Impacts of evaporative cooling on tropical cyclone rainband

Impacts of evaporative cooling from raindrops on tropical cyclone (TC) rainbands are examined using nonhydrostatic model with 2-km mesh under an idealized condition. Rainbands are actively formed outside the eyewall in the experiment including evaporative cooling, while they are hardly seen in the experiment without evaporative cooling. Rainbands propagate in the counter clockwise and radially outward direction. The formation and propagation of rainband are closely related to cold pools. New convective cells are successively generated at the upstream end of a cold pool through horizontal convergence between low-level inflows and cold outflows. This formation process is called hereafter as the upstream development. The upstream development organizes spiral-shape rainbands along low-level streamline azimuthally averaged and propagates them radially outward.

Cold pool also induces pressure anomaly near the surface and it causes asymmetric flows from azimuthally averaged low-level wind. The asymmetric flows advance cold pool fronts, in the normal direction to rainbands, called hereafter as cross-band propagation. The cross-band propagation deflects movement of each cell away from the low-level streamlines and rotates it in the counter clockwise direction. Cross-band propagation is favorable movement for maintenance of convective cells, giving a positive feedback to the maintenance of rainbands.

The active rainbands provide large amount of condensational heating, which enhances the inward transport of absolute angular momentum through strong secondary circulation. As a result, TC size and its kinetic energy gradually increase with time when evaporative cooling is included.