ManilaTC StormBasics: Storm Formation
What are the Conditions for a Tropical Cyclone to form.
There are six main requirements for tropical cyclogenesis: Sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to sustain a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.
While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.
Please bear with us, some of the information presented will be TECHNICAL in nature. We will try to do our best to explain!
While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form.
Please bear with us, some of the information presented will be TECHNICAL in nature. We will try to do our best to explain!
... the surface of the ocean must be warm
Warm ocean waters (of at least 26.5°C [80°F]) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
A typhoon has been known to increase in strength rather quickly, a phenomenon known as Rapid Intensification, once the storm goes over warm waters and has no wind shear affecting it... It can go from a 130kph typhoon to a 200kph monster within 24 hours! |
... there must be a forming circulation in the Low Levels
A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
Whether it be a depression in the intertropical covergence zone (ITCZ), a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. This feature is known to be a Low Level Circulation Center. Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge. |
... atmospheric instability or Humidity
An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
Relatively moist layers near the mid-troposphere (5 km [3 mi]). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity. |
... an Outflow or exhaust point must be established
What comes in must come out they say. Same goes in a tropical cyclone.
The winds blow in cyclonically in the lower level of a typhoon and then burst out in the opposite direction in the upper levels of the typhoon... thus providing exhaust... It also is termed as Divergence. High divergence levels in the upper reaches of the atmosphere show that the storm is properly stacked upright like a pancake! Once this occurs, intensification is much possible. |
... that the developing storm must be 5 deg from the equator
A minimum distance of at least 500 km [300 mi] from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
The Coriolis force imparts rotation on the flow and arises as winds begin to flow in toward the lower pressure created by the pre-existing disturbance. In areas with a very small or non-existent Coriolis force (e.g. near the Equator), the only significant atmospheric forces in play are the pressure gradient force (the pressure difference that causes winds to blow from high to low pressure and a smaller friction force; these two alone would not cause the large-scale rotation required for tropical cyclogenesis. The existence of a significant Coriolis force allows the developing vortex to achieve gradient wind balance. This is a balance condition found in mature tropical cyclones that allows latent heat to concentrate near the storm core; this results in the maintenance or intensification of the vortex if other development factors are neutral. There have been cases though over the last three years that storms have formed within 500km of the equator, most recently Super Typhoon Bopha (Pablo) |
... wind shear must be below 20-30 kph
Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.
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Sources:
NOAA AOML Hurricane Research Division FAQ
English version Wikipedia - Tropical Cyclones portal
Sky-Chaser.com of Chris Collura
NOAA OSEI.
NOAA AOML Hurricane Research Division FAQ
English version Wikipedia - Tropical Cyclones portal
Sky-Chaser.com of Chris Collura
NOAA OSEI.