A Lagrangian investigation of hot and cold temperature extremes in Europe

Abstract

Since temperature extremes have a strong impact on environment and society, it is crucial to understand their underlying mechanisms. While their relationship to the largescale atmospheric circulation has been comprehensively investigated, the role of physical processes in the formation of air masses with extreme temperatures is less well understood. This study presents a Lagrangian analysis of the 0.1% most extreme six-hourly hot and cold events in three European regions (UK, Central Europe, Balkans) for the time period 1989-2009. The results provide insight into typical transport patterns and physical processes (adiabatic compression, radiation, surface heat fluxes) occurring along the trajectories of extreme temperature air masses. Cold events in Europe are most frequently induced by advection of cold air masses from the Arctic and Russia. The transport to the target region is characterized by a temperature increase caused by adiabatic compression and, in the maritime setting of the UK, also by diabatic heating due to surface sensible heat fluxes. Despite the warming along the trajectories, the extremeness of the associated 2 m temperature increases, due to the dislocation of the air mass to regions with a milder climate. Hot events are generally associated with weaker horizontal transport, but strong adiabatic warming and local temperature increase caused by enhanced radiation and surface heat fluxes. This in situ warming is particularly strong in Central Europe. Evaluating the temperature evolution along the trajectories reveals that hot and cold extremes develop on a similar time-scale of 2-3 days. This time-scale is mostly set by physical processes for hot extremes and controlled by advective transport for cold extremes. The diagnostics applied in this study lead to an improved process understanding that can provide a basis for more accurate predictions of temperature extremes.

Characterising the relationship between weather extremes in Europe and synoptic circulation features

“…Ireland. The important role of cold air advection for cold extremes in Ireland is also evident from the trajectory calculations of Bieli et al (2014). Nevertheless, the anomalies in Fig.…”

“…In addition, the spatial shift between blocking centre and target location results in a easterly flow of typically warm continental air masses towards the region where the temperature extremes occur. A contribution of easterly transport to hot temperature extremes in the UK is also evident from the Lagrangian analysis of Bieli et al (2014) (see their Fig. 4a).…”

“…During summer hot temperature extremes are associated with anticyclonic circulation anomalies (Cassou et al, 2005;Carril et al, 2008;Stefanon et al, 2012;Pfahl and Wernli, 2012c). In this case (and particularly for more northerly parts of Europe), not horizontal temperature advection, but adiabatic warming in sinking air masses and an anomalous clear-sky radiative forcing during anticyclonic conditions are thought to be the most important mechanism (Pfahl and Wernli, 2012c;Bieli et al, 2014). In addition, summer heat extremes can be amplified through soil moisture feedbacks (e.g.…”

Dependence of Present and Future European Temperature Extremes on the Location of Atmospheric Blocking

“…Blocking describes strong and stationary high‐pressure systems, which interrupt the climatological flow at midlatitudes for up to several weeks (Rex, 1950). They are often linked to the development of surface HWs (e.g., Pfahl & Wernli, 2012) due to their persistence, which allows the accumulation of heat in a certain area (Bieli et al, 2015; Perkins, 2015). The Russian HW in summer 2010 is one recent example of a block leading to a devastating HW, which affected large parts of Europe and Russia.…”

“…The role of the extraordinary atmospheric conditions caused by the blocking in the development and maintenance of the Russian HW was discussed by many studies in the aftermath (e.g., Galarneau et al, 2012; Lupo et al, 2012; Miralles et al, 2014; Schneidereit et al, 2012). Conversely, blocking has also been connected to winter CSs (e.g., Buehler et al, 2011; Sillmann et al., 2011), mainly through the advection of cold air (e.g., Bieli et al, 2015; Sousa et al, 2018). Recently, also the blocking impacts during the transition seasons such as spring have come into focus (e.g., Brunner et al, 2017; Cassou & Cattiaux, 2016).…”