Environmental Influences on the Lightning of Cold-based Continental Convection Simulated Numerically: Land-Ocean Contrast and Inverted-vs-Normal Charge Structure

Abstract:

A long-standing enigma has been the reasons for the observed preference of lightning for land globally. When lightning occurs over oceans, it is usually downwind from a nearby coastline. Two hypotheses have historically been proposed: the fact that updrafts are faster, over land for thermodynamic reasons (thermal hypothesis) and that there are more aerosols there (aerosol hypothesis).

Our aerosol-cloud model with emulated bin microphysics, with five microphysical species of hydrometeor and seven aerosol species, represents all known and empirically quantified modes of initiation of ice. New schemes are included for raindrop-freezing fragmentation and breakup in ice-ice collisions, for all permutations of colliding microphysical species. A simulation of a cold-based normal storm from Colorado has been validated both microphysically and electrically (lightning frequency and type, charge density).

Sensitivity tests are shown in this presentation elucidating the influences on the cold-based continental storm from environmental factors such as aerosols, temperature, moisture and shear. Lightning is determined by environmental factors controlling updraft speed (e.g. instability). Reducing ascent but not cloud-base causes lightning flashes to disappear, consistent with the classic Boccippio-Vonnegut theory. Since cloud-base is too cold (near 0 degC) for coalescence, the cloud condensation nucleus (CCN) aerosol concentration has only a weak effect. Moreover, secondary ice production provides a natural ‘buffer’, reducing sensitivity to changes in ice nucleus (IN) aerosol loading of the environment.

By including a maritime sounding, the land-sea contrast in lightning for such storms is predicted to arise from the vertical structure of environmental temperature and humidity. Ascent is slowed by lack of instability, and by the low warm cloud-base. Maritime aerosols cannot cause this suppression.

Finally, other sensitivity tests show the impact from microphysical processes such as breakup in ice-ice collisions. The normal charge structure arises from graupel falling out of the storm after being positively charged, leaving the storm with overall net negative charge, which then causes negative cloud-to-ground flashes. Inverted charge structure arises similarly from negative charging of graupel.