Aircraft Measurements of High Average Charges on Aerosol Particles Associated with Layer Clouds

The first reliable aircraft measurements of aerosol charges within clouds were obtained during the Lake-ICE project utilizing an aft-facing “interstitial” inlet to eliminate spurious contact charging that contaminated previous aircraft measurements. Aerosol charges were measured during level flight at 2.5 km elevation over Lake Michigan. Initial measurements were made through a 5 km wide Sc turret that extended well above the general Sc layer. The average interstitial aerosol charge varied from +20e to -10e (e = magnitude of electron charge). After exiting the cloud turret the aircraft emerged well above the Sc layer where the measured value of average aerosol charge varied between +10 and +20e. The typical values of space charge density of 1pC/L (calculated from the measured aerosol concentration) are comparable to the space charge density carried by raindrops in thunderstorms! We used the measured size distributions and the theory for equilibrium charges on aerosol particles and cloud drops to help evaluate the source of these remarkably high space charges. We have also considered the broader implications of our observed electrical environment on the microphysical processes within non-thunderstorm clouds.