall. This is an important point to remember because flooding caused by the surge can destroy roads and bridges making evacuation before the storm impossible. Since thunderstorms accompany hurricanes, and these storms can strike inland areas long before the hurricane arrives, water draining from the land in streams and estuaries may be impeded by the storm surge that has pushed water up the streams and estuaries.

It is also important to remember that water that is pushed onto the land by the approaching storm the flood surge will have to drain off after the storm has passed. Furthermore after passage of the storm the winds typically change direction and push the water in the opposite direction. Damage can also be caused by the retreating surge, called the ebb surge. Along coastal areas with barrier islands offshore, the surge may first destroy any bridges leading to the islands, and then cause water to overflow the islands.

Barrier islands are not very safe places to be during an approaching hurricane! Hurricane Damage Hurricanes cause damage as a result of the high winds, the storm surge, heavy rain, and tornadoes that are often generated from the thunderstorms as they cross land areas. Strong winds can cause damage to structures, vegetation, and crops, as described in the Saffir-Simpson scale discussed previously.

The collapse of structures can cause death. The storm surge and associated flooding, however, is what is most responsible for casualties. Extreme cases of storm surge casualties have occurred as recently as and in Bangladesh and in Myanmar.. In a cyclone struck Bangladesh during the highest high tides full moon.

The storm surge was 7 m 23 ft. Another cyclone in created a storm surge 6 m high and resulted in , deaths. The May cyclone in Myanmar is estimated to have killed , The amount of damage caused by a tropical cyclone is directly related to the intensity of the storm, the duration of the storm related to its storm-center velocity, as discussed above , the angle at which it approaches the land, and the population density along the coastline.

The table below shows how damages are expected to increase with increasing tropical storm category. Like the Richter scale for earthquakes, damage does not increase linearly with increasing hurricane category.

Prediction of hurricane intensity wind speed is more problematic as too many factors are involved. Hurricanes are continually changing their intensity as they evolve and move into different environments. Without the ability to know which environmental factors are going to change, it is very difficult to expect improvement on intensity forecasting. Hurricane Katrina was expected to loose intensity as moved out of the warmer waters of the Gulf of Mexico.

But, it showed a more rapid drop in intensity just before landfall because a mass of cooler dry air was pulled in from the northwest. Some progress has been made in predicting the number and intensity of storms for the Atlantic Ocean by Dr. William Gray of Colorado State University. He has shown that there is a correlation between the frequency of intense Atlantic hurricanes with the amount of rainfall in western Africa in the preceding year.

This correlation has allowed fairly accurate forecasts of the number of storms of a given intensity that will form each year. Nevertheless, Dr.

Gray's predictions are closely watched, and have been otherwise fairly accurate. Reducing Hurricane Damage There is plenty of historical data on hurricane damage in the United States so that it is not difficult to see ways that damage from hurricanes can be reduced. In terms of protection of human life, the best possible solution is to evacuate areas before a hurricane and its associated storm surge reaches coastal areas. Other measures can be undertaken to reduce hurricane damage as well.

The problem, however, is that it may not always be possible to issue such a warning in time for adequate evacuation of these areas. Because the storm surge and even gale force winds can reach an area many hours before the center of the storm, warnings must be issued long enough before the storm strikes that the surge and winds do not hinder the evacuation process.

The effectiveness of the warning systems also depends on the populace to heed the warning and evacuate the area rather than ride out the storm, and the state of preparedness of local government agencies in terms of evacuation and disaster planning. New Orleans is a particularly notable example.

Since most of the city is at or below sea level, a storm surge of 6 meters 20 feet from a category 4 or 5 hurricane would most certainly flood the city and choke all evacuation routes. Even with 24 hours notice of the approaching surge which would mean as soon as the storm entered the Gulf of Mexico it would be difficult to evacuate or convince people to evacuate within that 24 hour period. A hurricane approaching New Orleans was a disaster waiting to happen as we can all testify.

Hurricane Donna in shows the effects of the land decreasing the intensity of a hurricane. Donna hit the southern tip of Florida as a category 4 hurricane. It then took a northeastward track across Florida, loosing strength as it crossed the land. On re-entering the Atlantic Ocean it again increased in intensity due to the warm ocean waters, took a track along the east coast and eventually hit Long Island, New York.

Tropical Cyclones Hurricanes Fall Atmospheric Circulation The troposphere undergoes circulation because of convection. If the Earth were not rotating, this would result in a convection cell, with warm moist air rising at the equator, spreading toward the poles along the top of the troposphere, cooling as it moves poleward, then descending at the poles, as shown in the diagram above.

Once back at the surface of the Earth, the dry cold air would circulate back toward the equator to become warmed once again. The Coriolis Effect - Again, the diagram above would only apply to a non-rotating Earth. Since the Earth is in fact rotating, atmospheric circulation patterns are much more complex.

The reason for this is the Coriolis Effect. The Coriolis Effect causes any body that moves on a rotating planet to turn to the right clockwise in the northern hemisphere and to the left counterclockwise in the southern hemisphere.

The effect is negligible at the equator and increases both north and south toward the poles. Low Pressure Centers - In zones where air ascends, the air is less dense than its surroundings and this creates a center of low atmospheric pressure, or low pressure center. Winds blow from areas of high pressure to areas of low pressure, and so the surface winds would tend to blow toward a low pressure center. But, because of the Coriolis Effect, these winds are deflected.

In the northern hemisphere they are deflected to toward the right, and fail to arrive at the low pressure center, but instead circulate around it in a counter clockwise fashion as shown here. In the southern hemisphere the circulation around a low pressure center would be clockwise. Such winds are called cyclonic winds. High Pressure Centers - In zones where air descends back to the surface, the air is more dense than its surroundings and this creates a center of high atmospheric pressure.

Since winds blow from areas of high pressure to areas of low pressure, winds spiral outward away from the high pressure.

But, because of the Coriolis Effect, such winds, again will be deflected toward the right in the northern hemisphere and create a general clockwise rotation around the high pressure center. In the southern hemisphere the effect is just the opposite, and winds circulate in a counterclockwise rotation about the high pressure center. Such winds circulating around a high pressure center are called anticyclonic winds.

Because of the Coriolis Effect, the pattern of atmospheric circulation is broken into belts as shown here. The rising moist air at the equator creates a series of low pressure zones along the equator. Water vapor in the moist air rising at the equator condenses as it rises and cools causing clouds to form and rain to fall. After this air has lost its moisture, it spreads to the north and south, continuing to cool, where it then descends at the mid-latitudes about 30 o North and South.

Descending air creates zones of high pressure, known as subtropical high pressure areas. Because of the rotating Earth, these descending zones of high pressure veer in a clockwise direction in the northern hemisphere, creating winds that circulate clockwise about the high pressure areas, and giving rise to winds, called the trade winds , that blow from the northeast back towards the equator.

In the southern hemisphere the air circulating around a high pressure center is veered toward the left, causing circulation in a counterclockwise direction, and giving rise to the southeast trade winds blowing toward the equator. Near the equator, where the trade winds converge, is the Intertropical Convergence Zone ITCZ Air circulating north and south of the subtropical high pressure zones generally blows in a westerly direction in both hemispheres, giving rise to the prevailing westerly winds.

These westerly moving air masses again become heated and start to rise creating belts of subpolar lows. Meeting of the air mass circulating down from the poles and up from the subtropical highs creates a polar front which gives rise to storms where the two air masses meet. In general, the surface along which a cold air mass meets a warm air mass is called a front. The position of the polar fronts continually shifts slightly north and south, bringing different weather patterns across the land.

Powell, M. Houston, and T. Contributed by Chris Landsea First, the "right side of the system" is defined with respect to the system's motion: if the cyclone is moving to the west, the left side would be to the south of the system; if the cyclone is moving to the north, the left side would be to the west of the system, etc In general, the strongest winds in a cyclone are found on the left side of the system because the motion of the cyclone also contributes to its swirling winds. For tropical cyclones in the Northern Hemisphere, these differences are reversed: the strongest winds are on the right side of the system.

This is because the winds swirl anticlockwise south of the equator in tropical cyclones. Subject D4 How much energy does a hurricane release? One can look at the energetics of a hurricane in two ways: the total amount of energy released by the condensation of water droplets or It turns out that the vast majority of the heat released in the condensation process is used to cause rising motions in the thunderstorms and only a small portion drives the storm's horizontal winds.

More rain falls in the inner portion of hurricane around the eyewall, less in the outer rainbands. Converting this to a volume of rain gives 2.

A cubic cm of rain weighs 1 gm. Using the latent heat of condensation, this amount of rain produced gives 5. This is equivalent to times the world-wide electrical generating capacity - an incredible amount of energy produced!

Method 2 - Total kinetic energy wind energy generated: For a mature hurricane, the amount of kinetic energy generated is equal to that being dissipated due to friction. The dissipation rate per unit area is air density times the drag coefficient times the windspeed cubed See Emanuel for details. One could either integrate a typical wind profile over a range of radii from the hurricane's center to the outer radius encompassing the storm, or assume an average windspeed for the inner core of the hurricane.

This is equivalent to about half the world-wide electrical generating capacity - also an amazing amount of energy being produced! Either method is an enormous amount energy being generated by hurricanes.