Q. J. R. Meteorol. Soc. (2005), 131, pp. 1695–1712 doi: 10.1256/qj.03.217 Aircraft observations of the influence of electric fields on the aggregation of ice crystals By P. J. CONNOLLY 1 ∗ , C. P. R. SAUNDERS 1 , M. W. GALLAGHER 1 , K. N. BOWER 1 , M. J. FLYNN 1 , T. W. CHOULARTON 1 , J. WHITEWAY 2 and R. P. LAWSON 3 1 UMIST, Manchester, UK 2 Department of Earth and Space Science and Engineering, York University, Canada 3 SPEC Inc., Boulder, USA (Received 8 December 2003; revised 1 December 2004) SUMMARY 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. KEYWORDS: Aggregates EMERALD Microphysics 1. I NTRODUCTION The process of aggregation of ice crystals in clouds is a fundamental mechanism for the generation of large precipitation-sized particles. In thunderstorms, the aggregation process not only affects precipitation throughout the troposphere but also affects the microphysical properties of the extensive anvil cirrus outflow region which, in the Tropics, tends to reside between 10 km and the tropopause height of 15 to 18 km. Cirrus clouds, due to their extensive global coverage, play important roles in various atmospheric processes and their microphysical properties are important modulators of incoming solar and outgoing terrestrial radiation as described by Liou (1973), Ackermann et al. (1988), Stephens et al. (1990), Hartmann et al. (1992) and McFarquhar and Heymsfield (1997). Cirrus clouds may also influence the location of the tropopause and dehydrate air entering the stratosphere (Danielson 1982); hence they are important components of the earth-climate system. Atmospheric heating rates in the Tropics demonstrate a large sensitivity to the altitude of cirrus layers. It is because of this sensitivity that there are several potential important effects of ice-particle properties in cirrus clouds. The microphysical properties may influence heating rates in a direct way—by the interaction with the earth’s radiation fluxes, for example—or in various indirect ways. One such example of an indirect effect that may be relevant to thick cirrus is that the size, morphology and hence fallspeed of ice crystals in cirrus clouds play an important role in determining cloud lifetime and ∗ Corresponding author: Department of Physics, UMIST, Manchester M60 1QD, UK. e-mail: P. Connolly@postgrad.manchester.ac.uk c  Royal Meteorological Society, 2005. 1695