Master thesis defense by Jonas Hermann Damsbo

Title: A Case Study of Severe Cloudburst Events in Denmark

Abstract: Research shows that extreme rain, including cloudbursts (more than 15 mm precipitation in 30 minutes), has become more frequent in Denmark.

In this thesis we investigate how cloudburst events occur under Danish weather conditions, with analyses of seven different cases of cloudburst events from the past 25 years and. The overall purpose of the thesis is to make an analysis of the development of cloudbursts in Denmark and then propose an overview of how these weather phenomena can occur in Denmark.

Cloudbursts occur during Deep moist convection and are possible to cause major damage, such as floods.

The analysis is primarily based on weather maps, satellite images, radar images and soundings.

In the analysis we show that cloudbursts in Denmark is a phenomenon that primarily occurs in the period May to September - typically in the afternoon or evening where sun forcing creating favorable conditions for convection during quasi-geostrophic forcing.

We further investigate that jet streaks also prove to be an important trigger for cloudbursts. Extratropical cyclogenesis in connection with quasi-geostrophic forcing is a factor in one of the cases. In some cases, orographic forcing also affects how the air interacts at the convergence lines. In Denmark, the severe cloudbursts generally organize as multicells, but in case 7 the cloudburst was likely a low-topped supercell.

We find that the synoptic situation that is particularly favorable for cloudbursts, is a deepening low pressure west of Denmark with cold and dry air, and a high pressure east of Denmark with warm and humid air. These air flows create very good conditions for potential or conditional instability when these air masses collide over Denmark. Thus, the conditions are set for thunder cells to form along a frontal zone. These thunder cells have the potential to develop cloudbursts.

Supervisors: Aksel Walløe Hansen and Niels Woetmann Nielsen

Censor: Peter Aakjær

Participating by using the following zoom link: https://ucph-ku.zoom.us/j/63170995363