Keri Nicoll : PhD research
Phd Title - Coupling between the Global Atmospheric Electric Circuit and Clouds
The Global Atmospheric Electric Circuit (GEC) is a conceptual model used to describe the electrical characteristics of the atmosphere. In the GEC the two highly conducting surfaces of the ionosphere and the Earth's surface act as two plates of a spherical capacitor, with a leaky dielectric (air) in between.
The air is weakly conducting due to ionisation from cosmic rays and, near the surface, the Earth's natural radioactivity. Charge separation inside thunderstorms generates a potential difference, Vi, between the ionosphere and the Earth's surface, causing a small current, known as the air-Earth conduction current, Jz, to flow vertically between the two regions. The upper figure shows a conceptual representation of the GEC.
The main aim of this project was to investigate the effect of GEC current flow on stratiform clouds of large horizontal extent. These clouds are abundant in the atmosphere, and cover ~40% of the planet at any one time (1).
Gauss' law of electrostatics predicts that charge should be generated on the upper and lower eges of a cloud if current flows vertically through it (2). The charge will be transferred to cloud droplets in these regions, and the droplets may become highly charged. Theory predicts that cloud microphysical processes may be affected if the individual cloud droplets and CCN are charged (3),(4),(5).
Very few measurements of charge inside stratiform cloud exist to test this theory, and the typical magnitudes of charge in these regions is also uncertain. Therefore a major part of this project developed and tested instruments to measure charge aloft inside stratiform clouds. Several balloon based sensors were developed and flown through a variety of stratiform clouds. Charge was detected near the edges of stratiform cloud on many flights, in approximately the same magnitude as that predicted by theory.
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(1)Klein, S.A. and D.L. Hartmann. (1993).The seasonal cycle of low stratiform clouds. J. Climate, 6,1587-1606.
(2)Zhou, L. and B.A. Tinsley. (2007). Production of space charge at the boundaries of layer clouds. J. Geophys. Res. 112, D11203, DOI:10.1029/2006JD007998.
(3)Tinsley, B.A., R.P. Rohrbauch, M. Hei, K.V. Beard. (2000). Effects of image chrages on the scavenging of aerosol particles by cloud droplets and on droplet charging and possible ice nucleation processes. J. Atmos. Sci., 57, 2118-2134.
(4)Tripathi, S.N. and R.G. Harrison. (2002). Enhancement of contact nucleation by scavenging of charged aerosol particles. Atmos. Res. 62, 57-70.
(5)Khain, A., M. Arkihpov, M. Pinsky, Y. Feldman, Y. Ryabov. (2004). Rain enhancement of fog elimination by seeding with charged droplets. Part 1: theory and numerical simulations. J. Appl. Meteorol., 43, 1513-1529.