Master thesis defense by Lærke Halkjær

Title: Detecting and assessing a climatological baseline of potential supercells in DANRA 

Abstract: 
Supercells are among the most hazardous convective phenomena, capable of producing damaging winds, large hail, and tornadoes. Despite their destructive potential, a comprehensive climatology for supercells in Northern Europe and Denmarkislargelynon-existent due to them being rare and relying oneyewitness reports and radarobservations. Thisstudyestablishes the first climatological baseline of supercell activity based on the Danish Reanalysis (DANRA) from the Danish Meteorological Institute (DMI), covering parts of Northern Europe in the period from 1990 to 2023 at a spatial resolution of 2.5 km. Athreshold-based detection method has been developed and validated against known Danish meso cyclonic tornado events. The results identify a positive trend in supercell potential across the DANRA domain from period 1 (1991-2007) to period 2 (2008-2023), where there is a relative increase in the number of supercells by +47.0% in Denmark and a spatial mean frequency rise of +58.5%. The high est frequencies are observed in the Benelux countries, Northern Germany, and Poland, with a notable seasonal shift toward more frequent events in August. An analysis of the atmospheric parameters reveals that the increasing frequencyis primarily explained by changes in vertical wind shear, while thermodynamic instability such as CAPE is close to unchanged. Danish supercells are found to differ from those described by American-based indices; with a median SBCAPE around 750 J/kg and low Bulk Richardson Numbers (BRN), these events are predominantly mini-supercells. Validation of the 2021 Snertinge tornado case confirms that the DANRA model possesses the phys ical ability to reproduce classic supercell structures, including ”flying eagle” signatures in the vertical velocity. While the method detects the theoretical potential for supercells rather than a 1:1 observational record, the findings still underscore a growing need for specialized forecasting tools to support opera tional meteorologists in potentially warning against the risks posed by increasingly frequent and intense supercell events in Denmark.

Supervisors: Jens Hesselbjerg Christensen (hesselbjerg@nbi.ku.dk), Aksel Walløe Hansen (awh@nbi.ku.dk)