Ab

Figure 4. Schematic diagram showing the strength of the forcings on a tropical disturbance. The abscissa is the state of the system, Si denotes the cloud cluster state and S3 the TC. Curve A is the adiabatic heating and line B is the cooing due to adiabatic upward motion in the core region. See text for more details.

Figure 4. Schematic diagram showing the strength of the forcings on a tropical disturbance. The abscissa is the state of the system, Si denotes the cloud cluster state and S3 the TC. Curve A is the adiabatic heating and line B is the cooing due to adiabatic upward motion in the core region. See text for more details.

SST increases), curve A moves upward and S1 and S2 merge and then both disappear, i.e. the cloud cluster can no longer be maintained. The system rapidly moves toward the TC state in a "free fall" pulled by a forcing greater than A-B. There can be overshooting.

The remaining task is to explain the shape of curve A. As S increases, curve A increases, because a higher S causes a stronger meridional circulation, which brings in more moisture and causes more evaporation through the associated stronger surface winds, thus resulting in more convective heating. The rate of increase of A is initially modest, but becomes greater as a result of the increase in wind speed in the boundary layer and the corresponding increase in evaporation at the surface as the vortex spins up. A increases at a reduced rate as the state gets close to S3, as a result of the higher temperature in the core region, which reduces vertical instability and hinders further increase in con-vective activity. Moreover, with increased convergence at low levels and increased divergence at high levels, inertial stability provides further inhibition of meridional circulation. Also, as the boundary-layer air approaches saturation, the evaporation rate cannot keep increasing, thus limiting the further intensification of convective activity.

The foregoing interpretation of tropical cyclogenesis as a catastrophe was presented by Chao et al. (2003) and needs to be supported with modeling effort.

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