Cyclonic Activity In The Mediterranean Region From A High-Resolution Perspective Using Ecmwf Era5 Dataset

Abstract

This study focuses on developing a new Cyclone Detection and Tracking Method (CDTM) to take advantage of the recent availability of a high-resolution reanalysis dataset of ECMWF ERA5. The proposed algorithm is used to perform a climatological analysis of the cyclonic activity in the Mediterranean Region (MR) into a 40-year window (1979–2018). The tuning of the new CDTM was based on the comparison with currently available CDTMs and verified through careful subjective analysis to fully exploit the finer details of MR cyclones features. The application of the new CDTM to the ERA5 high-resolution dataset resulted in an increase of 40% in the annual number of cyclones, mainly associated with subsynoptic and baroclinic driven lows. The main cyclogenetic areas and seasonal cycle were properly identified into the MR context, including areas often underestimated, such as the Aegean Sea, and emerging new ones with cyclogenetic potential such as the coast of Tunisia and Libya. The better cyclone features description defined three distinct periods of cyclonic activity in the MR with peculiar and persistent characteristics. In the first period (Apr-Jun), cyclones develop more frequently and present higher velocities and deepening rates. In the second (Jul-Sep), the cyclonic activity is governed by thermal lows spreading slowly over short tracks without reaching significant depths. In the last and longest season (Oct-Mar), cyclones become less frequent, but with the highest deepening rates and the lowest MSLP values, ranking this period as the most favourable to intense storms.

Origin of low-tropospheric potential vorticity in Mediterranean cyclones

“…The values in Fig. 2 are in accordance with a recent study by Aragão and Porcù (2022), who quantified the cyclone activity in the Mediterranean using ERA5. Considering seasonality, we find the largest number of cyclones in winter (DJF) and about half that number during summer (JJA), representing a seasonal cycle similar to the one presented in Flaounas et al (2015).…”

“…Furthermore, this is about twice the climatological PV value over the Mediterranean Sea between 975-700 hPa, and thus, we consider it as a good threshold for the lower-tropospheric positive PV anomaly. We compute backward trajectories using LAGRANTO (Wernli and Davies, 1997;Sprenger and Wernli, 2015) until 48 h prior to the mature stage of the cyclone (t = −48 h), which is an adequate time period considering the short life span of Mediterranean cyclones (Trigo et al, 1999;Aragão and Porcù, 2022). Along these, we trace surface pressure, PV, and in the IFS simulations the available PV rates due to different diabatic processes.…”