Master Thesis Defense by Rune Magnus Koktvedgaard Zeitzen

Title: The Winds of Change - A Sensitivity Study of a Powerful Extratropical Cyclone Using the HARMONIE-AROME Numerical Weather Model

Abstract:
Extratropical cyclones, which are an everyday weather phenomena in western Europe, can at times develop into powerful European windstorms, that in the worst cases may cause damages in excess of EUR 1 billion. Despite their damage potential, questions still remain on their sensitivities to the changes brought on by anthropogenic climate change, which is critical in a mitigation context.

In this study, cyclone Anatol, a powerful European windstorm which impacted Denmark on December 3rd, 1999, is investigated through a Pseudo Global Warming (PGW) approach, using the HARMONIE-AROME numerical weather model. The sensitivity study focuses both on a storyline approach, and understanding what sensitivities the cyclone may have. This thesis builds on the work by, utilising the same methods, where the PGW approach is used to investigate the 2011 extreme cloudburst of Copenhagen, through an older update cycle of the HARMONIE-AROME model. 

This study concludes that a spatially uniform PGW modification leads to locally enhanced winds from the cyclone over Denmark, potentially driven by mesoscale instabilities in the cyclone core, but leads to insignificant differences on the synoptic scale. The mesoscale instabilities are identified as barotropic instabilities along the back-bent front of the cyclone, which are a feature only resolvable in high resolution kilometre-scale simulation, which demonstrates the advantages in utilising high resolution modelling. Additional experiments reveal a sensitivity in the development of the windstorm to the lower-tropospheric lapse rates, leading to an enhanced cyclone and winds if the static stability is decreased. The lapse rates are also linked to enhanced gusts over land, with a reduction in static stability leading to enhanced momentum transfer from low level jets to the surface. 

Through the storyline approach, the results of the study bear significance in a climate impact mitigation context, as cyclone Anatol is widely regarded as one of the most potent European windstorms to impact Denmark, and in this study, it is enhanced.  

Supervisors: Jens Hesselbjerg Christensen and Henrik Vedel (DMI)
Censor: Peter Aakjær (DTU)