Remote sensing analysis of a Mediterranean thundersnow and low-altitude heavy snowfall event

Abstract

On the 8 of March 2010 a heavy snowfall accompanied by lightning occurred over Catalonia (NE Spain), in the Western Mediterranean. Total lightning observations included 101 cloud-to-ground flashes and 169 intra-cloud flashes. Precipitation amounts in 24h exceeded 100mm and snow depths over low altitude terrain, where snow is rare, surpassed 30cm. Snow accumulations collapsed the regional communication transport network and the border with France was closed several hours. Occurrence of wet snow combined with increasingly strong winds caused widespread damage over large forest areas estimated in more than 20MEur and affected dramatically the high voltage power line distribution grid due to ice accretion, particularly in NE Catalonia where 33 high power electrical towers were knocked down.The meteorological framework at synoptic scale was dominated at low levels by a northern flow over Iberia due to a blocking high pressure system on the British Isles, and an upper level cold trough, which favoured a rapid cyclogenesis over the Mediterranean (9.2hPa drop in 12h). Weather radar observations indicated predominance of stratiform precipitation and some low-topped convection, with maximum reflectivities and tops mostly below 40dBZ and 4km respectively. The presence of mesoscale gravity waves, caused by wind-shear instability, is suggested as a triggering element for convection and subsequent lightning. Comparison of accumulated precipitation and lightning maps indicated clusters of lightning data unrelated to precipitation maxima. Further investigation of total lightning characteristics and co-located radar observations suggested a triggering effect by tall telecommunication towers inducing cloud‐to‐ground flashes and subsequent intra-cloud lightning.