Abstract
We present a numerical investigation of the processes that influenced the contrasting rapid intensity changes in Tropical Cyclones (TC) Phailin and Lehar (2013) over the Bay of Bengal. Our emphasis is on the significant differences in the environments experienced by the TCs within a few weeks and the consequent differences in their organization of vortex-scale convection that resulted in their different rapid intensity changes. The storm-relative proximity, intensity, and depth of the subtropical ridge resulted in the establishment of a low-sheared environment for Phailin and a high-sheared environment for Lehar. Our primary finding here is that in Lehar’s sheared vortex, the juxtaposition in the azimuthal phasing of the asymmetrically distributed downward eddy flux of moist-entropy through the top of the boundary layer, and the radial eddy flux of moist-entropy within the boundary layer in the upshear left-quadrant of Lehar (40–80 km radius) establishes a pathway for the low moist-entropy air to intrude into the vortex from the environment. Conversely, when the azimuthal variations in boundary layer moist-entropy, inflow, and convection are weak in Phailin’s low-sheared environment, the inflow magnitude and radial location of boundary layer convergence relative to the radius of maximum wind dictated the rapid intensification.
Highlights
We present a numerical investigation of the processes that influenced the contrasting rapid intensity changes in Tropical Cyclones (TC) Phailin and Lehar (2013) over the Bay of Bengal
Phailin rapidly intensified by about 36 m/s between 10 and 11 October 2013 and Lehar decayed by about 15 m/s between 27 and 28 November 2013 before weakening a further 9 m/s (∼17.5 knots) over the 12 hours (Fig. 1c,d)
TC rapid intensity changes are the result of a combination of external and internal factors
Summary
We present a numerical investigation of the processes that influenced the contrasting rapid intensity changes in Tropical Cyclones (TC) Phailin and Lehar (2013) over the Bay of Bengal. Given a reasonably favorable environment and a vortex structure, all TCs are expected to intensify unless they are subject to vertical wind shear (hereafter ‘shear’), dry air or relatively low sea surface temperatures[7,9,10,11]. Tang and Emanuel[33,34] using an axisymmetric model, concluded that inward eddy moist-entropy fluxes at mid-levels (through vortex-Rossby waves excited by shear-vortex interactions), were responsible for the ventilation of the TC-inner core with environmental air, and subsequent weakening of the TCs. On the other hand, Riemer and Laliberte[35] using a trajectory analysis in an asymmetric framework, affirmed the conclusion of Riemer et al.[31] that the dominant pathway of the environmental air to intrude into the TC inner-core was through the frictional inflow layer, and not through the mid-levels. Real TCs warrant further understanding where the asymmetric structure of winds in three dimensions is much more complex than those prescribed in the above studies
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