How tropical cyclones form and other terms and information

A tropical cyclone is a meteorological term for a storm system characterized by a low pressure center and thunderstorms that produces strong wind and flooding rain. A tropical cyclone feeds on the heat released when moist air rises and the water vapor it contains condenses. They are fueled by a different heat mechanism than other cyclonic windstorms such as nor’easters, European windstorms, and polar lows, leading to their classification as “warm core” storm systems.

Tropical cyclogenesis- Any development or strengthening of a low level cyclonic circulation in the atmosphere.

Conditions for tropical cyclogenesis:
1. Warm-water: 80 degrees Fahrenheit plus
2. Low shear 0-10kts in some rare cases near 25kts.
3. Organized Convection- thunderstorm activity
4. Low level circulation- Coriolis Effect; which causes the rotation of the system.

All of these conditions must be met for a tropical storm to form.

Unfavorable conditions:
1. Water below 80 degrees
2. High shear
3. Little to no convection
4. No evident low level circulation
5. Proximity to land
6. Dry Air

Key Terms-
Cyclogenesis- any development or strengthening of a tropical system.

Convection- Thunder Storm activity. (The transfer of heat or other atmospheric properties by massive motion within the atmosphere, especially by such motion directed upward.)

Low pressure- an area where the atmospheric pressure is lowest interrelation to the surrounding area.

Mid level low- a low pressure in the mid levels of the atmosphere

Upper level low- a low pressure system in the upper levels of the atmosphere.

High Pressure- an area in which the pressure is greater than the surrounding area. Often steer tropical systems depending on their location.
Bermuda High-The semi-permanent subtropical high of the North Atlantic Ocean, especially when it is located in the western part of that ocean area. Often the determining factor for an active season in the lower 48s (U.S.A)

Dry air- an area where there is little moisture and therefore inhibits tropical cyclone formation.

Shear- winds in the upper levels of the atmosphere that is able to blow the tops (convection) of tropical systems. Imagine coffee with cream over the top and as your blowing over the cream you see the coffee. The coffee represents the ocean, the cream represents the cloud tops, and blowing represents the upper level winds. Due to this factor high shear is unfavorable for tropical cyclone formation.

Trough- an elongated area of low pressure which can often steer tropical systems. If you look back to the 2009 season you remember tropical system starting out as waves from Africa developing but then curving out to sea. Trough can carry tropical systems out to sea and therefore deflect them from certain area.

Front- the dividing line between two air masses. Front can steer tropical systems as well. If you remember Hurricane Wilma we note that as it entered the Gulf of Mexico (GOM) it turned towards Florida. The front approached came into Florida and basically picked up the system and steered it along the frontal boundary.

Saharan Air Layer dust (SAL) - an intensely dry, warm and sometimes dust-laden layer of the atmosphere which often overlays the cooler, more-humid surface air of the Atlantic Ocean.

Inter-tropical convergence Zone (ITCZ)-is a belt of low pressure girdling Earth at the equator. It is formed by the vertical ascent of warm, moist air from the latitudes above and below the equator.

Eye-wall-the most severe weather of a cyclone occurs. The cyclone’s lowest barometric pressure occurs in the eye, and can be as much as 15% lower than the atmospheric pressure outside the storm.
eye-a region of mostly calm weather found at the center of strong tropical cyclones

Eyewall replacement cycle- also called concentric eye-wall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h (115 mph), or major hurricanes (Category 3 or above). When tropical cyclones reach this threshold of intensity and the eyewall contracts or is already sufficiently small, some of the outer rain bands may strengthen and organize into a ring of thunderstorms an outer eyewall that slowly moves inward and robs the inner eyewall of its needed moisture and angular momentum.

Rapid Intensification-is a condition that occurs when the minimum sea-level atmospheric pressure of a tropical cyclone decreases drastically in a short period of time. The National Weather Service describes rapid deepening as a decrease of 42 millibars in less than 24 hours. [1] However, this phrase is liberally applied to most storms undergoing rapid intensification.

Necessary conditions-In order for rapid deepening to occur, several conditions must be in place. Water temperatures must be extremely warm (near or above 30C, 86F), and water of this temperature must be sufficiently deep such that waves do not churn deeper cooler waters up to the surface. Wind shear must be low; when wind shear is high, the convection and circulation in the cyclone will be disrupted. Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well for extremely low surface pressures to develop, air must be rising very rapidly in the eyewall of the storm, and an upper-level anticyclone helps channel this air away from the cyclone efficiently.

Projected path- shows where a tropical system is going. This is based on computer model analysis as well as the current motion of the tropical cyclone.

Upper Level Outflow- clouds that form outward from a tropical cyclone. The outflow can indicate the health of the system.

Diffluence-(common recent term used by NHC) lowing of all sides; fluidity

Orography-(common recent term used by NHC)is the average height of land, measured in geopotential meters, over a certain domain.

Anticyclone-An extensive system of winds spiraling outward from a high-pressure center, circling clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Upwelling-A process in which cold, often nutrient-rich waters from the ocean depths rise to the surface.

Upper air dynamics- The effect of the upper level environment in relation to storms at the surface.

El Niño-A warming of the ocean surface off the western coast of South America that occurs every 4 to 12 years when upwelling of cold, nutrient-rich water does not occur. It causes die-offs of plankton and fish and affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world. El Niño causes more active season in the pacific and a less active season in the Atlantic.

La Nina-A cooling of the ocean surface off the western coast of South America, occurring periodically every 4 to 12 years and affecting Pacific and other weather patterns. This situation cause a more active season in the Atlantic Basin and a quieter season in the Pacific. La Nina is expected for the 2010 Atlantic Hurricane Season weak to moderate. A La Niña can follow after an El Nino which is happened between the 2009 Hurricane season (El Nino) to the 2010 hurricane season (la nina).

MJO: The Madden-Julian Oscillation (MJO) is an equatorial traveling pattern of anomalous rainfall that is planetary in scale.

The MJO is characterized by an eastward progression of large regions of both enhanced and suppressed tropical rainfall, observed mainly over the Indian Ocean and Pacific Ocean. The anomalous rainfall is usually first evident over the western Indian Ocean, and remains evident as it propagates over the very warm ocean waters of the western and central tropical Pacific. This pattern of tropical rainfall then generally becomes very nondescript as it moves over the cooler ocean waters of the eastern Pacific but reappears over the tropical Atlantic and Indian Ocean. The wet phase of enhanced convection and precipitation is followed by a dry phase where convection is suppressed. Each cycle lasts approximately 30-60 days.

The MJO is also known as the 30-60 day oscillation, 30-60 day wave, or intrseasonal oscillation.

Cape Verde Season- the time where, climatologically waves of Africa start developing.

Upper level divergence-Upper level divergence occurs when a mass of air is pulled away from a region faster than that mass can be replaced. This most commonly occurs when the upper level wind field is strong and meridional (high amplitude upper level waves).

Low level convergence- can result from fronts, low pressure frictional convergence, WAA, moisture advection, orographic convergence, and mesoscale convergence boundaries (sea breeze, temperature discontinuities).
An important ingredient that goes along with low level convergence is low level moisture and RH. Since convergence causes rising air, the RH will increase therefore causing the air to come closer to saturation as it rises

Tropical wave- A tropical wave is normally preceded by an area of sinking, intensely dry air, oriented as a northeast wind. With the passage of the trough line, the wind veers to the southeast, the humidity abruptly rises, and the atmosphere destabilizes, producing widespread showers and thunderstorms, occasionally severe. As the wave moves off westward, the showers gradually diminish. Simply a broad area of low pressure associated with heavy convection. They often times come off the coast of Africa and can form into tropical systems. Steering currents (High Pressure, Fronts-Troughs, upper level lows), determine where the storm goes and how fast it goes. If the steering currents are week then the system moves slowly.

Tropical Disturbance- An area of low pressure associated with convective activity.

Invest- An area of disturbed weather in which meteorologist at the NHC are interested in and may want to see more model runs of the disturbance.

Tropical Depression- a warm- core low pressure system with a defined surface circulation that has organized with winds less than 38 mph.

Tropical storm- a warm-core low pressure system with a defined surface circulation with winds greater than 38 mph and less than 75 mph

Hurricane- a warm-core low pressure system with a defined surface circulation with winds greater than 75 mph. Commonly have a broad outflow and a well defined surface rotation. These systems have an eye which is the center of circulation. The eyewall reflects the strongest winds and heaviest rains. A major Hurricane (category 3 on the Saffir Simpson Scale) has the potential to destroy homes, buildings, etc.

Why do systems weaken over land?
Simply because Hurricanes Fuel on warm water (80 degrees plus).With out warm water they cannot survive.

What are the best computer models to use in the tropics?
The best hurricane forecasting models we have are “global” models that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called “dynamical” models. The four best hurricane forecast models–ECMWF, GFS, UKMET, and CMC–are all global dynamical models.