Aerosol Effects on Lightning and Intensity of Landfalling Hurricanes

N. Cohen and A. Khain

Abstract Intense and persistent lightning in landfalling hurricanes takes place within the 250-300 km-radius ring around the hurricane center. At the same time the lightning activity in the eye wall takes place only during comparatively short periods of tropical cyclone (TC) intensification related to the replacement of old eyewall by the new one. As soon as the hurricane weakens, the lightning in the eye wall disappears. The mechanisms responsible for most of the phenomena are unknown. In this study we provide some observational evidence and numerical estimations to show that lightning in hurricanes approaching or penetrating the land, especially at their periphery, arises under the influence of continental aerosols, which affect the microphysics and dynamics of clouds in TCs. Numerical simulations using a 2-D mixed phase cloud model with spectral microphysics show that aerosols that penetrate cloud base of maritime clouds dramatically increase the amount of supercooled water as well as ice content and vertical velocities. As a result, in clouds developing in dirty air ice crystals, graupel, frozen drop/hail and supercooled water can coexist within the same cloud zone which allows for collisions and charge separation. Simulation of the possible effects of aerosols on landfalling tropical cyclone has been carried out using a 3-km resolution Weather Research and Forecasting (WRF) mesoscale model. It is shown that aerosols change cloud microstructure in a way that allows one to attribute observed lightning structure to effects of continental aerosols. It is shown also that aerosols, invigorating clouds at 250-300 km from TC center decrease the convection intensity in the TC center leading to some TC weakening. The results suggest that aerosols change the intensity and spatial distribution of precipitation in landfalling TCs. These results can serve as a justification of the observed weekly cycle of intensity and precipitation of landfalling TCs caused, supposedly, by the weekly variation of anthropogenic aerosol concentration.

Keywords: Tropical cyclones, lightning, cloud-aerosol interaction, numerical modeling.

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