Relationships between cloud-to-ground lightning and surface rainfall during 1992–1996 in the Spanish Basque Country area

The relationship between rainfall surface amounts and cloud-to-ground lightning stroke activity was studied during 1992–1996 in three locations in the Spanish Basque Country. In this area the surface distribution of ground strokes presents an average density of about 1 flash km−2 year−1 and the mean annual rain depth is about 103 mm. Most of the electrical storms analyzed (87%) were produced under two separate meteorological conditions: Oceanic and Continental. Continental events present the highest number of daily CG counts: counts that can reach values on the order of 5×103. Rainfall yield estimates during the periods researched were 23, 38, 21×104 m3 per CG flash at the three locations used in this paper: Bilbao, San Sebastian and Vitoria, respectively. The rainfall yield was evaluated from the ratio between total precipitation collected at every station and total number of CG counts per unit of surface measured inside a pixel of 20×20 km2 centred over the collecting points. It was observed that daily rainfall yield values present great disparity with standard deviation values of 122, 268 and 218×104 m3 per CG flash for Bilbao, San Sebastian and Vitoria, respectively. Associated mean daily rainfall yield was 68, 136 and 120×104 m3 per CG flash. These observations clearly indicate that daily rain depth and daily CG counts do not bear linear correlation. Rainfall yield in the Oceanic and Continental class present significant different mean values at the three locations and is always higher in Oceanic events. Likewise, rainfall yield in both situations—Oceanic and Continental—generally evolves during the year, reducing in value during the hot season, indicating that the environmental conditions of cloud formation could play a major role in the lightning–rainfall relationship. However; in two cases, Oceanic events in Bilbao during February and Continental events in Vitoria during July, rain depths and CG counts are correlated with r values of 0.980 and 0.854, respectively.