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Dynamics of a local Alpine flooding event in October 2011: moisture source and large‐scale circulation
Author(s) -
Piaget Nicolas,
Froidevaux Paul,
Giannakaki Paraskevi,
Gierth Franziska,
Martius Olivia,
Riemer Michael,
Wolf Gabriel,
Grams Christian M.
Publication year - 2014
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.2496
Subject(s) - trough (economics) , climatology , ridge , geology , precipitable water , advection , rossby wave , gulf stream , precipitation , potential vorticity , flooding (psychology) , moisture , atmospheric circulation , subtropical ridge , geography , vorticity , meteorology , vortex , psychology , paleontology , physics , economics , psychotherapist , macroeconomics , thermodynamics
Alpine heavy precipitation events often affect small catchments, although the circulation pattern leading to the event extends over the entire North Atlantic. The various scale interactions involved are particularly challenging for the numerical weather prediction of such events. Unlike previous studies focusing on the southern Alps, here a comprehensive study of a heavy precipitation event in the northern Alps in October 2011 is presented with particular focus on the role of the large‐scale circulation in the North Atlantic/European region. During the event exceptionally high amounts of total precipitable water occurred in and north of the Alps. This moisture was initially transported along the flanks of a blocking ridge over the North Atlantic. Subsequently, strong and persistent northerly flow established at the upstream flank of a trough over Europe and steered the moisture towards the northern Alps. Lagrangian diagnostics reveal that a large fraction of the moisture emerged from the West African coast where a subtropical upper‐level cut‐off low served as an important moisture collector. Wave activity flux diagnostics show that the ridge was initiated as part of a low‐frequency, large‐scale Rossby wave train while convergence of fast transients helped to amplify it locally in the North Atlantic. A novel diagnostic for advective potential vorticity tendencies sheds more light on this amplification and further emphasizes the role of the ridge in amplifying the trough over Europe. Operational forecasts misrepresented the amplitude and orientation of this trough. For the first time, this study documents an important pathway for northern Alpine flooding, in which the interaction of synoptic‐scale to large‐scale weather systems and of long‐range moisture transport from the Tropics are dominant. Moreover, the trapping of moisture in a subtropical cut‐off near the West African coast is found to be a crucial precursor to the observed European high‐impact weather.