A Lagrangian analysis of upper-tropospheric anticyclones associated with heat waves in Europe

Philipp Zschenderlein,Stephan Pfahl,Andreas H Fink,Heini Wernli

doi:10.5194/egusphere-egu2020-4574

Abstract

&lt;p&gt;Heat waves impose large impacts on various sectors. Meteorologically, these events are co-located to upper-tropospheric anticyclones. In order to elucidate the formation of these anticyclones and the role of diabatic processes, we trace air masses backwards from the upper-tropospheric anticyclones and quantify the diabatic heating in these air parcels. We analyse anticyclones that are connected to summer heat waves at the surface during the period 1979 &amp;#8211; 2016 in different European regions. Around 25-45% of the air parcels are diabatically heated during the last three days prior to their arrival in the upper-tropospheric anticyclones and this amount increases to 35-50% for the last seven days. The influence of diabatic heating is larger for heat wave anticyclones in northern Europe and western Russia and smaller in southern Europe. Interestingly, the diabatic heating occurs in two geographically separated air streams. Three days prior to arrival, one heating branch (western branch) is located above the western North Atlantic and the other heating branch (eastern branch) is located to the southwest of the target upper-tropospheric anticyclone. The diabatic heating in the western branch is related to the warm conveyor belt of a North Atlantic cyclone upstream of the evolving upper-level ridge. In contrast, the eastern 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. Central Europe is influenced by both branches, whereas western Russia is predominantly affected by the eastern branch. The formation of the upper-tropospheric anticyclone, and therefore of the heat wave, is highly depended on the western branch, whereas its maintenance is more affected by the eastern branch. For long-lasting heat waves, the western 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 time scales.&lt;/p&gt;

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

Introduction

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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