Master Thesis Defense by Mikkel Noye Knudsen

Title: Linking Atmospheric Blocking to Precipitation Extremes: Insights from ERA5 Reanalysis with a Focus on Europe

Abstract:
Atmospheric blocking is a persistent large-scale circulation pattern that disrupts the mid-latitude westerlies and is frequently associated with high-impact weather extremes. While the link between blocking and extreme precipitation is widely recognized, it remains insufficiently quantified in terms of spatial structure and physical mechanisms. This study investigates the relationship between atmospheric blocking and extreme precipitation events over Europe during boreal summer (JJA), with a particular focus on the spatial dependence of this connection. 

Using ERA5 reanalysis data for geopotential height at 500 hPa and daily precipitation from the GPCP dataset for the period 2000–2021, extreme precipitation events are defined as the 99th percentile of 3-day accumulated precipitation. Atmospheric blocking is identified using two established but fundamentally different indices: an anomaly-based index following Dole and Gordon (1983) and a gradient-based index following Scherrer et al. (2006). The co-occurrence of blocking and extreme precipitation is quantified through conditional probabilities and expressed using a Risk Ratio framework.

The results reveal a pronounced spatially dependent relationship between blocking and extreme precipitation. Within the immediate vicinity of blocking centers (radii < 10°), the likelihood of extreme precipitation is significantly reduced, consistent with subsidence and atmospheric stability in the core of blocking anticyclones. In contrast, at synoptic distances of 10°–20°, the presence of blocking substantially increases the probability of extreme precipitation, with Risk Ratios exceeding 3–5 in several regions. This enhancement reflects the role of blocking as a remote dynamical driver that stalls or deflects synoptic-scale weathersystems, prolonging moisture convergence and rainfall duration.

The findings demonstrate that atmospheric blocking acts as a dual mechanism: locally suppressing precipitation while remotely amplifying the risk of long-duration, high-impact rainfall events. The comparison of blocking indices further suggests that anomaly-based definitions may be more effective for identifying circulation regimes relevant to flood risk. Overall, this study highlights the importance of large-scale circulation persistence in shaping the most extreme precipitation events in Europe and provides insight into the dynamical conditions underlying hydrological extremes.

Supervisor Jens Hesselbjerg Christensen
Co-Supervisor Aksel Walløe Hansen