Tropical Cyclone Lightning Characteristics as Revealed by the World Wide Lightning Location Network (WWLLN)

Lightning occurrences within tropical cyclones have been used to infer not only aspects of the electrical nature of these storms, but also their dynamical and microphysical characteristics. There have also been efforts to link tropical cyclone lightning activity with cyclogenesis and intensification. Given the relatively rare and episodic nature of lightning in tropical cyclones, robust conclusions can only be found in samples spanning several storms, yet most studies have been limited to a few cyclones because of the challenges that lightning detection over the open ocean presents.

The World Wide Lightning Location Network (WWLLN) is a developing, long-range, ground-based network that locates flashes across the globe. Our previous research has shown an increasing detection efficiency of the WWLLN, currently reaching values above 10%, and that this efficiency holds a strong statistical proportionality with more established lightning detection systems. With this confidence in the WWLLN's detection abilities, we have studied the lightning flashes of the 74 tropical cyclones that occurred in the Atlantic basin from 2004 to 2007, and the 99 tropical cyclones that occurred in the eastern North Pacific between 2004 and 2009. Specifically, we have: 1) explored the possible relationship between tropical cyclone intensity, intensity change and lightning activity, as well as its modulation by sea surface temperatures, 2) compared and contrasted the radial and azimuthal distribution of flashes in the two basins, and 3) investigated the diurnal cycle of lightning flashes.

Our preliminary results show that in the Atlantic and eastern Pacific basins, convective activity is maximized in the downshear left quadrant in the core, and downshear right quadrant in the outer rainbands. A much weaker asymmetry with respect to storm motion was found, which is largely an artifact of the vertical wind shear signature and depends on the relative directions of vertical shear and storm motion. Tropical cyclone intensity and intensity change were not found to hold a simple relationship with flash density, but Atlantic basin intensifying tropical cyclones exhibited more flashes than non-intensifying storms, and larger flash densities were observed in weaker, versus stronger, tropical cyclones. Thus, flash density in the inner core may have potential for distinguishing between intensifying and non-intensifying TCs in the Atlantic, while in the Pacific the strongest, non-intensifying TCs exhibited the highest flash rates. This difference between the Atlantic and eastern North Pacific storms may be related to the underlying sea surface temperatures the tropical cyclones experience. Climatologically, Atlantic basin storms spend most of their lifetimes over relatively warm waters (> 26° C), while in the eastern Pacific they tend to occur over a wider range of sea surface temperatures.