Lagrangian Trajectory Modelling for a Person lost at Sea during Adriatic Scirocco Storm of 29 October 2018 Matjaž Liˇ cer 1 , Solène Estival 2 , Catalina Reyes-Suarez 3 , Davide Deponte 3 , and Anja Fettich 4 1 National Institute of Biology, Piran, Slovenia 2 École Nationale Supérieure de Techniques Avancées, Paris, France 3 Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Sgonico, Italy 4 Slovenian Environment Agency, Ljubljana, Slovenia Correspondence: Matjaž Liˇ cer (matjaz.licer@nib.si) Abstract. On 29 October 2018 a windsurfer’s mast broke about 1 km offshore during a severe Scirocco storm in the Northern 1 Adriatic Sea. He was drifting in severe marine conditions until he eventually beached alive and well in Sistiana (Italy) 24 2 hours later. We conducted an interview with the survivor to reconstruct his trajectory and to gain insight into his swimming 3 and paddling strategy. We then attempted a Lagrangian simulation of his trajectory in two ways. Firstly by performing a lee- 4 way simulation using the OpenDrift tracking code using two object types: Person-in-Water-1 and Person-powered-vessel-2. 5 Secondly, we model the trajectory using our own Lagrangian tracking code FlowTrack. In both cases a high-resolution (1 km) 6 setup of NEMO v3.6 circulation model was employed for the surface current component and a 4.4 km operational setup of the 7 ALADIN atmospheric model was used for wind forcing. OpenDrift yields best results using Person-powered-vessel-2 object 8 type, indicating a relatively broad search and rescue area which covers 45 km 2 after six hours and rises to 380 km 2 after 24 9 hours. The simulated most probable SAR area envelops the reconstructed drift trajectory and is also temporaly consistent with 10 the reconstruction. FlowTrack yields a search and rescue area with a comparable lateral extent but with much less downwind 11 spread. While both Lagrangian models were able to envelop the reconstructed drift trajectory during this validation, we recom- 12 mend using OpenDrift for similar search-and-rescue missions in the future due to its flexibility and drifting object dependent 13 calibration on empirical data. 14 1 Introduction 15 Lagrangian particle tracking of objects lost at sea is an important branch of ocean forecasting. Maritime search and rescue 16 (SAR) or other types of civil service responses depend on timely and reliable estimates of the most probable areas which 17 contain the drifting object. These estimates generally require prior computation of ocean currents, waves and winds in the area, 18 which are most often provided by numerical circulation, wave and atmosphere models. 19 The wind force contribution to the objects drift is termed its leeway and has both downwind (drag) and crosswind (lift) com- 20 ponent (Breivik and Allen, 2008). The object’s drift therefore generally deviates from the wind direction by some divergence 21 angle L α (Allen and Plourde, 1999), related to the downwind and crosswind components. Specific values of the object’s down- 22 wind and crosswind drift are determined by the balance of the wind (lift and drag) force on the overwater part of the object 23 1 https://doi.org/10.5194/nhess-2019-362 Preprint. Discussion started: 6 January 2020 c  Author(s) 2020. CC BY 4.0 License.