Tropical Meteorology I Weather Center Event #4 Tropical Meteorology What is Tropical Meteorology? – The study of cyclones that occur in the tropics.

Tropical

Meteorology IWeather 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 mid-latitude 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. Favorable ocean conditions

2. A deep mixed layer

3. Proximity to the equator

4. A pre-existing disturbance

5. Moist conditions in the mid-troposphere

6. 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. Tropical disturbance.

2. Tropical Depression.

3. Tropical Storm.

4. 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 short-waves 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.