The effect of liquid water on thunderstorm charging

Abstract

Laboratory studies have shown that thunderstorm charging caused by the interactions of ice crystals and graupel pellets is affected in sign and magnitude by temperature and cloud liquid water content; the presence of water droplets is a requirement for substantial charge transfer. Relationships showing the dependence of charge transfer on ice crystal size and velocity have previously been reported and now, in a continuation of the laboratory studies, the effect of liquid water content on the charge transfer has been investigated. The experiments show that positive graupel charging occurs at temperatures above a “charge sign reversal temperature” with negative charging at lower temperatures. The reversal temperature moves to lower temperatures when the liquid water content is increased. However, at low values of liquid water content, the sign of the graupel charging is inverted being positive at low temperatures and negative at higher temperatures. A discussion is presented of the various charge transfer theories. The results are consistent with the idea of two competing mechanisms whose relative success depends on the temperature and liquid water content. Positive graupel charging occurs when the graupel surface grows from the vapor and the crystals interact with a negative surface charge caused by a temperature gradient across the rime ice surface layer. Negative graupel charging occurs when the surface growth effect is swamped by freezing droplets which create either a pseudo contact potential with which the crystals interact, or a positive surface charge, due to dislocations in the rime ice, which is removed during glancing crystal interactions. Relationships between charge transfer, liquid water content, temperature, ice crystal size, and velocity have been determined and the equations may be used in numerical models of the development of thunderstorm electric fields. A one‐dimensional model indicates that the charge separation rates noted here are adequate to account for thunderstorm electrification. Use of the equations with cloud parameter values obtained in a thunderstorm research flight, leads to a predicted charge reversal level around −13°C, which is in agreement with the analysis of the electric field in the thunderstorm studied.

On the relationships between lightning frequency and thundercloud parameters of regional precipitation systems

“…[30] Because the charging zone may involve a temperature range from −5°C to −30°C [Takahashi, 1978;Saunders et al, 1991;Williams et al, 1991;Takahashi and Miyawaki, 2002], it is of interest to find the temperatures where the best correlation between radar echo and lightning occurs. The 11 year TRMM database can be used to narrow the range of possibilities.…”

“…In NIC, it is hypothesized that particle-scale charge separation occurs through rebounding collisions between precipitationsized ice particles and abundant small ice crystals in the presence of supercooled liquid water [Takahashi, 1978;Saunders et al, 1991;Saunders, 1993;MacGorman and Rust, 1998]. Observations from different weather regimes suggest that robust mixed-phase processes in the mixedphase region (0°C to −40°C) are necessary to generate storm electrification and initiate lightning [Stolzenburg et al, 1998a;1998b;Lang and Rutledge, 2002;Atlas and Williams, 2003;MacGorman et al, 2005].…”

Numerical Simulation of Electrical Space Charge Density and Lightning by using a 3-Dimensional Cloud-Resolving Model

“…In the case study, the T-method (Takahashi 1984) and the S-method (Saunders et al 1991) cannot simulate the normal tripole structure of the electrical space charge density. On the other hand, the HF-method (Helsdon and Faley 1987) can simulate the normal tripole charge structure, but the appearance rate of the simulated cloud-ground lightning with positive polarity is considerably too much in comparison with the observation.…”

“…One is based on the experimental studies of Saunders et al (1991) (Smethod), and the others on the numerical studies of Helsdon and Farley (1987) (HF-method). The value of the generating electrical charge in the separation process between graupel and cloud ice (or snow) in the HF-method is replaced 200 × 10 15 C with 3 × 10 15 C, which is the averaged value in the T-method.…”

Total Lightning Flash Activity Response to Aerosol over China Area

“…The intimate connection between electrical activity and some factors in the mixed-phase region was mainly due to the non-inductive charging process where rebounding collisions between big and small ice particles lead to appreciable charge separation. The magnitude and sign carried by ice particles relied on the droplet size spectrum, cloud water content, rime accretion rate, temperature and so on [9,10].…”