Mechanisms of lightning formation in deep maritime clouds and hurricanes (The HAMP contribution)

In maritime clouds over warm surface most droplets formed at the cloud base fall out without ever reaching the freezing level. Nevertheless, lightning takes place sometimes in the eye walls of hurricanes, where clouds are, supposedly, the most maritime in the world. In this presentation we address the following question: "Why does lightning take place in deep maritime convective clouds in the Intertropical Convergence zone and in hurricane eyewalls at all?" Numerical simulations using the spectral microphysics Hebrew University cloud model show that the formation of lightning in maritime clouds requires two conditions to be satisfied: a) significant vertical velocities and a large cloud-depth, and b) the existence of small aerosols with the radii lower than about 0.05 in diameter in the cloud condensational nuclei (CCN) size spectra. Both factors are necessary components for in-cloud nucleation well above the cloud base and to formation of a significant amount of supercooled water. Small aerosols form over the ocean, supposedly, due to chemical reactions followed by particle collisions to create the aerosol accumulation mode. Small aerosols can be of continental origin and may penetrate into oceanic regions, like sulphate pollution or African dust. The latter may explain the existence of intense lightning in the ITCZ over the ocean near the African coast. The potential importance of small CCN in creation of supercooled water at the upper levels, high ice crystals concentrations in cloud anvils and in other microphysical features of deep tropical convection is discussed.