## 5 Sea Conditions Analysis of the Sea Storm Event of 28 December 2020  
The coastal area of Naples hit by the sea storm is showed in Figure 3a. The entire waterfront of Via Caracciolo/Via Partenope was affected by the storm, but the effects appear to increase in magnitude from Mergellina towards Castel dell'Ovo (from West to East), likely due to the relatively lower protection offered by the Posillipo promontory to the eastern part of the waterfront with respect to specific direction of wave motion (SSW-NNE) (Figure 9). Via Partenope was the waterfront area most affected by damage,  
Figure 8: Lago Patria, Capo Posillipo, and Port of Naples ISMAR weather stations wind diagrams in reference to the time interval: 28 December 2020-H00.00 to H24:00 UTC. Figure 7: **(a)** Satellite image (EUMETSAT 2020) and (**b**) sea level barometric map (CFS Reanalysis) relative to 28 December 2020-H18:00 UTC. The intense cyclonic system over France is characterized by a complex system of occluded fronts (magenta lines). Blue lines are cold fronts, and red lines are warm fronts. as highlighted in yellow in the inlet image of Figure 10. The main wave exposure angle is the angular sector encompassing the directions from which the largest waves can affect the study area. This angle (in reference to the north) is determined by taking the half-lines tangent to the promontories that mostly restrict the direction to the open sea, in our case, the western margin of Capri Island (named Punta Arcera) and Capo Posillipo (Figure 9). Considering the coastline point P\({}_{0}\), in correspondence of \"Via Partenope\", we defined the main wave exposure angle relative to point P\({}_{1}\), where it is possible to consider undisturbed the wave motion, due to the absence of wave energy dissipation by interference with the seafloor. These last elements modify the direction of propagation of wave fronts due to diffraction phenomena. This point is located at the intersection between the axis of the main wave exposure angle and a water depth H of \(-34\) m (Figure 10). Figure 10: Position of the point P\({}_{0}\) (coastal waterfront point) and P\({}_{1}\) (offshore point, with seafloor at \(-34\) m), significant in analysis of wave motion. Base map by Reference [47]. Figure 9: Main wave exposure angle relative to point P\({}_{1}\) in front of Via Partenope. The value H = \(-\)34 m is equal to the maximum characteristic semi-wave length (L/2) measured during the observed meteorological event:  
\[\text{L}=1.56\ \text{T}_{\text{s}}{}^{2}=1.56\ \cdot 6.6^{2}=68\ \text{m}. \tag{3}\]  
In the case of the study area, the geographical fetch is wide. Most of the directions included in the sector relative to point P1 intercept the coasts of North Africa (with fetch extending over 600 km) and the northern coasts of Sicily (with fetch extending approximately 300 km). This means that a large possibility exists of wind energy transfer to the sea surface and consequent increase of wave dimensions (length and height). In the Mediterranean area, the barometric lows generating the major wind events, and greatest wave heights, have not a full compliance with the definition of fetch. Particularly, wind usually does not have constant direction and intensity over the entire length of the fetch. For this reason, the geographical fetch has been locally limited to the value of 250 nautical miles (about 460 km) (Table 4). The barometric conditions on 28 December over the central Mediterranean showed a high rate of exploitation of fetches in the directions of the main wave exposure angle (Figure 11), as the anemometric event discussed in this study has constantly maintained a direction of about 210\({}^{\circ}\). In this condition, the role of wave motion from angular sectors external to the main wave exposure angle is not significant, which makes the evaluation of the effective fetch unnecessary. The wind event triggering the sea storm began in the early morning of 28 December.