3.2 Tropical Cyclone Formation Requirements

There are six necessary conditions for TC development. The following is a list of these parameters (Bergeron, 1954; Riehl, 1979).
a. A Threshold Value of Earth's Vorticity - It has been observed that TCs do not form within 3 degrees latitude from the equator. The value of Coriolis parameter, the vertical component of the earth's angular speed (or the earth's vorticity), vanishes at the equator and increases to extremes at the north and south poles. Apparently a threshold value of Coriolis parameter must exist for a TC to form.
b. Low-Level Relative Vorticity - Tropical cloud clusters or ensembles, which develop into tropical depressions, are situated in regions of low-level positive vorticity (i.e. cyclonic rotation).
c. Vertical Wind Shear - Strong vertical wind shear prevents the formation of TCs.
d. SST and Mixed Layer Depth - Numerous studies have used exact numbers as a minimum SST criterion for development. Typically these are in the range of 26-28C. The warm ocean water must exist over a sufficient depth (i.e., 200 feet).
e. Potentially Unstable Atmosphere - The troposphere must be potentially unstable to sustain convection for an extended period of time.
f. Tropospheric Humidity - A necessary value of relative humidity is typically 50-60% at the lower to middle troposphere.

Section 3.1 Section 3.3

Chapter 2