Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

Abstract

A statistically significant link is presented between atmospheric blocking located over the eastern North Atlantic and northern Europe and warm‐season thunderstorm activity over western and central Europe. Lightning data from 2001 to 2014 were used to identify thunderstorm days and blocking events were extracted from the ERA‐Interim reanalysis using an objective identification algorithm. The statistical link between the two phenomena is established through odds ratio analysis. Two areas – one over the eastern part of the North Atlantic and one over the Baltic Sea – were identified as locations where blocking influences the occurrence of deep moist convection in parts of western and central Europe. Based on the mean ambient conditions on days with blocking in these two areas, well‐known dynamic and thermodynamic mechanisms supporting or suppressing the development of thunderstorms were confirmed. The anticyclonic circulation of a block over the eastern part of the North Atlantic leads to a northerly to northwesterly advection of dry and stable air masses into Europe on the eastern flank of the block. In addition, these environmental conditions are on average associated with large‐scale subsidence of air masses (convection‐inhibiting conditions). In contrast, the southerly to southwesterly advection of warm, moist and unstable air masses on the western flank of a block over the Baltic Sea results in convection‐favouring conditions over western and central Europe. Both blocking situations are on average associated with weak wind speeds at mid‐tropospheric levels and with weak wind shear. As a consequence, thunderstorms related to atmospheric blocking over the Baltic Sea tend to be on average less organised.

Investigation of the temporal variability of thunderstorms in central and western Europe and the relation to large‐scale flow and teleconnection patterns

“…Most of the WMILX days in the respective sub-regions are dominated by a southerly to southwesterly upper-level flow as observed in other studies concerning thunderstorm or hail events in Europe (e.g. Kapsch et al, 2012;Merino et al, 2014;Wapler and James, 2015;Nisi et al, 2016;Mohr et al, 2019). Figure 3 illustrates the manifold forms of these southwesterly flow patterns for four exemplary sub-regions (indicated by the red box).…”

“…Most of the WMILX days in the respective sub‐regions are dominated by a southerly to southwesterly upper‐level flow as observed in other studies concerning thunderstorm or hail events in Europe (e.g. Kapsch et al ., ; Merino et al ., ; Wapler and James, ; Nisi et al ., ; Mohr et al ., ).…”

“…Similar conditions were observed by Mohr et al . (), who identified that Scandinavian blocking, which corresponds to SCAND + (Cassou, ), is frequently associated with an increased odds of thunderstorm occurrence due to convection‐favouring conditions on its western flank (southwesterly advection of warm, moist and unstable air masses).…”

“…Across the domain, convection‐favouring conditions primarily occur ahead of upper‐level troughs, consistent with similar studies (Kapsch et al ., ; Merino et al ., ; Punge and Kunz, ; Westermayer et al ., ; Mohr et al ., ; Taszarek et al ., ). In some regions, however, further flow configurations were shown to provide favourable pre‐convective environments such as, for example, a broad upper‐level low over the Adriatic Sea favouring thunderstorm occurrence in southern Poland.…”

“…While there have been a number of studies addressing this topic over the USA for severe convection (hail and tornadoes), and a few efforts elsewhere globally (Allen and Karoly, 2014;Pinto, 2015;Dowdy, 2016;Blamey et al, 2017), there have been comparatively few ones considering the drivers of thunderstorm variability over Europe (e.g. Mohr et al, 2019). Piper and Kunz (2017), hereinafter referred to as PK17, identified a connection between lightning activity and the North Atlantic Oscillation (NAO), which they attributed to variations in the large-scale forcing leading to the development of thunderstorms.…”