Cloud-to-ground Lightning Warnings Using Electric Field Mill and Lightning Observations

1. Introduction Automated lightning warning systems are a critical component of many day-today operations. This is particularly true of airports but also applies in some industrial applications, such as mining and the manufacture, testing, and handling of explosives and weapons. Many automated lightning warning systems use a combination of lightning detection information and one or more electric field mills (EFMs). In previous studies (e.g. Murphy and Holle, 2006), we have analyzed the capability of various lightning detection systems to provide warning prior to the first cloud-to-ground (CG) lightning flash within an Area Of Concern (AOC) around a given point or facility requiring lightning safety information. The lack of continuously recorded EFM data was the primary hindrance to including EFMs as part of those previous studies. The purpose of this paper is to remedy that problem, at least for one particular location in the world where continuous EFM data are available. Unlike lightning detection systems, which respond to fast transients in the electromagnetic field and/or optical signals generated by lightning, EFMs detect the electrostatic field and relatively slow changes in that field. They detect the presence of charge separation and net charge directly above and in the immediate surroundings of the sensor. Depending on where the charge is located, the effective detection range of an EFM varies from a few km to perhaps