Identification of favorable environments for thunderstorms in reanalysis data

The relations between lightning occurrence over Europe from the EUCLID network (2008-2013) and parameters derived from ERA-Interim reanalysis data were studied to increase the understanding of the conditions under which thunderstorms form. The objective was to identify relevant factors beyond instability and convective inhibition, in order to better model thunderstorms using numerical weather prediction or climate model data. It was found that latent instability is only required up to a certain amount of approximately 200-400 J kg(-1) CAPE. For higher values of CAPE (similar to 800-2800 J kg(-1)), the relative frequency of lightning is rather constant. Relative humidity in the low to mid-troposphere has a major influence on storm occurrence with low relative humidity strongly suppressing thunderstorm development. For an average 850-500 hPa relative humidity below 50%, the frequency of lightning decreases to below 15%, even when CIN is negligible and CAPE sufficient. A subtle dependency on wind shear was found in which two regimes of higher frequency of lightning were identified. For very weak and for high shear the probability was higher than for intermediate values of both deep-layer and low-level shear.