Contribution to the climatological study of lightning

In this work, a new parameterization of the thunderstorm cloud electrical flash frequency is obtained on the basis of simple dimensional arguments. The flash frequency is expressed as a power function of both the cloud top height and the cloud droplet concentration, thus partly taking into account the role of the microphysical cloud characteristics. Since droplet concentrations are not predicted by General Circulation Models (GCMs) and further show great variability, two different “standard” droplet concentrations, respectively valid for continental and maritime thunderstorms, are used. A numerical experiment has been made by using the METEO-France GCM “ARPEGE” in relation to both, this new parameterization and the Price and Rind (1992) parameterization, in order to determine the global characteristics of electrical flashes. The analysis of the results reveals that the mean annual global flash frequency as well as the total number of flashes that strike annually the globe are higher when the new parameterization is used but are in better agreement with the Optical Transient Detector (OTD) (Christian et al., 1996) observations, thus indirectly attesting for its validity. Further, the good agreement between monthly and diurnal global flash numbers and the corresponding OTD observations confirms the ability of “ARPEGE” of correctly representing the global thunderstorm development. Finally, the new parameterization has been used in a second numerical experiment in order to infer the role of a doubling of CO2 on the global distribution of electrical flashes. A 10% increase of the mean global annual flash frequency is obtained for a surface warming of roughly 2°C.