SEMINAR SERIES and COLLOQUIA

Cloud-to-ground (CG) lightning bursts in the eyewall of mature tropical cyclones (TCs) are believed to be good indicators of imminent intensification of these systems. While numerous well-documented observational cases exist in the literature, no modeling studies of the electrification processes within TCs have been made so far. Towards this goal, a numerical cloud model featuring a 12-class bulk microphysics scheme and a three-dimensional branched lightning module was utilized to simulate the evolution of the microphysics fields and subsequent electrical activity in an idealized hurricane like vortex over ocean.

In order to test the reliability of the model in maritime tropical environment, an idealized simulation of a well-documented TOGA COARE squall line case was carried out first. This experiment was then carried out at three additional horizontal grid spacings to test potential resolution dependencies.

The simulated tropical squall line exhibited updraft speeds much weaker than their continental counterparts which was in turn consistent with shallow 30 dBZ echo tops and lower content of graupel and supercooled water droplets at midlevels. This reduction of graupel and supercooled water was mainly caused by a rapid depletion of liquid water by enhanced coalescence ahead of the line. This resulted in a system producing overall little lightning.

As the horizontal resolution increased from 600 m to 3 km, the qualitative aspects of the simulated squall line dynamics, microphysics and lightning were overall similar, thereby suggesting that the hurricane simulations presented in this study could still provide a good qualitative insight of the storm's dynamical, microphysical and electrical properties. Preliminary results of the TC simulation showed that the highest total lightning flash rate were primarily found within the eyewall but seldom within the stronger cells forming the outer rainbands where updraft speeds rarely exceeded 10 m s-1 and 15 m s-1, respectively. As expected, these regions of the storm were generally characterized by moderate total graupel mixing ratio and moderate cloud water content. The inner eyewall region exhibited a normal tripole charge structure while a normal dipole was observed in the outer eyewall stratiform region and in the strongest cells forming the outer rainbands.

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