Contribution to the climatological study of lightning

In this work. a new parameterization of the thunderstorm cloud electrical f lash frequency is obtained on the basis of simple dimensional arguments. Th e flash frequency is expressed as a power function of both the cloud top he ight and the cloud droplet concentration, thus partly taking into account t he role of the microphysical cloud characteristics. Since droplet concentra tions are not predicted by General Circulation Models (GCMs) and further sh ow great variability, two different "standard" droplet concentrations, resp ectively valid for continental and maritime thunderstorms, are used. A nume rical experiment has been made by using the METEO-France GCM "ARPEGE" in re lation to both, this new parameterization and the Price and Rind (1992) par ameterization, in order to determine the global characteristics of electric al flashes. The analysis of the results reveals that the mean annual global flash frequency as well as the total number of flashes that strike annuall y the globe are higher when the new parameterization is used but are in bet ter agreement with the Optical Transient Detector (OTD) (Christian et at., 1996) observations, thus indirectly attesting for its validity. Further, th e good agreement between monthly and diurnal global flash numbers and the c orresponding OTD observations confirms the ability of "ARPEGE' of correctly representing the global thunderstorm development. Finally, the new paramet erization 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 flas hes. A 10% increase of the mean global annual flash frequency is obtained f or a surface warming of roughly 2 degrees C.