Charge transfer in graupel/crystal collisions under different microphysical conditions

E. E. AVILA; F. NOLLAS; R.A. LIGHEZZOLO; N.E. CASTELLANO; R.G. PEREYRA; R.E. BÜRGESSER

Congreso; America Geophysical Union, Meeting of the Americas; 2013

Laboratory measurements of the interactions between ice crystals and riming ice particles have shown that the charge transfer per rebounding collision could be sufficient to explain thunderstorm electrification. These studies have shown that the magnitude and sign of the charge transferred to riming graupel particles during interactions with ice crystals is sensitive to the cloud microphysical conditions such as the cloud temperature and liquid water content, distribution of cloud droplet size, ice crystal size, and impact velocity. A laboratory investigation of the electric charge separated in collisions between vapor grown ice crystals and a target growing by riming is presented in this work, with the goal of studying the performance of the non-inductive mechanism under different microphysical conditions. In particular in this work we explore the microphysical conditions present in: (a) the stratiform regions of the Mesoscale Convective Systems and (b) severe storms. For the stratiform regions, a series of experiments were conducted by using a rimer 2 mm in diameter, for ambient temperatures between -7°C and -13°C, effective liquid water content between 0.05 and 0.5 g/m3 and impact speeds between 1 and 3 m/s. The measurements for severe storms were performed by using a rimer 10 mm in diameter, temperatures between -6°C and -30°C, effective liquid water content between 0.3 and 4.5 g/m3 and impact speed of 11 m/s.Charge diagrams of the sign of the rimer charging as a function of the ambient temperature and the effective liquid water content for different velocities and rimer sizes are presented. The results show that in stratiform conditions the riming target acquires a charge preferentially positive; while for severe storm conditions there is a positive rimer charging zone at temperatures above -15°C and a negative zone at lower temperatures. The implications of these results to the electrification processes are discussed.