Abstract
AbstractTropical cyclone formation and evolution at, or near, the Equator is explored using idealized three‐dimensional model simulations, starting from a prescribed, initial, weak counterclockwise rotating vortex in an otherwise quiescent, nonrotating environment. Three simulations are carried out in which the maximum tangential wind speed (5 m s) is specified at an initial radius of 50, 100, or 150 km. After a period of gestation lasting between 30 and 60 hr, the vortices intensify rapidly, the evolution being similar to that for vortices away from the Equator. In particular, the larger the initial vortex size, the longer the gestation period, the larger the maximum intensity attained, and the longer the vortex lifetime. Beyond a few days, the vortices decay as the cyclonic vorticity source provided by the initial vortex is depleted and negative vorticity surrounding the vortex core is drawn inwards by the convectively driven overturning circulation. In these negative vorticity regions, the flow is inertially/centrifugally unstable. The vortex evolution during the mature and decay phases differs from that in simulations away from the Equator, where inertially unstable regions are much more limited in area. Vortex decay in the simulations appears to be related intimately to the development of inertial instability, which is accompanied by an outward‐propagating band of deep convection. The degree to which this band of deep convection is realistic is unknown.
Highlights
The majority of tropical cyclones form at latitudes beyond 5◦ from the Equator, where it is assumed the Earth's background rotation is sufficient to support vortex spin-up (e.g., Anthes, 1982, p. 48; Gill, 1982, p. 476; Houze, 1993, p. 406; Pielke and Pielke, 1997, p. 82; Wallace and Hobbs, 2006, p. 369)
We provide a broadscale overview of metrics describing vortex evolution in the three idealized numerical simulations with zero Coriolis parameter, comparing them with the three simulations of KS17 in which the Coriolis parameter is that for 10◦N
The left panels show the evolution of the three idealized numerical simulations with zero Coriolis parameter, while for comparison the right panels show the evolution of the three idealized numerical simulations at 10◦N taken from KS17
Summary
The majority of tropical cyclones form at latitudes beyond 5◦ from the Equator, where it is assumed the Earth's background rotation is sufficient to support vortex spin-up (e.g., Anthes, 1982, p. 48; Gill, 1982, p. 476; Houze, 1993, p. 406; Pielke and Pielke, 1997, p. 82; Wallace and Hobbs, 2006, p. 369). This storm made landfall two days later along the southeastern coastline of the Malaysian Peninsula at 1.6◦N, about 50 km north of Singapore (Chang et al, 2003; Chang and Wong, 2008) Another noteworthy entry in the IBTrACS dataset is Typhoon Agnes (1984), which formed essentially on the Equator, at 0.1◦N, 148.5◦E. Agnes intensified as it tracked to the northwest, attaining a maximum wind speed of 52.5 m s−1 (189 km hr−1) at a latitude of 11.2◦N. It made landfall at near-peak intensity on Samar Island in the Philippines, and later reintensified over the South China Sea before making landfall for a second time over Vietnam. Most of the loss of life and damage due to Agnes occurred in the Philippines
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