Fluid Physics

Tropical Cyclogenesis

(Tropical cyclone formation
and intensification)

A tropical cyclone (TC) is a large rotating self-sustaining warm-water high-energy storm that when reaching proximity to land, creates new coastal bathymetry and topography (optimizing coastal hydrology).

Figure 1: Typhoon Neoguri, 8 July 2014. [NASA]

Tropical cyclones may be referred to as typhoons, hurricanes, tropical storms, or cyclones.

“Severe tropical cyclones have local names. They are called hurricanes in the Atlantic and eastern North Pacific Oceans, typhoons in the western North Pacific Ocean, and cyclones in the South Pacific and Indian Oceans. They are all the same phenomenon.”

—

Robert A. Houze Jr., “Clouds in Tropical Cyclones”, Monthly Weather Review, Vol. 138, Feb. 2010

Tropical cyclones are formed over warm tropical waters, drawing moisture as evaporation (latent heat) in a vertical vortex cylinder that spreads it back out to precipitate and cycle through the TC again.

Figure 2: Section view of a tropical cyclone (TC). [NOAA]

The formation and intensification of tropical cyclones is called tropical cyclogenesis. While conditions required for tropical cyclogenesis are known, the precise mechanisms are not fully understood, leading to substantial new research now underway as anthropogenic climate change increases TC damage to underdeveloped civil infrastructure.

Conditions

Tropical cyclone (TC) formation, and TC intensification, require the following conditions:

Warm ocean water surface.
Moisture at mid-atmosphere.
Not too close to the Equator.
Weak vertical wind shear.
Atmospheric instability suitable for convection.

The sea surface temperature (SST) must be at least 26°C (79°F). That is warmer than “room temperature” (which is 20°C to 22°C). The upper limit of TC intensity depends on the SST.

There must be moist air near the mid-level of the troposphere. If dry air blows into a TC, the storm will weaken.

TCs need to form away from the Equator, to be able to spin enough to create a vortex. As explained in the Storm Rotation page of this article (link below), overall TC rotation is a result of the Coriolis force, which is minimal near the Equator.

Vertical flow in the troposphere must not be interrupted by strong wind shear. For example, strong easterly Trade Winds shearing with strong westerly El Niño will usually break the upward air flow necessary to form a TC in the Caribbean Sea (more hurricanes form there when there is no El Niño).

Atmospheric instability is also required, to support convection. A stable atmosphere does not have enough temperature gradient to support upward moist air flow, because the temperature differences are not enough to overcome thinning of air as altitude increases. With atmospheric instability, the temperature differences are enough to support bouyancy of moisture. Atmospheric instability may be caused by regional turbulence.

Figure 3: Convergrence and divergence of the Trade Winds north of South America provide atmospheric instability suitable for TC formation. [NASA]

Figure 4: Hypothetical atmospheric instability (vertical section drawing of horizontal layers). [Wiki]

Ocean Upwelling

Sometimes, when regional weather prevents a TC from drifting much, the TC will continue cooling the surface water in the same place, bringing up cooler deeper water (called ocean upwelling), weakening the storm. The weakening of the storm allows warm water to partially fill back in under the TC to resume storm intensification.

Ocean upwelling occurs out at sea, and contributes to overall warming of the oceans.