QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 133: 1453–1457 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.133 Field identification of a unique globally dominant mechanism of thunderstorm electrification J. Latham, 1,2 * W. A. Petersen, 3 W. Deierling 1 and H. J. Christian 3 1 NCAR, P.O. Box 3000, Boulder, CO 80301, USA 2 also at Centre for Atmospheric Science, The University of Manchester, PO Box 88, Manchester M60 1QD, UK; ESSC and NSSTC 3 University of Alabama in Huntsville, 320 Sparkman Drive, Huntsville, AL 35899, USA ABSTRACT: Two wholly distinct studies involving TRMM-satellite global data were conducted. One involved the relationship between lightning frequency f and brightness temperature, the other between f and ice-water-path. Both studies demonstrate that globally valid relationships exist between f and thundercloud ice-precipitation content, from which it follows that graupel pellets play a crucial role in thundercloud charging. Ground-based field studies provide further support for this conclusion and show that f is also strongly dependent upon the ice crystal content. All these findings are consistent with the non-inductive charging mechanism, but not with any other proposed mechanism of thunderstorm electrification. We conclude that the non-inductive mechanism dominates electric field growth and lightning production in all seasons–for both oceanic and terrestrial thunderstorms–on a global scale. Copyright  2007 Royal Meteorological Society KEY WORDS lightning; charge transfer; thunderstorm electrification; ice; non-inductive mechanism Received 31 January 2007; Revised 25 May 2007; Accepted 14 June 2007 1. Introduction Intensive research over the last few decades (e.g. Chris- tian et al., 1980; Latham, 1981; Dye et al., 1988; Saun- ders, 1993; McGorman and Rust, 1998) has led to the generally accepted view that although several charge transfer processes could contribute – sometimes in a sig- nificant way – to the development of strong electric fields in thunderstorms, culminating in lightning, the dominant charging mechanism is probably the so-called non-inductive process (e.g. Reynolds et al., 1957; Taka- hashi, 1978; Jayaratne et al., 1983; Avila et al., 1988; Norville et al., 1991; Saunders et al., 1991; McGorman and Rust, 1998; Saunders and Peck, 1998; Takahashi and Miyawaki, 2002; Mansell et al., 2005). The non- inductive process involves separation of opposite polar- ities of charge in rebounding collisions between grau- pel pellets growing by accretion of supercooled water droplets and small ice crystals growing by vapour dif- fusion. The particle-scale charge separation is followed by gravitationally driven separation of the oppositely charged hydrometeors and associated growth of cloud- scale electric fields. The assertion that non-inductive pro- cesses dominate thunderstorm electrification is based at least partially on circumstantial evidence, however, and since most field experiments have been conducted within a restricted geographical region (i.e. mainly in the USA) we have to date possessed negligible information as to the predominant thunderstorm electrification mechanism(s) * Correspondence to: J. Latham, NCAR, P.O. Box 3000, Boulder, CO 80301. E-mail: latham@ucar.edu over most parts of the world. Herein we describe recent findings which help remedy this deficiency. Sections 2 and 3 are concerned with satellite and ground-based field studies respectively. Overall conclu- sions are presented in Chapter 4. 2. Satellite studies Our ability to obtain information on thunderstorms and lightning activity on a global scale has been trans- formed over the past decade by the NASA Marshall Space Flight Center development of two satellite-borne lightning-detection instruments – the Optical Transient Detector (OTD) (Christian et al., 2003) and the Lightning Imaging Sensor (LIS) (Christian et al., 1999). The OTD made continuous measurements from 1995 – 2000 over an almost global domain (∼70 ° orbital inclination), while the LIS has made lightning measurements from 1997 to the present over the tropics (latitude range ±35 degrees). The LIS is mounted on the Tropical Rainfall Measuring Mission (TRMM) satellite (Kummerow, 1998) which car- ries other instruments including the TRMM Microwave Imager (TMI) and the TRMM Precipitation Radar (PR). The field evidence presented in this section was provided by these three TRMM devices. The data set from the LIS consists of the optical events observed while making overpasses of active thunder- storms. Data recorded by the LIS include the time of lightning events, their radiant energy and location. The LIS instrument is a compact staring imager, and the 600 × 600 km 2 field-of-view allows the sensor to observe Copyright  2007 Royal Meteorological Society