Decaying Stage

A tropical cyclone decays for several reasons:

(a) it moves inland, where the cyclone's inflow is no longer made humid by the surface, and surface friction is greater than over the sea,

(b) it moves to an area of the sea which is too cool,

(c) the TC remains stationary, steadily cooling the sea beneath, until it is too cold to supply the required heat,

(d) it shifts to higher latitudes, where the ground's vorticity is greater (Note 12.K), reducing the cyclone's spin relative to the ground,

(e) the upper divergence (which sucks the central core upwards) becomes detached by the tug of upper winds, allowing upper westerlies to encroach into the system.

Attempts have been made to use cloud seeding to promote the decay of tropical cyclones which threaten coastal areas (Note 9.E). For instance,

Figure 13.14 The track of tropical cyclone Oscar in February 1983, showing the areas near Fiji swept by storm winds, gale-force winds and hurricane winds, respectively.

Figure 13.14 The track of tropical cyclone Oscar in February 1983, showing the areas near Fiji swept by storm winds, gale-force winds and hurricane winds, respectively.

seeding outside the radius of strong winds might encourage cloud growth and deep convection there. The latent heat thus released at the periphery of the tropical cyclone would lessen that in the eyewall, and hence weaken its updraught. Unfortunately, this does not seem to work.

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