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MEPS prepress abstract   -  DOI:

Thermal soaring over the North Sea and implications for wind farm interactions

Jens van Erp*, Elspeth Sage, Willem Bouten, Emiel van Loon, Kees C. J. Camphuysen, Judy Shamoun-Baranes

*Corresponding author:

ABSTRACT: Seabirds use several flight modes at sea, including thermal soaring, in which thermal uplift is used to gain altitude and save energy. An increase in flight altitude may have consequences for wind farm interactions if it results in birds spending more time within the rotor swept zone (RSZ). To understand conditions under which thermal soaring occurs and potential implications for wind farms interactions, we investigated thermal soaring of seabirds in relation to atmospheric conditions in June and July at two study areas on the North Sea, west and north of the Dutch coast. We developed algorithms that identified thermal soaring in GPS tracks of lesser black-backed gulls and radar tracks of seabirds. By combining species-specific three-dimensional information on flight behaviour from bio-logging with the continuous spatiotemporal coverage of radar positioned at wind parks we obtained a more comprehensive overview of thermal soaring at sea than either method would obtain alone. Our results showed that birds flew at higher altitudes during thermal soaring than non-soaring flight, increasing the proportion of flight time within the RSZ. Thermal soaring occurred inside offshore wind farms to a similar degree as outside. Thermal soaring was positively correlated with positive temperature differences (∆T) between sea surface and air (at 2 m above sea level), and north and north-westerly winds. We show that the probability of thermal soaring over the North Sea, inside and outside wind farms, increases with larger temperature differences, resulting in increased time spent within the RSZ and an increased collision risk for seabirds.