Tropical Storms & Hurricanes
Part 2
Hurricane KatrinaAugust
2005
Lecture #17 (April 7, 2010, Wednesday)
All tropical cyclone tracks (1945-2006).
While moving westward, tropical disturbances intensify as surface ocean waters warm beneath them.
Take about 1 week to traverse the Atlantic as average migrating speeds are about 15–35 kmph (10–20 mph).
Only ~ 10% intensify into more organized, rotating storms.
When at least one closed isobar is present on weather map, the disturbance is classified as a tropical depression. A tropical depression is an organized system of clouds and thunderstorms with a defined, closed surface circulation and maximum sustained winds of less than 17 m s-1 (33 knot) or 39 mph (63 km/h). It has no eye and does not typically have the organization or the spiral shape of more powerful storms. However, it is already a low-pressure system, hence the name "depression".
Hurricane Formation
Further intensification, to wind speeds of 63 km/h (39 mph), place the storm in the category of tropical storm. A tropical storm is an organized system of strong thunderstorms with a defined surface circulation and maximum sustained winds between 17 metres per second (33 kn) (39 miles per hour (63 km/h)) and 32 metres per second (62 kn) (73 miles per hour (117 km/h)). At this point, the distinctive cyclonic shape starts to develop, although an eye is not usually present. Government weather services, other than the Philippines, first assign names to systems that reach this intensity (thus the term named storm).
Hurricane status is gained when winds reach or exceed 120 kmph (74 mph). A hurricane or typhoon (sometimes simply referred to as a tropical cyclone, as opposed to a depression or storm) is a system with sustained winds of at least 33 metres per second (64 kn) or 74 miles per hour (119 km/h). A cyclone of this intensity tends to develop an eye.
Hurricane Formation
Three tropical cyclones at
different stages of
development.
The weakest (left),
demonstrates only the
most basic circular
shape.
A stronger storm (top
right) demonstrates spiral
banding and increased
centralization.
The strongest (lower
right) has developed an
eye.
Should a tropical cyclone form in the North Atlantic Ocean or the
Northeastern Pacific Ocean, then it will classified using one of the
categories in the Saffir-Simpson Hurricane Scale.
In the Western Pacific, tropical cyclones will be ranked using the
Japan Meteorological Agency's scale.
The Regional Specialized Meteorological Centre in New Delhi, India
also uses a different scale to assess the maximum sustained winds
of a tropical cyclone.
In the Southern Hemisphere, the Météo-France forecast center on La
Reunion in France uses a scale that covers the whole of the South
West Indian Ocean. Both the Australian Bureau of Meteorology and
the Regional Specialized Meteorological Centre in Nadi, Fiji use the
Australian tropical cyclone intensity scale.
Tropical cyclone scales
Hurricane Intensity ScaleThe Saffir-Simpson hurricane scale classifies hurricanes into 5 categories
based on central pressure, maximum sustained wind speed, storm surge.
Some scientists, including Kerry Emanuel and Lakshmi
Kantha, have criticized the scale as being too simplistic,
indicating that the scale does not take into account the
physical size of a storm, nor the amount of precipitation it
produces. Additionally, they and others point out that the
Saffir-Simpson scale, unlike the Richter scale used to
measure earthquakes, is not open-ended, and is quantized
into a small number of categories. Proposed replacement
classifications include the Hurricane Intensity Index, which
is based on the dynamic pressure caused by a storm's
winds, which bases itself on surface wind speeds, the
radius of maximum winds of the storm, and its
translational velocity. Both of these scales are continuous,
akin to the Richter scale; however, neither of these scales
have been used by officials.
Criticism
After the series of powerful storm systems of the 2005 Atlantic
hurricane season, a few newspaper columnists and scientists brought
up the suggestion of introducing Category 6, and they have suggested
pegging Category 6 to storms with winds greater than 174 or 180 mph
(78–80 m/s; 150–155 knots; 280–290 km/h). Only a few storms in
history have reached into this hypothetical category. Many of these
storms were West Pacific super typhoons, most notably Typhoon Tip
in 1979 with sustained winds of 190 mph.
According to Robert Simpson, there is no reason for a Category 6 on
the Saffir-Simpson Scale because it is designed to measure the
potential damage of a hurricane to human-made structures. If the wind
speed of the hurricane is above 155 mph (250 km/h), then the damage
to a building will be "serious no matter how well it's engineered".
However, the result of new technologies in construction leads to the
suggestion that an increase in the number of categories is necessary.
Category 6
High winds: excessive damage even to well-built buildings
Heavy rainfall and flood: large amounts of property damage
Storm surge: large amounts of damage along coastal regions
Storm surges occur as water piles up due to both heavy winds and low atmospheric pressure
Hurricane Camille (1969) caused a storm surge of 7 m (23 ft) along the Mississippi coast.
Additionally, high surf occurs atop the surge, increasing damage
Winds and surge are typically most intense in the right front quadrant of the storm where wind speeds combine with the speed of the storm’s movement to create the area of highest potential impact
Tornadoes: This area also produces the greatest frequency of tornadoes within the hurricane due to frictional drag of lower atmospheric winds upon landfall.
Destruction by Hurricanes
The National Hurricane Center forecasts storm surge using the SLOSH
model, which stands for Sea, Lake and Overland Surges from Hurricanes.
Wind speed variations by quadrant
Max storm surge
Water levels may
decrease
Tornadoes
most often
form in the
right
quadrants
relative to
hurricane
storm center
in the
direction of
storm
movement.
Tropical systems, while generally located equatorward of the 20th
parallel, are steered primarily westward by the east-to-west winds on
the equatorward side of the subtropical ridge—a persistent high
pressure area over the world's oceans.
In the tropical North Atlantic and Northeast Pacific oceans, trade
winds—another name for the westward-moving wind currents—steer
tropical waves westward from the African coast and towards the
Caribbean Sea, North America, and ultimately into the central Pacific
ocean before the waves dampen out. These waves are the precursors
to many tropical cyclones within this region.
In the Indian Ocean and Western Pacific (both north and south of the
equator), tropical cyclogenesis is strongly influenced by the seasonal
movement of the Intertropical Convergence Zone and the monsoon
trough, rather than by easterly waves.
Tropical cyclones can also be steered by other systems, such as other
low pressure systems, high pressure systems, warm and cold fronts.
Movement and track
Movement depends upon the stage of development.
Tropical disturbances and depressions are largely regulated by trade wind and simply move westward.
For tropical storms and hurricanes, upper-level winds and ocean temperatures gain importance in determining their movements.
In the Atlantic, storms that gain latitude recurve toward the northeast due to the influence of surface and upper-level westerlies.
Movement is essentially parabolic. However, movement may be highly erratic in particular storms.
Hurricane Movement and Dissipation
Subtropical High
(Bermuda-Azores High)
The Tropical Setting
Warm waterCold water
Trade wind inversion
Hurricane
Movement and
Dissipation
Examples
of erratic
hurricane
paths
Hurricanes die when
They move over waters that cannot supply warm
moist air.
They move over land.
They reach a location where the flow aloft is
unfavorable (e.g., strong wind shear).
Hurricane Movement and Dissipation
The National Hurricane Center (NHC) is responsible for predicting and tracking Atlantic and east Pacific hurricanes.
Data are gathered through satellite observations, surface observations, and aircraft using dropsondes.
Statistical, dynamic, and hybrid computer modelsrunning on supercomputers assist in future track position and storm intensity predictions
Future positions are given along 6-hour trajectories with accuracy decreasing as lead time increases.
Hurricane Forecasts and Advisories
A watch is administered if an approaching
hurricane is predicted to reach land in more
than 24 hours.
If the time frame is less, a warning is given.
The erratic nature of the systems leads to
difficulties in exact prediction, warning, and
evacuation of prone areas.
Hurricane Watches and Warnings
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