Origin of low-tropospheric potential vorticity in Mediterranean cyclones

Scherrmann, Alexander; Wernli, Heini; Flaounas, Emmanouil

doi:10.5194/wcd-4-157-2023

Cited by 7 publications

(3 citation statements)

References 50 publications

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“…In this study, we choose composite tracks of confidence level 5, denoting the agreement of at least five cyclone detection methods upon the presence of a cyclone for at least one time step in its lifetime. At this confidence level, the seasonal cycle attains a distinct cycle with a minimum in summer and a maximum in spring, compared to confidence levels of four or less, suggesting the removal of shallow heat lows that are of lesser interest from a dynamic perspective (Scherrmann et al, 2023). The cyclone's center position and sea-level pressure (SLP) minimum are tracked throughout its lifetime.…”

Section: Combined Cyclone-tracking Algorithmmentioning

confidence: 85%

Process-based classification of Mediterranean cyclones using potential vorticity

Givon,

Hess,

Flaounas

et al. 2024

Weather Clim. Dynam.

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Abstract. Mediterranean cyclones (MCs) govern extreme weather events across the Euro-African Basin, affecting the lives of hundreds of millions. Despite many studies addressing MCs in the last few decades, their correct simulation and prediction remain a significant challenge to the present day, which may be attributed to the large variability among MCs. Past classifications of MCs are primarily based on geographical and/or seasonal separations; however, here we focus on cyclone genesis and deepening mechanisms. A variety of processes combine to govern MC genesis and evolution, including adiabatic and diabatic processes, topographic influences, land–sea contrasts, and local temperature anomalies. As each process bears a distinct signature on the potential vorticity (PV) field, a PV approach is used to distinguish among different “types” of MCs. Here, a combined cyclone-tracking algorithm is used to detect 3190 Mediterranean cyclone tracks in ECMWF ERA5 from 1979–2020. Cyclone-centered, upper-level isentropic PV structures in the peak time of each cyclone track are classified using a self-organizing map (SOM). The SOM analysis reveals nine classes of Mediterranean cyclones, with distinct Rossby-wave-breaking patterns, discernible in corresponding PV structures. Although classified by upper-level PV structures, each class shows different contributions of lower-tropospheric PV and flow structures down to the surface. Unique cyclone life cycle characteristics, associated hazards (precipitation, winds, and temperature anomalies), and long-term trends, as well as synoptic, thermal, dynamical, seasonal, and geographical features of each cyclone class, indicate dominant processes in their evolution. Among others, the classification reveals the importance of topographically induced Rossby wave breaking to the generation of the most extreme Mediterranean cyclones. These results enhance our understanding of MC predictability by linking the large-scale Rossby wave formations and life cycles to coherent classes of under-predicted cyclone aspects.

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Section: Combined Cyclone-tracking Algorithmmentioning

confidence: 85%

Process-based classification of Mediterranean cyclones using potential vorticity

Givon,

Hess,

Flaounas

et al. 2024

Weather Clim. Dynam.

Self Cite

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“…Later, accompanied by an upper-level PV-cutoff that detached from the trough, it swayed north-eastward, toward the Mediterranean Sea, where it deepened and stalled. At its peak, to the north of the Nile Delta, the cyclone reached a minimum pressure of 994 hPa, which ranks it among the deepest Mediterranean cyclones (e.g., Scherrmann et al, 2023). This was also the cyclone stage leading to most of the rainfall during its passage through the northern Sahara, implying that Mediterranean-origin moisture may be involved in this HPE.…”

Section: Early Spring Hpe In the Dry Core Of The Saharamentioning

confidence: 92%

Saharan Rainfall Climatology and its Relationship with Surface Cyclones

Armon¹,

Vries

Marra

et al. 2023

Preprint

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“…Furthermore, they do not involve the unique geographical features of the Mediterranean basin, which have been shown to be a key factor for cyclone dynamics in the Mediterranean (e.g. Flaounas et al, 2022;Scherrmann et al, 2023).…”

Section: Introductionmentioning

confidence: 99%

The upstream-downstream mechanism of North Atlantic and Mediterranean cyclones in semi-idealized simulations

Scherrmann,

Wernli,

Flaounas

2023

Preprint

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Abstract. Cyclogenesis in the Mediterranean typically follows an archetypal scenario with the intrusion of a potential vorticity (PV) streamer over the Mediterranean, which results from a preceding Rossby Wave breaking (RWB) upstream over the North Atlantic. The RWB is amplified by the presence of warm conveyor belts (WCBs) in at least one North Atlantic cyclone about 4 days prior to Mediterranean cyclogenesis. This scenario has been found in several case studies of intense Mediterranean cyclones with a variety of different North Atlantic cyclone tracks and intensities, and different locations of PV streamers intruding the Mediterranean. While four main events (namely North Atlantic cyclone, WCBs, RWB and the resulting PV streamers) are suggested to be necessary for Mediterranean cyclogenesis, they rarely occur in a spatially consistent, fully repetitive pattern. To more systematically study this link between the upstream North Atlantic cyclone dynamics and the Mediterranean cyclogenesis, we perform a set of semi-idealized simulations over the Euro-Atlantic domain. For these simulations, we prescribe a constant climatological atmospheric state in the initial and boundary conditions. To trigger the downstream Mediterranean cyclogenesis scenario, we perturb the climatological polar jet through the inversion of a positive upper-level PV anomaly. The amplitude of this perturbation determines the intensity of the triggered North Atlantic cyclone. This cyclone provokes RWB, the intrusion of a PV streamer over the Mediterranean, and the formation of a Mediterranean cyclone. Therefore, our results show a direct causality between the presence of a North Atlantic cyclone and the downstream intrusion of a PV streamer into the Mediterranean, which causes cyclogenesis about 4 days after perturbing the polar jet, which we refer to as the upstream-downstream mechanism of cyclogenesis. To investigate the sensitivity of this mechanism, we vary the position and amplitude of the upper-level PV anomaly. In all simulations, cyclogenesis occurs in the Mediterranean. Nevertheless, the evolution, track and intensity of the Mediterranean cyclones are sensitive to the dynamical structure and intensity of the intruding PV streamer, which itself is sensitive to the interaction of the upstream cyclone and the RWB. By applying different seasonal climatological atmospheric states as initial conditions we show that the seasonal cycle of Mediterranean cyclogenesis indeed depends on the large-scale atmospheric circulation. In particular, we show that the Mediterranean cyclones in these semi-idealized simulations show characteristics that agree with the observed climatology of Mediterranean cyclones in the respective season.

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Origin of low-tropospheric potential vorticity in Mediterranean cyclones

Cited by 7 publications

References 50 publications

Process-based classification of Mediterranean cyclones using potential vorticity

Process-based classification of Mediterranean cyclones using potential vorticity

Saharan Rainfall Climatology and its Relationship with Surface Cyclones

The upstream-downstream mechanism of North Atlantic and Mediterranean cyclones in semi-idealized simulations

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