On the dynamical coupling between atmospheric blocks and heavy precipitation events: A discussion of the southern Alpine flood in October 2000

Abstract

In October 2000, a high-impact lake flood event occurred in southern Switzerland. During the month prior to the flood event three heavy precipitation events (HPEs) occurred. The first two events preconditioned the catchment and brought the lake close to its flood level. During the third event the lake level rose above the flood threshold. At the same time, anomalously high blocking activity was observed in the northern North Atlantic/European region. This study describes the synoptic development during the month prior to the flood and investigates the role of atmospheric blocking in the formation of the HPEs using ERA-Interim data. Atmospheric blocks are identified as persistent negative potential vorticity (PV) anomalies in the upper troposphere. All three heavy precipitation events were forced by upper-level equatorward elongated streams of stratospheric high-PV air (PV streamers). These PV streamers formed in the strong deformation field upstream and downstream of single blocks or in between two blocks. During the third and most persistent heavy precipitation episode the eastward propagation of the PV streamer was prevented by a downstream block for several days leading to a stationary upper-level northeastward flow and a prolonged period of heavy precipitation over the catchment. The study identifies and quantifies a potential feedback between heavy precipitation and blocks via diabatic depletion of PV. It is shown that a substantial fraction of the diabatically modified low-PV air (63%) that reached and strengthened the blocks over the Atlantic and Europe during this month experienced heating in HPE areas.

Relationship between atmospheric blocking and warm‐season thunderstorms over western and central Europe

“…; Lenggenhager et al . ; Lenggenhager and Martius, ). For example, the central European flooding of June 2013 (Schröter et al .…”

“…In addition, warm air advection to the western sector of the block can contribute to warm anomalies within blocks (e.g., the Russian heatwave in 2010; Trenberth and Fasullo, 2012;Quandt et al, 2017). A relation also exists between blocking and heavy precipitation or flood events (including flash floods; Sousa et al, 2017;Lenggenhager et al, 2018;Lenggenhager and Martius, 2019). For example, the central European flooding of June 2013 (Schröter et al, 2015) was influenced by an Atlantic blocking regime that resulted in cool and unusually wet conditions due to repeated Rossby wave breaking over Europe (Grams et al, 2014).…”

Size of the Atmospheric Blocking Events: Scaling Law and Response to Climate Change

“…Atmospheric blocking events are large-scale, quasi-stationary, high-pressure systems that persist for extended periods (e.g., several days to weeks) and block or divert the midlatitude jet (Rex, 1950;Woollings et al, 2018). These anomalous weather patterns, whose dynamics are still not well understood (Hassanzadeh & Kuang, 2015;Nakamura & Huang, 2018;Woollings et al, 2018), can cause or contribute to extreme events such as heat waves, droughts, and cold spells (Barriopedro et al, 2011;Dole et al, 2011;Lau & Kim, 2012;Lenggenhager et al, 2019;Trigo et al, 2005;Woollings et al, 2018). As a result, there has been significant interest in understanding how blocking events might change with climate change (Davini et al, 2012;Matsueda & Endo, 2017;Masato et al, 2013;Woollings et al, 2018).…”

Reconstruction and simulation of an extreme flood event in the Lago Maggiore catchment in 1868

“…During this recent event, several episodes of heavy precipitation occurred in close succession (Barton et al, 2016). The final episode lasted for several days with a downstream blocking anticyclone over central and eastern Europe, preventing an eastward progression of the breaking wave and PV-streamer (Lenggenhager et al, 2018).…”