Lagrangian Trajectory Modelling for a Person lost at Sea duringAdriatic Scirocco Storm of 29 October 2018

Abstract. On 29 October 2018 a windsurfer's mast broke about 1 km offshore during a severe Scirocco storm in the Northern Adriatic Sea. He was drifting in severe marine conditions until he eventually beached alive and well in Sistiana (Italy) 24 hours later. We conducted an interview with the survivor to reconstruct his trajectory and to gain insight into his swimming and paddling strategy. We then attempted a Lagrangian simulation of his trajectory in two ways. Firstly by performing a leeway simulation using the OpenDrift tracking code using two object types: Person-in-Water-1 and Person-powered-vessel-2. Secondly, we model the trajectory using our own Lagrangian tracking code FlowTrack. In both cases a high-resolution (1 km) setup of NEMO v3.6 circulation model was employed for the surface current component and a 4.4 km operational setup of the ALADIN atmospheric model was used for wind forcing. OpenDrift yields best results using Person-powered-vessel-2 object type, indicating a relatively broad search and rescue area which covers 45 km2 after six hours and rises to 380 km2 after 24 hours. The simulated most probable SAR area envelops the reconstructed drift trajectory and is also temporaly consistent with the reconstruction. FlowTrack yields a search and rescue area with a comparable lateral extent but with much less downwind spread. While both Lagrangian models were able to envelop the reconstructed drift trajectory during this validation, we recommend using OpenDrift for similar search-and-rescue missions in the future due to its flexibility and drifting object dependent calibration on empirical data.