A composite approach to produce reference datasets for extratropical cyclone tracks: Application to Mediterranean cyclones

Abstract

Abstract. Many cyclone detection and tracking methods (CDTMs) have been developed in the past to study the climatology of extratropical cyclones. However, all CDTMs have different approaches in defining and tracking cyclone centers. This naturally leads to cyclone track climatologies of inconsistent physical characteristics. More than that, it is typical for CDTMs to produce a non-negligible amount of bogus tracks which can be perceived as “false positives”, or more generally as CDTM artifacts, i.e. tracks of weak atmospheric features that do not correspond to large or mesoscale vortices. Lack of consensus in CDTM outputs and the inclusion of significant amounts of bogus tracks therein, has long prohibited the production of a commonly accepted reference dataset of extratropical cyclone tracks. Such a dataset could allow comparable results on the analysis of storm track climatologies and could also contribute to the evaluation and improvement of CDTMs. To cover this gap, we present a new methodological approach that combines overlapping tracks from different CDTMs and produces composite tracks that concentrate the agreement of more than one CDTM. In this study we apply this methodology to the outputs of 10 well-established CDTMs which were originally applied to ERA5 reanalysis in the 42-year period of 1979–2020. We tested the sensitivity of our results to the spatio-temporal criteria that identify overlapping cyclone tracks, and for benchmarking reasons, we produced five reference datasets of subjectively tracked cyclones. Results show that climatological numbers of composite tracks are substantially lower than the ones of individual CDTM, while benchmarking scores remain high (i.e. counting the number of subjectively tracked cyclones captured by the composite tracks). This suggests that our method is able to filter out a large portion of bogus tracks. Indeed, our results show that composite tracks tend to describe more intense and longer-lasting cyclones with more distinguished early, mature and decay stages than the cyclone tracks produced by individual CDTMs. Ranking the composite tracks according to their confidence level (defined by the number of contributing CDTMs), it is shown that the higher the confidence level, the more intense and long-lasting cyclones are produced. Given the advantage of our methodology in producing cyclone tracks with physically meaningful, distinctive life stages and including a minimum number of bogus tracks, we propose composite tracks as reference datasets for climatological research in the Mediterranean. The supplementary material provides the composite Mediterranean tracks for all confidence levels and in the conclusion we discuss their adequate use for scientific research and applications.

A New Refinement of Mediterranean Tropical‐Like Cyclones Characteristics

“…The WCC tracks are provided by a composite approach aimed at producing a reference data set for Mediterranean cyclones (Flaounas et al., 2023). This method combines overlapping tracks from different cyclones' detection and tracking methods and produces composite tracks also containing the information about the agreement of the different methods.…”

“…The cyclone tracking is available through Flaounas et al. (2023) and all data for the cyclone characterization have been obtained through Hersbach et al., 2023a; Hersbach et al., 2023b.…”

Linking compound weather extremes to Mediterranean cyclones, fronts and air-streams

“…The cyclone tracks are a result of the efforts of the MedCyclones COST Action 4 to produce a reference dataset of cyclone intensity and position (i.e., the value and location of the SLP minimum) within the Mediterranean region. The composite tracking approach described in Flaounas et al (2023) combines ten different cyclone detection and tracking methods applied to the ERA5 reanalysis (Hersbach et al, 2020) to identify cyclones spending at least 24 h in a broad Mediterranean domain (20 • -50 • N and 20 • W-45 • E). Although the tracks are computed on the original ERA5 temporal and spatial resolution, in this study, for consistency with the compound data described in Section 2.1, we interpolate the position onto a 0.5 • horizontal grid and select the time steps coinciding with the hours 00, 06, 12, 18 UTC.…”

“…As in recent articles using the Flaounas et al (2023) cyclone dataset (see Givon et al, 2023;Rousseau-Rizzi et al, 2023), we select the tracks with confidence-level of 5, i.e., those satisfying an agreement among a minimum of five detection methods for at least 12 h in the cyclone's lifetime. The selection yields 3190 cyclones from 1979 to 2020, excluding many shallow heat lows occurring in summer or in the transition seasons (Givon et al, 2023).…”

Surface compound extremes and Mediterranean cyclones

“…The co-occurrence of extreme events, resulting generally in stronger surface impacts compared to isolated extreme events, constututes a hazard for the public and can be associated with severe damage to buildings, infrastructure and means of transport. Within the context of the MedCyclone initiative (see e. g. the Mediterranean cyclone track dataset defined in [1]), we investigate the "impact area" of the cyclones crossing the Mediterranean region by linking their passage with the appearance of hazards at the surface, in particular extremes of accumulated precipitation, near surface wind intensity, height of combined swell and wave. This analysis is expected to produce an estimate of the fraction of compound extremes -combinations of the extremes listed aboveassociated with the MedCyclones tracks in the recent climatological period, including geographical and seasonal details.…”