fmars-08-645762 March 8, 2021 Time: 12:6 # 1 ORIGINAL RESEARCH published: 10 March 2021 doi: 10.3389/fmars.2021.645762 On the Performance of High Frequency Radar in the Western Mediterranean During the Record-Breaking Storm Gloria Pablo Lorente1,2*, Jue Lin-Ye1,3, Manuel García-León1,3, Emma Reyes4, Maria Fernandes5, Marcos Garcia Sotillo1, Manuel Espino3, Maria Isabel Ruiz1, Vicente Gracia3, Susana Perez1, Roland Aznar1,2, Andres Alonso-Martirena5 and Enrique Álvarez-Fanjul1 1 Área de Medio Físico, Puertos del Estado, Madrid, Spain, 2 Nologin Consulting SL, Zaragoza, Spain, 3 Laboratori d’Enginyeria Marítima, Universitat Politècnica de Catalunya, BarcelonaTech, Barcelona, Spain, 4 SOCIB – Edited by: Balearic Islands Coastal Ocean Observing and Forecasting System, Palma, Spain, 5 Qualitas Remos, Madrid, Spain Alejandro Jose Souza, Center for Research and Advanced Studies - Mérida Unit, Mexico Storm Gloria (January 19–24, 2020) hit the NW Mediterranean Sea with heavy rainfall, Reviewed by: strong easterly winds, and very high waves, causing structural damages and 13 Jorge Alejandro Kurczyn Robledo, National Autonomous University of fatalities. The low-lying Ebro Delta (ED) region was severely inundated, ruining rice Mexico, Mexico fields and seaside promenades. A variety of Copernicus Marine Environment Monitoring Christian M. Appendini, Service (CMEMS) modeling and observational products were jointly used to examine National Autonomous University of Mexico, Mexico the fingerprint of Gloria and the response of the upper oceanic layer. According to *Correspondence: the results, Gloria can be interpreted as a high-impact once-in-a-decade metocean Pablo Lorente event where various historical records were beaten. The 99th percentile of several [email protected] parameters (wind speed, significant wave height, wave period, and surface current Specialty section: velocity), derived from long-term observational time series, was persistently exceeded. This article was submitted to The atmospheric surge, albeit not negligible, exerted a secondary role in ED. The Coastal Ocean Processes, a section of the journal ability of a high-frequency radar deployed in this region (HFR-ED) to characterize Frontiers in Marine Science the striking features of the storm was quantified from both waves and circulation Received: 23 December 2020 aspects. Consistent radar current observations were subsequently compared against Accepted: 16 February 2021 the 5-day-ahead forecast of CMEMS Iberia-Biscay-Ireland (IBI) regional ocean model to Published: 10 March 2021 determine, from an Eulerian perspective, the strengths and shortcomings in its predictive Citation: Lorente P, Lin-Ye J, capabilities. Time-averaged maps of surface circulation, superimposed with fields of García-León M, Reyes E, Instantaneous Rate of Separation (IROS), were derived to resolve flow features and Fernandes M, Sotillo MG, Espino M, Ruiz MI, Gracia V, Perez S, Aznar R, identify areas of elevated particles dispersion, respectively. The mean and P99 values Alonso-Martirena A and of IROS almost doubled the historical statistics in the vicinity of the northern Ebro Álvarez-Fanjul E (2021) On hemidelta. Although IBI predicted moderately well basic features of the storm-induced the Performance of High Frequency Radar in the Western Mediterranean circulation, results suggests that coastal transport processes, likely modulated by wave- During the Record-Breaking Storm current interactions, were not fully captured. Furthermore, current estimations from other Gloria. Front. Mar. Sci. 8:645762. doi: 10.3389/fmars.2021.645762 two radar systems, overlooking immediate choke points like the Ibiza Channel and the Frontiers in Marine Science| www.frontiersin.org 1 March 2021| Volume 8| Article 645762 fmars-08-645762 March 8, 2021 Time: 12:6 # 2 Lorente et al. Monitoring Gloria With HF Radars Strait of Gibraltar, evidenced Gloria’s remote-effect in the anomalous circulation patterns observed, that altered the usual water exchanges between adjacent sub-basins. Finally, three-dimensional outcomes from IBI were used to elucidate the impact of this moving storm at different depth levels. Data analyses illustrated that Gloria caused a large increase in kinetic energy and a significant deepening of the mixed layer depth. Keywords: storm gloria, HF radar, Western Mediterranean Sea, CMEMS, ocean model and observations comparison INTRODUCTION fine-scale processes in intricate nearshore areas. In turn, models can reciprocally offer a seamless predictive picture of the three- Any portion of the worldwide coastline can suffer the threat dimensional sea state. of a plethora of metocean hazards, ranging from storm surges, The storm-induced ocean transport, monitored by a HFR and flash-floods and tsunamis, to landfalling hurricanes and tropical simulated by a forecast model, can be compared by adopting typhoons. Natural disasters might impose significant and long- two main approaches. Some methods rely on Lagrangian lasting pressures on coastal ecosystems, resulting frequently time-dependent information, while others require only an in a chronic degradation of marine protected areas (Adger Eulerian snapshot of the velocity field. The former, which et al., 2005; Spalding et al., 2014). Equally, their destructive encompass Lyapunov exponents and Lagrangian Coherent effects in human infrastructures may disrupt essential services, Structures, provide a robust framework to compare transport damage economical assets and even cause casualties, especially properties between different ocean current datasets, allowing in densely populated urban coastal areas (Tintoré et al., 2019; the resolution of coherent flow patterns (Haller, 2015; García- Wolff et al., 2020). Sánchez et al., 2021). However, Lagrangian techniques are often Therefore, the accurate monitoring, understanding and time consuming and computationally more expensive as they prediction of natural weather-related hazards are not only crucial require trajectory integrations (Nolan et al., 2020). The latter, to prompt a wealth of anticipatory adaptation strategies, but also albeit useful to quantify particle dispersion (Futch, 2009 and of great economic value for the maritime sector (Melet et al., 2019; Haza et al., 2010; Poje et al., 2010), cannot distinguish 2020). Such preventive approaches can aid to mitigate devastating between coherent and transient features. Eulerian methods effects and, eventually, strengthen the community resilience at include the computation of various kinematic diagnostics such multiple scales (Klein et al., 2003; Linnenluecke et al., 2012; as the vorticity, the horizontal divergence (DIV), the strain, Summers et al., 2018). Moreover, there is an emerging question or the Instantaneous Rate of Separation (IROS) (Futch, 2019; about the triggering role of both anthropogenic pressures and Schaeffer et al., 2017; Kim, 2010; Archer, 2016). In this global warming on the frequency, duration, spatial extent, and context, there seems to be consensus about the convenience intensity of present and future extreme sea states (Konisky et al., of combining both approaches, when possible, to adequately 2015; Sánchez-Arcilla et al., 2016; Grases et al., 2020). explore transport processes at (sub) mesoscale ranges. A halfway Despite of the increasing complexity and steady upgrade of approach, denominated Eulerian Coherent Structures, has been regional ocean forecast models in terms of accuracy to reproduce recently developed to connect Eulerian quantities to short-term large-scale physical processes, local phenomena are still poorly Lagrangian transport (Nolan et al., 2020; Serra et al., 2020). replicated at coastal scales. This is especially true for extreme Storm Gloria hit the Western Mediterranean Sea (WMS; events (Rockel, 2015; Lorente et al., 2019a). Over the last decades, Figure 1A) from January 19–24, 2020. Eastern parts of Spain High-Frequency radar (HFR) has emerged as a useful tool to were ravaged with heavy rainfall, strong easterly winds, and unveil hydrodynamic aspects still uncovered by current state- high waves that beat historical records (Amores et al., 2020), of-the-art regional ocean models. HFRs provide, in near real causing widespread flooding, severe structural damages and 13 time, reliable wave information and fine-resolution maps of the fatalities. The low-lying Ebro river Delta, a large wetland with surface circulation over broad coastal areas (Rubio et al., 2017; a rich ecology, was dramatically affected by this unprecedented Roarty et al., 2019). HFRs present a wide range of practical episode (Figures 1B,C). Despite the significant cloud cover, applications, among others: analysis of transport processes (for Sentinel-2 L1C Satellite imagery could adequately portray the search and rescue operations, oil spill emergencies, etc.) or the adverse impact of Gloria at local scale. Daily snapshots of the effective monitoring of extreme events, such as storm surges, Normalized Difference Water Index (NDWI), an adimensional tsunamis, typhoons, and hurricanes (Suzuki, 2015; Saramul, parameter proposed by McFeeters(1996), revealed an abrupt 2017; Miles et al., 2017; Toguchi et al., 2018; Dao et al., 2019; change on specific water bodies from January 18 (Figure 1B) Lipa et al., 2019; Serra et al., 2020). Given the consolidation to January 23 (Figure 1C), during Gloria’s passage. Although of this remote sensing technology, the synergistic integration of both images do not discriminate between storm surge (i.e., salty HFRs and hydrodynamic models has demonstrated