Validation of Seatrack Web using surface drifters in the Gulf of Finland and Baltic proper

Seatrack Web (STW) is a numerical model for oil spill drift forecasting widely used by oil spill combating authorities around the Baltic Sea. Seatrack Web gives an opportunity to simulate oil slick trajectory 5 days ahead and 30 days back in time with 1 hour interval. It is possible to choose different substances, such as oil, chemicals or floating object. An option to use satellite detected oil spills is implemented in the model. For modeling of oil slick trajectory with STW forcing fields are needed. Wind and current are taken from High Resolution Operation Model for the Baltic (HIROMB). Currents and wind are used with 3 hour interval. The horizontal resolution of current velocities is 3 nautical miles. Vertical resolution of HIROMB model is 50 different depth layers. Seatrack Web uses the wind forecasts from 10 meters height. Lightweight, long-life and low-cost drifters are developed by Tallinn University of Technology and local engineering companies in Estonia. The drifters have rugged and simple housing construction and are equipped with GPRS based two-way communication. Drifters are constructed so that about 95% of it's cylinder like body is below waterline and during drifting the buoy shows that way well surface current. In operation, data are transferred and visualized in real time through special web based software. In this study, the drifters are used for the validation of Seatrack Web. The drifters were used in 13 cases being deployed in variety of locations. The experiments have different duration ranging from 8 hours up to 8 days. The longest distance covered during single experiment was 52 nautical miles. The simulations with Seatrack Web were performed using fresh oil and floating objects with different extra wind factors, as it was not clear how well the drifters respond to the direct wind drift. Besides, two drifters were released at the same location to study the spatial spreading. There was insignificant difference when fresh oil or floating object with 0 extra wind factors has been used in STW. In certain cases adding of extra wind factor improved the comparison between STW and drifter's trajectory. The best coincidence between STW and surface drifters gave average departure about 40 m/h. In general the departure was slightly in excess of 100 m/h. Thus, the analyses show that drifters of suggested construction can be used for monitoring of the pollution drift on the sea surface.