On the Growth of the Hurricane Depression

Why do cyclones form in a conditionally unstable tropical atmosphere whose vertical thermal structure is apparently more favorable to small-scale cumulus convection than to convective circulations of tropical cyclone scale? It is proposed that the cyclone develops by a kind of secondary instability in which existing cumulus convection is augmented in regions of low-level horizontal convergence and quenched in regions of low-level divergence. The cumulus- and cyclone-scale motions are thus to be regarded as cooperating rather than as competing–the clouds supplying latent heat energy to the cyclone, and the cyclone supplying the fuel, in the form of moisture, to the clouds. A scale-analysis indicates that it is appropriate to use the balance equations of Eliassen for the macro-motion; in this case the effect of friction in the boundary-layer may be incorporated as a condition on the vertical velocity at the top of the boundary layer. It is argued that the mean humidity in a system of convecting cumulus clouds in statistical equilibrium with the cyclone-scale circulation is appreciably less than its saturation value. The atmosphere is then gravitationally stable for the macro-scale convective process even though it is gravitationally unstable for the micro-scale convective process. The amplification of the disturbance is due to the surface frictionally induced convergence of moisture and liberation of latent heat in the center of the cyclone.