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Earth System Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/esd-2020-37
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/esd-2020-37
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  08 Jul 2020

08 Jul 2020

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This preprint is currently under review for the journal ESD.

A New View of Heat Wave Dynamics and Predictability over the Eastern Mediterranean

Assaf Hochman1, Sebastian Scher2, Julian Quinting1, Joaquim G. Pinto1, and Gabriele Messori2,3 Assaf Hochman et al.
  • 1Department of Tropospheric Research, Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Germany
  • 2Department of Meteorology and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • 3Department of Earth Sciences, Uppsala University, Uppsala, Sweden

Abstract. Skillful forecasts of extreme weather events have a major socio-economic relevance. Here, we compare two complementary approaches to diagnose the predictability of extreme weather: recent developments in dynamical systems theory and numerical ensemble weather forecasts. The former allows us to define atmospheric configurations in terms of their persistence and local dimension, which inform on how the atmosphere evolves to and from a given state of interest. These metrics may be used as proxies for the intrinsic predictability of the atmosphere, which depends exclusively on the atmosphere’s properties. Ensemble weather forecasts inform on the practical predictability of the atmosphere, which primarily depends on the performance of the numerical model used. We focus on heat waves affecting the Eastern Mediterranean. These are identified using the Climatic Stress Index (CSI), which was explicitly developed for the summer weather conditions in this region and differentiates between heat waves (upper decile) and cool days (lower decile). Significant differences are found between the two groups from both the dynamical systems and the numerical weather prediction perspectives. Specifically, heat waves show relatively stable flow characteristics (high intrinsic predictability), but comparatively low practical predictability (large model spread/error). For 500 hPa geopotential height fields, the intrinsic predictability of heat waves is lowest at the event’s onset and decay. We relate these results to the physical processes governing Eastern Mediterranean summer heat waves: adiabatic descent of the air parcels over the region and the geographical origin of the air parcels over land prior to the onset of a heat wave. A detailed analysis of the mid-August 2010 record-breaking heat wave provides further insights into the range of different regional atmospheric configurations conducive to heat waves. We conclude that the dynamical systems approach can be a useful complement to conventional numerical forecasts for understanding the dynamics of Eastern Mediterranean heat waves.

Assaf Hochman et al.

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Short summary
Skillful forecasts of extreme weather events have a major socio-economic relevance. Here, we compare two approaches to diagnose the predictability of Eastern Mediterranean heat waves: one based on recent developments in dynamical systems theory and one leveraging numerical ensemble weather forecasts. We conclude that the former can be a useful and cost-efficient complement to conventional numerical forecasts for understanding the dynamics of Eastern Mediterranean heat waves.
Skillful forecasts of extreme weather events have a major socio-economic relevance. Here, we...
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