Recently it was shown (Goswami and Rao 1993) that the process of intensification of tropical disturbances depends on the size of the incipient vortex in a rather nonlinear fashion. Among vortices of size ranging from 100 to 450 km (radius), embedded in the same large scale condition, it is the vortex with size about 250 km that intensifies to the most severe system. These results also showed a strong correspondence between the maximum intensity reached and the initial (3–6 hour) low level convergence field near the centre. The purpose of the present work is to identify the process(es) responsible for this scale selective intensification of tropical disturbances. It is proposed that diffusion is likely to play a crucial role in bringing about this selective intensification. In the present work a series of experiments with an axisymmetric numerical model of tropical cyclone (Wada’s model) is carried out to determine the relative roles of horizontal diffusion of momentum, moisture, heat and vertical diffusion. The results show that diffusion significantly affects the process of intensification and scale selection. While moderate diffusion does not alter the magnitude of intensification significantly, the scale selection is quite sensitive to the strength of diffusion. Interestingly, these diffusion processes, of momentum, moisture, heat and vertical do not affect the scale selection in the same fashion. The scale selection process turns out to be a result of a combined effect of these diffusion processes. However, no single diffusion process alone can give rise to a sharp selection of scale at the size of 250 km.
Volume 128 | Issue 8
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