Aircraft observations of the influence of electric fields on the aggregation of ice crystals

Aircraft observations of ice-crystal size and habit distributions in the cirrus outflow from deep convection at several geographic locations are reported. In situ measurements were made in the outflow from maritime thunderstorms near Kwajalein, part of the Marshall Islands and of thunderstorms with more continental aerosol concentrations both in the United States and near Darwin, Australia over the Tiwi Islands. Images of chain-like aggregates of small ice crystals, some with plate-like shapes were observed with a state-of-the-art microphysics probe in the outflow regions of continental storms that were typically highly electrified, displaying lightning. The ‘chains’ were not found in the outflow regions of maritime storms that are typically less electrically active. The striking similarity between these images and previous laboratory measurements of ice aggregation in electric fields are remarked upon. This evidence is used to support the theory that chain aggregates of ice crystals may be common in fully glaciated regions of continental thunderstorms, where ice-particle number densities are high, and their presence is due to the electric field alignment of ice crystals with subsequent enhancement of the aggregation process by dipole induction resulting in short-range attractive inter-particle forces. It is not confirmed where in the storm the aggregates were typically formed; however, in the Darwin thunderstorms they were noted to occur with the highest frequency towards the cirrus outflow base when the cirrus base altitude was high, and generally decreased in frequency with increasing distance from the storm. The potential consequences of electrically enhanced aggregation in continental storms and related electric field mechanisms along with the role of homogeneous freezing in intense thunderclouds are discussed.