Abstract
Abstract. This study presents a Lagrangian analysis of upper-tropospheric anticyclones that are connected to surface heat waves in different European regions for the period 1979 to 2016. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air parcels backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. Around 25 %–45 % of the air parcels are diabatically heated during the last 3 d prior to their arrival in the upper-tropospheric anticyclones, and this amount increases to 35 %–50 % for the last 7 d. The influence of diabatic heating is larger for heat-wave-related anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams; 3 d prior to arrival, one heating branch (remote branch) is located above the western North Atlantic, and the other heating branch (nearby branch) is located over northwestern Africa and Europe to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the remote branch is related to warm conveyor belts in North Atlantic cyclones upstream of the evolving upper-level ridge. In contrast, the nearby branch is diabatically heated by convection, as indicated by elevated mixed-layer convective available potential energy along the western side of the matured upper-level ridge. Most European regions are influenced by both branches, whereas western Russia is predominantly affected by the nearby branch. The remote branch predominantly affects the formation of the upper-tropospheric anticyclone, and therefore of the heat wave, whereas the nearby branch is more active during its maintenance. For long-lasting heat waves, the remote branch regenerates. The results from this study show that the dynamical processes leading to heat waves may be sensitive to small-scale microphysical and convective processes, whose accurate representation in models is thus supposed to be crucial for heat wave predictions on weather and climate timescales.
Highlights
Among various kinds of natural hazards, temperature extremes and especially heat waves during summer impose large impacts on human health (Horton et al, 2016; Watts et al, 2018)
Most European regions are influenced by both branches, whereas western Russia is predominantly affected by the nearby branch
We only present the density maps for heat waves in central Europe, western Russia and Greece–Italy because they exhibit the largest differences
Summary
Among various kinds of natural hazards, temperature extremes and especially heat waves during summer impose large impacts on human health (Horton et al, 2016; Watts et al, 2018). Anthropogenic climate change has already increased the number of heat wave days during the last decades (Perkins et al, 2012), which is in line with an overall global-scale temperature increase (Horton et al, 2015). E.g. Europe, encounter changes in the frequency, persistence and maximum duration of regional circulation patterns associated with extreme temperatures (Horton et al, 2015). It is crucial to understand the processes that lead to the formation and maintenance of these circulation patterns. Zschenderlein et al (2019) provided an analysis of European heat waves, and in all subregions considered, from the Iberian Peninsula to western Russia, these heat waves were associated with either an upper-tropospheric ridge or a blocking flow pattern. Several earlier studies emphasised that heat waves in the midlatitudes are typically colocated with atmospheric blocking
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