Diabatic Heating as a Pathway for Cyclone Clustering Encompassing the Extreme Storm Dagmar

C Weijenborg,T Spengler

doi:10.1029/2019gl085777

Abstract

AbstractContrary to the general notion that extratropical cyclones reduce baroclinicity, the baroclinicity is found to be enhanced in the wake of the extreme winter storm Dagmar. Thus, individual storms can increase baroclinicity, yielding a pathway to secondary cyclogenesis and cyclone clustering. We use a recently introduced diagnostic for baroclinicity—the tendency equation for the isentropic slope—and found that strong diabatic heating due to moisture supply from the subtropical Atlantic led to the enhanced baroclinicity in the rear of Dagmar. Storms ensuing Dagmar benefited from this increased baroclinicity. In contrast to previous studies on the mechanisms of cyclone clustering, we only find weak evidence for Rossby wave breaking and thus propose diabatic heating as an alternative pathway to cyclone clustering.

Highlights

The extratropical cyclone Dagmar that developed over the North Atlantic around 24 December 2011 was one of the most extreme storms for western Norway (Martius et al, 2016; NVE, 2012)
We complement our analysis with a comparison of the isentropic slope framework to the more traditional energy frameworks of Lorenz (1955) and Orlanski and Katzfey (1991) together with an overview of Rossby wave breaking during the event
We diagnosed a cyclone clustering period associated with the extreme storm Dagmar using the isentropic slope framework that provides a tendency equation for the kinematic and diabatic contributions to changes in baroclinicity (Papritz & Spengler, 2015)

Summary

Introduction

The extratropical cyclone Dagmar that developed over the North Atlantic around 24 December 2011 was one of the most extreme storms for western Norway (Martius et al, 2016; NVE, 2012). Previous studies on clustering focus mainly on the spatiotemporal occurrence of cyclones in the Northern Hemisphere, finding that the exit region of the North-Atlantic jet is one of the main regions for cyclone clustering (Mailier et al, 2006; Vitolo et al, 2009) In this area, clustering of cyclones is often accompanied by an intensified eddy-driven jet that is zonally extended towards western Europe (Pinto et al, 2014; Priestley et al, 2017), kept in place by two-sided Rossby wave breaking (Barnes & Hartmann, 2012). Using the isentropic slope diagnostic, we investigate the life cycle of Dagmar with particular focus on the evolution of the baroclinicity over the North Atlantic storm track and the associated period of cyclone clustering. We complement our analysis with a comparison of the isentropic slope framework to the more traditional energy frameworks of Lorenz (1955) and Orlanski and Katzfey (1991) together with an overview of Rossby wave breaking during the event

Methodology and Data

Synoptic Evolution Around the Storm Dagmar

Build-up of Baroclinicity Across the North Atlantic

Energetics of the Evolution

Cyclone Clustering

Discussion and Concluding