Using 20 years of lightning data in ground flash density statistics in France

Most of lightning statistics used by the lightning protection community are derived from lightning locating systems. Such systems collect flash data over large regions and during long period of time. In their life, those systems are likely to undergo several major changes as the state of the art in remote sensing techniques improves leading to some impacts on the data. In addition the flash data measured by lightning locating systems underestimate the actual lightning risk since it accounts only for one ground contact per flash that is well known nowadays not correct. Then the different evolutions in observing systems and underestimation introduced by flash data must be addressed and compensated in order to give a more accurate and relevant information for lightning risk assessment. The French national lightning locating system operated by Meteorage, has collected more than 20 years of lightning data all over the country. This system is no exception since from its inception several major changes, in either technology or system settings, have significantly modified the lightning detection performances affecting the homogeneity of the data and the relevancy of the GFD statistics. The work presented in this paper is based on this long duration French dataset. It attempts to define a method which comes around the inhomogeneity introduced by lightning detection performance evolutions and suggests the use of the flash ground contacts multiplicity instead of flash data only. To achieve this goal the cumulative peak current distribution method developed by the CIGRE task force C404 is used to determine the compensation factors to correct the statistics for detection efficiency effect. In addition, it is suggested the use of ground contacts data instead of flash data for lightning risk statistics. This parameter is derived from the lightning data collected in France on 2011 with a program developed by Meteorage based on a clustering algorithm so called ´k-means´. The meth- d suffers from some necessary assumptions depending on the Meteorage´s LLS history and operational background, but the final new ground contacts density parameter derived from the longest observation period available in France looks more realistic and reliable. This work is a first attempt that must be extended in the future to compute high spatial resolution statistics supporting new applications for lightning risk assessment.