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