Hurricanes

Hurricanes are large, tropical storm systems that form and develop over the warm waters near the equator.

They are responsible for weather that can devastate entire communities:

Heavy rain -- Flooding
Strong Winds
Very Large Waves and Storm Surge
Possibly Tornadoes


To begin our study of hurricanes we must first look at the typical weather in the tropics.

The sun typically remains high in the sky all year -- little variation in temperature with seasons.
The strong sun heats the water and enhances the evaporation of the water.
The general flow of air near the equator is out of the east -- Trade Winds.


The trade winds blow from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere.
A region of convergence (Intertropical Convergence Zone -- ITCZ) creates a band of thunderstorms near the equator.

Since pressure gradients are small near the equator, we need to look at the flow of wind to find trough regions.
A trough, or ripple, in the easterly flow is known as a "tropical wave."
Surface convergence occurs on the east side of this wave and surface divergence occurs on the west side.
Thunderstorms tend to form on the east side of the tropical wave.

Conditions for Tropical Wave Development

The wave must be north or south of the equator.
Coriolis force is zero at the equator.
Conditional Instability.
Weak vertical shear.
This is different from what we want in the mid-latitudes.
Warm sea-surface temperatures (SST)
Typically SSTs are greater than 26°C
 
Hurricane Development

Convergence at the surface leads to convergence of moisture.
The convection or thunderstorms that form release latent heat which intensifies the surface low.
The warm core column of air will create an upper level high pressure center.
The weak shear allows the storm to remain vertical and allows for the latent heat release to enhance the surface low.  (Positive feedback)
The energy for the growth of the storm comes from the ocean (evaporation).
Low shear will keep the heating in the core to remain above the surface convergence.
Strong shear would rip the storm apart.
As the winds increase, the ocean surface becomes "rougher" and friction is added to the balance of forces.
This friction enhances surface convergence.
This feedback mechanism continues as long as the favorable conditions for hurricane growth continue to exist.

This feedback mechanism is called CISK:

Conditional Instability of the Second Kind


Anatomy of a hurricane

The Eye is the central region of the hurricane.  It is often cloud free and has relatively calm winds and is associated with subsidence that, in the strongest storms, keeps the eye cloud-free.

The eyewall is the band of clouds surrounding the eye were the rain and winds are most intense.

Rain bands are the spiral arms of clouds of the hurricane as it pulls in tropical moisture.

Origin of hurricanes

Hurricane birthplaces are near the equator but between 5-15 degrees latitude away from the equator.  The paths tend to be easterly as the hurricanes develop and grow.

Tropical Wave Development

Tropical Disturbance
A collection of thunderstorms with a slight circulation.

Tropical Depression
Closed circulation
Wind speeds between 20-34 knots.

Tropical Storm
Closed circulation
Wind speeds between 35-64 knots.
The storm is first named at this stage.

Hurricane
Closed circulation
Sustained wind speeds in excess of 64 kts (~74 mph).

Saffir-Simpson Scale
A numerical scale (1-5) that describes the damage potential of a hurricane.
A quick and easy description of the strength of a hurricane.

Hurricane hazards

Strong Winds

Hurricane sustained wind speeds are in excess of 74 mph.
Occasionally can get as high as 155 kts.
Winds are typically stronger in Pacific storms -- storms have more time grow in the larger Pacific ocean.
Wind damage can be significant especially to weakly built houses.
Spin-up vortices can cause very strong and damaging wind gusts.


Hurricane winds are usually the strongest
on the right side of the storm.

The forward motion of the storm is added to the wind speeds on the right side of the storm to enhance the surface winds.

Inland Flooding

Torrential rains can, especially if the hurricane moves slowly inland, can cause substantial flooding.
E.g., Camille (1969)
Inland in Mississippi
Flooding in Virginia
E.g., Agnes (1972)
Inland in Florida
$6.3 Billion in damage along the East Coast
Flooding in Pennsylvania
E.g., Alberto (1994)
Never a hurricane!  (Tropical Storm)
Stalled over Georgia -- Americus, GA received 21" of rain!


Storm Surge
 

The abnormal rise in the ocean level associated with the hurricane landfall.
As the hurricane approaches the coast, it "pushes" a large mass of water in front of it.
When this pile of water reaches the coast, the water levels can rise as much as 7 meters (~22 feet).
Usually 80 - 160 km wide.
Not a "tidal wave" or tsunami.
The dramatic rise in the sea level can cause catastrophic damage.
The greatest storm surge is associated with:
Stronger hurricanes
High tide
Shape of the shore


So once a hurricane runs its course, how does it die?

Moves out of the warm, moist tropical air.
Moves over land.
Loss of moisture source
Increased surface friction
Temperature of the land is cooler than the warm ocean
Moved under unfavorable large scale flow.
High shear can rip a storm apart
Large scale subsidence can inhibit convection

A disaster in the making

45 million people now live along hurricane-prone regions in the U.S.
Gulf Coast population:
1960 -- 5.2 Million
1990 -- 10.1 Million
Doubled in only 30 years.
Florida to Virginia
1960 -- 4.4 Million
1990 -- 9.2 Million

So can we evacuate?

Hurricane evacuation times currently range from 15 - 30 hours depending on the locale.

Current warnings are only valid for 24 hours.

Can the transportation infrastructure handle such a mass exodus quickly enough?

Prediction Difficulties

We still have difficulty predicting the precise landfall of most hurricanes.

Some hurricanes can "loop."

What if we evacuate and the hurricane goes elsewhere -- the "Cry Wolf" problem.

False alarms are still high and many people become apathetic.
We see that here with tornado watches and warnings!