Investigating the impact of European heat waves on the lower atmosphere

Motivated by heat waves impacts on human health and the economy, research on this type of extreme event generally focuses on near surface variables. In this study, we broaden the view by looking at the effects on the vertical structure of the atmospheric boundary layer. Previous research by Miralles et al. (2014) reports extreme boundary layer heights during the severe heat waves of 2003 and 2010 and suggests a correlation between mean potential temperature in the lower atmosphere and the boundary layer height. We investigate whether these findings are common to European heat waves in general with the aim of getting insights into their formation and persistence. To get a comprehensive analysis, we compare summer time vertical profiles taken from reanalysis, namely CERRA and COSMO-REA6, as well as German radio sonde observations between 2014 and 2018. We follow the method of Szemkus et al. (in press) to extract extremal spatial patterns of two meter temperature from the reanalyses. This information is used to define heat waves. Furthermore, the atmospheric boundary layer heights in all three data sources are estimated either by the well established Bulk-Richardson-Number based method or a self-developed machine learning approach. We then compare empirical distributions of boundary layer heights during heat waves and normal conditions on a domain wide scale and grid point wise to account for regional differences. Additionally, we also extract extremal spatial patterns from the height data using the aforementioned method to compare them to the patterns found for temperature. The results of our work could possibly be used to improve the discriminability of different severity levels of heat waves or to formulate a heat wave measure that is not based solely on surface variables.