Large-scale drivers of the mistral wind: link to Rossby wave life cycles and seasonal variability

Yonatan Givon,Douglas Keller Jr,Romain Pennel,Romain Pennel,Romain Pennel,Romain Pennel,Philippe Drobinski,Philippe Drobinski,Philippe Drobinski,Philippe Drobinski,Shira Raveh-Rubin,Vered Silverman

doi:10.5194/wcd-2-609-2021

Abstract

Abstract. The mistral is a northerly low-level jet blowing through the Rhône valley in southern France and down to the Gulf of Lion. It is co-located with the cold sector of a low-level lee cyclone in the Gulf of Genoa, behind an upper-level trough north of the Alps. The mistral wind has long been associated with extreme weather events in the Mediterranean, and while extensive research focused on the lower-tropospheric mistral and lee cyclogenesis, the different upper-tropospheric large- and synoptic-scale settings involved in producing the mistral wind are not generally known. Here, the isentropic potential vorticity (PV) structures governing the occurrence of the mistral wind are classified using a self-organizing map (SOM) clustering algorithm. Based upon a 36-year (1981–2016) mistral database and daily ERA-Interim isentropic PV data, 16 distinct mistral-associated PV structures emerge. Each classified flow pattern corresponds to a different type or stage of the Rossby wave life cycle, from broad troughs to thin PV streamers to distinguished cutoffs. Each of these PV patterns exhibits a distinct surface impact in terms of the surface cyclone, surface turbulent heat fluxes, wind, temperature and precipitation. A clear seasonal separation between the clusters is evident, and transitions between the clusters correspond to different Rossby-wave-breaking processes. This analysis provides a new perspective on the variability of the mistral and of the Genoa lee cyclogenesis in general, linking the upper-level PV structures to their surface impact over Europe, the Mediterranean and north Africa.

Highlights

The mistral is a northerly gap wind regime located at the Rhône valley in southern France
This coupling between the wind speed in the Gulf of Lion (GOL) and cyclone in the Genoa region is considered the fundamental of lee cyclogenesis in the Alps (Drobinski et al, 2005; Flamant, 2003; Lebeaupin-Brossier et al, 2013; Burlando, 2009) and reassures that the mistral set of events under consideration is analogous to events of lee cyclogenesis, filtering out northerlies stemming from sea-breeze dynamics (Drobinski et al, 2018)
The clusters illustrate a thematic separation of the potential vorticity (PV) continuum responsible for mistral events, and one can envision how the waves propagate by switching from one cluster to another

Summary

Introduction

The mistral is a northerly gap wind regime located at the Rhône valley in southern France. The mistral outflow, composed of continental air masses, picks up moisture at intense evaporation rates over the GOL, before flowing towards the lee cyclone in the Gulf of Genoa (see Fig. 1) and further destabilizing it. Rainaud et al (2017) related strong mistral events to heavy precipitation events occurring along the European–Mediterranean coastline. This relationship is manifested mainly by the remoistening during mistral events and the following flow of this moist air towards the European mountain slopes scattered along the coast. Classified as a dry air outbreak (Flamant, 2003), the mistral brings relatively dry and cold continental air masses

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Conclusion