Download What is a tropical cyclone?

Tropical
Meteorology I
Weather Center Event #4
Tropical Meteorology
 What
–
is Tropical Meteorology?
The study of cyclones that occur in
the tropics.
What is a tropical cyclone?

A cyclone has low sea-level pressure and
cyclonic movement around the center.

A tropical cyclone is different than a midlatitude cyclone, since the heat source is the
warm oceans found in the tropics.

They form over warm ocean waters.
Mid-latitude cyclone north of North Dakota
Hurricane
Gloria
How does a tropical cyclone form?

Favorable conditions must be in place.

Spontaneous tropical cyclogenesis really does
not occur.
What types of conditions do you think would
support the formation of a tropical cyclone?
 There are six conditions that are important. 
Favorable conditions
1.
2.
3.
4.
5.
6.
Favorable ocean conditions
A deep mixed layer
Proximity to the equator
A pre-existing disturbance
Moist conditions in the mid-troposphere
A favorable wind shear pattern
Favorable ocean conditions

Ocean temperatures must be warm,
generally 26°C or 79°F, or so.

The depth of the ocean must also be
sufficient: generally >50m for development.

 A deep mixed layer. 
More on a Deep Mixed Layer

Mixed layer: a layer (of the ocean from the
surface down) that has fairly uniform
temperatures.

This layer should be deep, not shallow.

High winds will mix up the ocean, and in the
case of a tropical cyclone, will work to drive
cooler waters up towards the surface.
Deep mixed layer, continued

A deep mixed layer will not allow much
cooler water to make it to the surface.

Cool sea-surface temperatures are not
favorable tropical cyclones, so this is why a
deep mixed layer is important.
Proximity to the equator (NH)

Planetary vorticity (coriolis) drives the
circulation of a tropical cyclone.

At the equator, the coriolis effect is zero and
convection here may not result in rotation.

Although tropical cyclogenesis is favored in
the tropics, tropical cyclones generally form
at least a few degrees north of 0°.
Pre-existing disturbance

This disturbance or convection will work to
initialize tropical cyclogenesis.

Vorticity and convergence are required to
develop a cyclone.

Convection is a source for latent heat, which
works to drive the intensification.

A concentrated area is also important.
A moist mid-troposphere

Without this moisture, evaporation will occur,
leading to cooling and less convection.

Warm, moist air rises and leads to further
cloud development.
 Think of radar…dry slots can work to eat
away at a shield of precip…dry slots or air
aloft can eat away at a tropical cyclone. 
A favorable wind shear pattern

Similarly to dry air eating away at a tropical
cyclone, wind shear can tear it apart.

Wind shear is the change of direction of
winds with increasing height.

With weak wind shear, latent heat can
remain in one area and convection is able to
concentrate itself and further intensify.
Tropical cyclone classification
1.
2.
3.
4.
Tropical disturbance.
Tropical Depression.
Tropical Storm.
Hurricane.
 How are each of the classifications defined?
Tropical disturbance

An area of clouds and thunderstorms, often
referred to as a tropical wave.

A tropical disturbance is not terribly
organized, with a broad area low pressure
and modest winds.

Circulation is likely not clearly defined.
Tropical Depression

A more organized area of clouds and
thunderstorms.

Has a closed surface circulation, with a fairly
well defined area of low pressure.

Sustained winds are up to, but not greater
than 38mph.
Tropical Storm

A Tropical Storm if very well organized.

An area of low pressure is clearly defined.

Winds are stronger and are centralized
around the center of low pressure.

Sustained winds are between 39 and 73mph.
Hurricane

An organized, self-sustaining cyclone.

A central area of low pressure, that can
create a very sharp pressure gradient.

An eye may develop, in which the strongest
winds surround.

The sustained winds are 74 or more mph.
Tropical cyclone structure

A tropical cyclone is a warm-core low.

This warm core causes the thickness from the
surface on up to be greater.

The storm system is vertically stacked, but
high pressure occurs aloft.

The high causes air to diverge above the
storm, which can be seen as anti-cyclonic flow
on infrared satellite imagery.
Vertical structure
Consider the vertical structure of the mid-latitude
low
and
Hurricane
Gloria
Extra-tropical vertical structure
Tropical cyclone movement

Steering winds, generally caused by the
Earth’s rotation, move the tropical cyclones.

A subtropical ridge of high pressure is
generally present in the North Atlantic Ocean
during the hurricane season. (Bermuda high)

With storms forming on the equatorial side of
the high, they tend to move from east to west.
Steering winds
Steering winds continued

Other areas of low and high pressure also
affect the track of tropical cyclones.

If a trough of low pressure approaches from
the west, it will often work to curve the
system up to the north and then northeast.

Other areas of high pressure or local wind
flows can also work to steer a storm.
Forward speed

The forward speed of tropical cyclones is
generally fairly slow.

With the subtropical high and usual lack of a
tropical jet, there is no strong driving force.

Once tropical cyclones interact with shortwaves disturbances and other troughs, they
do tend to pick up forward speed.
Tropical cyclone dissipation


Tropical cyclones generally don’t last too
long…days and occasionally a week or two.
Since many conditions must be in place for
initial development, if any of those conditions
are no longer met, dissipation occurs.
What factors might lead to dissipation?
Tropical cyclone dissipation





Interaction with land.
Moving into cooler waters.
Remaining stationary for a long period.
Dry air interaction.
Wind shearing.
Land interaction

Although surface roughness (friction) plays a
role, it is not the main cause for dissipation.

Warm waters fuel tropical cyclones and land
masses are dry…limited latent heat source.

Moving into a mountain range or other higher
terrain can work to weaken a system faster.

Shallow water or brief land interactions
generally do not cause significant dissipation.
Cooler waters

As a cyclone moves over cooler waters, the
fuel for the warm core of the system is gone.

Cooler waters can be due to latitude, ocean
current or interaction with a previous storm.

The cooler the water, the more of a
weakening effect it will have on the cyclone.
Sea-surface temperature example
Remaining stationary

If a tropical cyclone remains stationary, it
begins to thoroughly mix up the ocean.

This mixing will eventually bring up cooler
water temperatures to the surface.

Also, remaining stationary will rob the
atmosphere of moisture and energy,
especially if the outflow is great enough.
Dry air interaction

Dry air works to dissipate a tropical cyclone,
since the warm, moist air keeps it “alive”.

Dry air may be filtered into the system, or the
storm may move into a very dry air mass.
Wind shearing

Wind winds begin to change direction with
increasing height, the storm gets torn apart.

Wind shear may also be due to velocity.

If an area of strong horizontal wind speed
works into a storm (or vice virsa), this will
also work to disorganize the cyclone.