Influence of offshore wind farms on the latent heat flux in the marine boundary layer

Unique airborne in-situ measurements were evaluated to investigate the influence of offshore wind farms on the latent heat flux in the marine boundary layer. 21 of the total 42 measurement flights carried out in the framework of the WIPAFF project over the German Bight in the year 2016 and 2017 enabled such an evaluation under different atmospheric conditions. The airborne data was collected with the research aircraft Dornier 128 belonging to the Technische Universität Braunschweig, Germany. The aircraft is equipped among others with high-resolution sensors for water vapor, three-dimensional  wind vector, temperature and pressure. The measurements of 15 flights showed a significant increase of the vertical upward latent heat flux over the offshore wind farm clusters Amrumbank West, Nordsee Ost, Meerwind Süd/Ost or the wind farm cluster Godewind. For thermally stable conditions, all nine measurement flights except one showed an enhanced latent heat flux downstream of the wind farms at hub height, with an increase of up to +70 W m^-2compared to the undisturbed flow. For flights during unstable thermal conditions, 8 out of 13 cases showed an increase, with the largest difference with respect to undisturbed flow of +400 W m^-2 above the wind farm. The results also suggest that not only the thermal stratification but also the moisture gradient plays a decisive role in whether the influence of the wind farm is visible in the latent heat flux.