Differentiating lightning in winter and summer with characteristics of the wind field and mass field

Abstract

Lightning in winter (December–January–February, DJF) is rare compared to lightning in summer (June–July–August, JJA) in central Europe north of the Alps. The conventional explanation attributes the scarcity of lightning in winter to seasonally low values of variables that create favorable conditions in summer. Here we systematically examine whether different meteorological processes are at play in winter. We use cluster analysis and principal component analysis and find physically meaningful groups in ERA5 atmospheric reanalysis data and lightning data for northern Germany. Two thunderstorm types emerged: wind-field thunderstorms and CAPE (convective available potential energy) thunderstorms. Wind-field thunderstorms are characterized by increased wind speeds, high cloud shear, large dissipation of kinetic energy in the boundary layer, and moderate temperatures. Clouds are close to the ground, and a relatively large fraction of the clouds are warmer than −10 ∘C. CAPE thunderstorms are characterized by increased convective available potential energy (CAPE), the presence of convective inhibition (CIN), high temperatures, and accompanying large amounts of water vapor. Large amounts of cloud-physics variables related to charge separation such as ice particles or cloud base height further differentiate both wind-field thunderstorms and CAPE thunderstorms. Lightning in winter originates in wind-field thunderstorms, whereas lightning in summer originates mostly in CAPE thunderstorms and only a small fraction in wind-field thunderstorms. Consequently, typical weather situations of wind-field thunderstorms in the study area in northern Germany are strong westerlies with embedded cyclones. For CAPE thunderstorms, the area is typically on the anticyclonic side of a southwesterly jet.