Tropical Storms & Hurricanes

Part 1

Lecture #16 (April 5, 2010, Monday)

August

1992

Tracks of rain accumulation

& clouds from Hurricane Rita

September

2005

Tropical Cyclones Around the Globe

―Hurricane―: N. Atlantic Ocean, NE Pacific Ocean, SE Pacific Ocean

―Typhoon―: NW Pacific Ocean

―Severe tropical cyclone―: SW Pacific Ocean, SE Indian Ocean

―Eevere cyclonic storm―: N Indian Ocean

―Tropical cyclone―: SW Indian Ocean

Why

none

here?

Why

none

here?

The relative sizes of the United States, Typhoon Tip and Cyclone Tracy

(the largest and one of the smallest tropical cyclones recorded,

respectively)

Global

satellite

image of

Typhoon

Tip near

peak

strength

Typhoon Tip

at its record

peak intensity

Category 5 on

October 12,

1979

Formed: October 4, 1979

Dissipated: October 19, 1979

Highest winds: 260 km/h (160 mph)

(10-minute sustained)

305 km/h (190 mph)

(1-minute sustained)

Lowest pressure: 870 hPa (mbar)

(Worldwide record low)

Fatalities: 86 direct, 13 indirect

Areas affected: Guam, Japan

Typhoon Tip

Storm path

Severe Tropical

Cyclone Tracy

Category 4

cyclone

(Australian

scale)

Category

3 cyclone

(SSHS)

Formed: 21 December 1974

Dissipated: 26 December 1974

Highest winds: 205 km/h

(125 mph)

(1-minute

sustained)

240 km/h

(150 mph)

(gusts)

Lowest pressure: 950 hPa (mbar)

Fatalities: 71

Damage: $1.1 billion (1974 USD)

$5 billion (2009 USD)

Areas affected: Tiwi Islands,

Darwin,

Northern

Territory

Severe Tropical

Cyclone Tracy

Storm path

Devastation brought by Cyclone Tracy

Cyclone Catarina, a rare South Atlantic tropical cyclone, viewed from

the International Space Station in March 2004

Catarina, a

category

2 tropical

cyclone, was

approaching

the Brazilian

coastline on

March 27, 2004

near peak

intensity.

Cyclone Catarina

at landfall

Formed: March 24, 2004

Dissipated: March 28, 2004

Highest winds: 155 km/h (100 mph)

(1-minute sustained)

Lowest pressure: 972 hPa (mbar)

(Worldwide record low)

Fatalities: 3-10 direct, 13 indirect

Damage: $350 million (2004 USD)

$399 million (2009 USD)

Areas affected: Santa Catarina and Rio

Grande do Sul, Brazil

Cyclone Catarina

Storm path

Structure of a tropical cyclone

A cross section diagram of a mature tropical cyclone,

with arrows indicating air flow in and around the eye

Hurricane Structure

Eye

wall

Northern

Hemisphere

L

H

A cross section of a typical hurricane

Tropical cyclones form when the energy released by

the condensation of moisture in rising air causes a

positive feedback loop over warm ocean waters.

A cross section of a typical hurricane

?

Katrina's Category 4 hurricane force winds were observed

by NASA’s QuikSCAT satellite on August 29, 2005.

1 knot = 1 .1508 mph = 1.852 kmph

Temperature distribution across a hurricane

Why the air

temperature is

much higher in

the eye?

(1) Latent heat is

released from

condensation in

the formation of

eye wall clouds.

(2)Sinking air in

the eye is

compressed and

warmed

adiabatically.

Eye: center area, descending air, light winds, average diameter of about 25 km (15 mi). A shrinking eye indicates intensification.

Eye wall: strongest winds and convergent uplifting, largest cumulonimbus clouds, heaviest precipitation as much as 2500 mm (100 inch) per day.

Weak uplift or even sinking air and low precipitation regions separate individual spiral cloud bands.

Lower portions: counter-clockwise rotation, pressure differences into the center of the storm are about twice as great as the average mid-latitude cyclones, plus smaller size, thus much greater pressure-gradient force resulting in strong sustained winds

Upper portions: clockwise rotation, the storm are also blanketed by a cirrus cloud cap due to overall low temperatures.

Hurricane Characteristics

Unlike mid-latitude cyclones, hurricanes are warm-core lows due to warm ocean surfaces.

As air converges into the surface low pressure center, adiabatic cooling due to expansion keeps the horizontal temperature differences moderate towards the eye.

Horizontal pressure gradient decreases slowly with altitude in the eye due to pressure decreases more slowly with increasing altitude in the warm core.

At about 400 mb (7.5 km), pressures within the storm are approximate the same as outside pressure.

From 400 mb to the tropopause, pressures within the storm exceed those outside upper portion of the storm rotates anticyclonically while lower portions rotate cyclonically.

Hurricane Characteristics

Typically, eyes are easy to spot using weather radar. This radar image

of Hurricane Andrew clearly shows the eye over southern Florida.

Radar Image of Hurricane Katrina as it approached LA coast

Eye

Eye wall

A satellite photo of Typhoon Amber of the 1997 Pacific typhoon

season, exhibiting an outer and inner eyewall while undergoing

an eyewall replacement cycle.

A picture of Hurricane Wilma's eye taken at 08:22 CDT (13:22 UTC)

October 19, 2005, by the crew aboard the International Space

Station. Wilma was at peak intensity at the time, with a minimum

central pressure of only 882 mbar (26.06 inHg), making it the

strongest Atlantic hurricane in history. Not only is this a classic

example of a pinhole eye, but also of the stadium effect, where the

eyewall slopes out and up.

Most powerful of all storms, energy unleashed by a single hurricane can exceed the annual electrical consumption of US & Canada. Most energy attained by hurricanes stems from latent heat release in the cloud formation process.

Sustained winds > 120 kmph (74 mph).

Generally of lesser intensity than tornadoes, but much larger in size and longer in life span makes hurricanes much more devastating.

Average diameters are approximately 600 km (350 mi), typically ~ 1/3 of midlatitude cyclones, but no fronts.

Central pressure averages about 950 mb & may be as low as 870 mb.

Hurricanes occur during the times of highest SSTs.

For the Northern Hemisphere, August and September are typically the most active months with highest SSTs.

Hurricane Characteristics

Extratropical storms are areas of low pressure which

exist at the boundary of different air masses. Almost all

storms found at mid-latitudes are extratropical in nature,

including classic North American nor'easters and

European windstorms. The most severe of these can have

a clear "eye" at the site of lowest barometric pressure,

though it is usually surrounded by lower, non-convective

clouds and is found near the back end of the storm.

Subtropical storms are cyclones which have some

extratropical characteristics and some tropical

characteristics. As such, they may have an eye, but are

not true tropical storms. Subtropical storms can be very

hazardous, with high winds and seas, and often evolve

into true tropical storms. As such, the National Hurricane

Center began including subtropical storms in their

naming scheme in 2002.

The North

American

blizzard

of 2006, an

extratropical

storm, showed

an eye-like

structure at its

peak intensity

(here seen just

to the east of

the Delmarva

Peninsula).

Extraterrestrial storms

A hurricane-like storm on the south pole of Saturn displaying an

eyewall tens of kilometers high

Tropical cyclogenesis is the technical term describing the

development and strengthening of a tropical cyclone in

the atmosphere.

The mechanisms through which tropical cyclogenesis

occurs are distinctly different from those through which

mid-latitude cyclogenesis occurs.

Tropical cyclogenesis involves the development of a

warm-core cyclone, due to significant convection in a

favorable atmospheric environment.

An average of 86 tropical cyclones of tropical storm

intensity form annually worldwide, with 47 reaching

hurricane/typhoon strength, and 20 becoming intense

tropical cyclones (at least Category 3 intensity on the

Saffir-Simpson Hurricane Scale or SSHS).

Requirements for tropical cyclone formation

1. Sufficiently warm sea surface temperatures

2. Atmospheric instability

3. High humidity in the lower to middle levels of the

troposphere

4. Enough Coriolis force to develop a low pressure

center

5. A pre-existing low level focus or disturbance

6. Low vertical wind shear.

These conditions are necessary for tropical cyclone

formation, but they do not guarantee that a tropical

cyclone will form.

SST ―

The Tropical Setting

The Tropical Setting

Depth of 26 °C isotherm on October 1, 2006

Normally, an ocean temperature of 26.5°C (79.7°F)

spanning through at least a 50-m depth is considered the

minimum to maintain the special mesocyclone that is the

tropical cyclone.

Hurricane Katrina, August 25-27, 2005. A hurricane needs

SSTs at 81-82 oF (~ 27-28 oC) or warmer to strengthen

(NHC).

On rare

occasions,

tropical

storms may

form or

strengthen in

mid-latitudes.

Hurricane Vince

formed in the

temperate

subtropics

during the 2005

Atlantic season

Subtropical High

(Bermuda-Azores High)

The Tropical Setting

Warm waterCold water

Trade wind inversion

An easterly wave

Hurricane Formation

Tropical disturbances: often begin in eastern ocean basins as

disorganized clusters of thunderstorms with weak pressure gradients.

Some form when mid-latitude troughs migrating into the tropics.

Some form as part of convection associated with ITCZ.

An

easterly

wave

Hurricane Formation

Most tropical disturbances entering western Atlantic and becoming

hurricanes originate in easterly waves, which are large undulations or

ripples in normal trade wind pattern

These waves typically stretch between 2000–3000 km

(1200–1800 mi)

Waves in the trade winds in the Atlantic Ocean—areas of converging

winds that move along the same track as the prevailing wind—create

instabilities in the atmosphere that may lead to the formation of

hurricanes.

The African easterly jet is a region of the lower troposphere over West Africa where the

seasonal mean wind speed is maximum and easterly. The jet develops because heating

of the West African land mass during the Northern Hemisphere summer creates a

surface temperature and moisture gradient from the Gulf of Guinea and the Sahara,

and the atmosphere responds by generating vertical wind shear to maintain thermal

wind balance. During the mature phase of the West African Monsoon (August to

September) maximum mean wind speeds in the jet of approximately 13 m/s are located

around 4°N—5°N at a height of 4 km (or 650 mb). The jet produces synoptic scale,

westward propagating disturbances known as African easterly waves ( tropical waves).

A small number of mesoscale storm systems embedded in these waves develop into

tropical cyclones after they move from west Africa into the tropical Atlantic.

Schematic

representation

of flow around

a low-pressure

area (in this

case, Hurricane

Isabel) in the

Northern

hemisphere.

The pressure

gradient force

is represented

by blue arrows,

the Coriolis

acceleration

(always

perpendicular

to the velocity)

by red arrows

A minimum distance of 500 km (300 miles) from the

equator is normally needed for tropical

cyclogenesis. The role of the Coriolis force is to

provide for gradient wind balance (curved isobars).

Infrared image of a powerful southern hemisphere cyclone, Monica,

near peak intensity, showing clockwise rotation due to the Coriolis

effect

Most hurricanes form between the latitudes of 5° and 20°

in tropical oceans, except over the South Atlantic and

Southeastern Pacific. Why?

Hurricanes form only over deep warm surface ocean layers with surface temperatures in excess of 27-28 oC (81-82 oF).

Energy is derived from evaporation of surface water and then latent heat is released when clouds are formed.

Poleward of about 20o, water temperatures are usually below this threshold.

Hurricanes are most frequent in late summer and early autumn during highest SSTs.

Coriolis force is an important factor for rotation to form, as such, hurricanes do not form equatorward below 5o latitude.

An unstable atmosphere is also necessary and this typically occurs toward the central to western ocean basins as trade wind inversions and cool ocean surfaces dominate over eastern ocean basins.

Strong vertical shear must be absent for hurricane formation because it disturbs supply of moisture to the hurricanes.

Conditions Necessary for Hurricane Formation