Impact of Environmental Parameters on Sea Surface Radar Backscatter and Air-Sea CO2 Fluxes in the Western Baltic

Long-term radar backscatter measurements, obtained from the research platform FINO-2 in the western Baltic Sea using the multi-frequency and multi-polarization scatterometer (Multi³Scat) of the University of Hamburg, are analyzed with respect to the influence of environmental parameters on the radar backscatter. The scatterometer measurements were obtained from the FINO-2 tower at a height of 25 meter above the sea surface. Auxiliary data, available from the platform, entail atmospheric (wind and air temperature), and oceanic (waves and water temperature) parameters, as well as CO₂ fluxes. In combination with auxiliary data, the measurements are used to infer air-sea CO₂ fluxes as a function of wind speed, air-sea temperature difference, and significant wave height. Results confirm that unstable atmospheric stratification increases surface wind stress during winter seasons, resulting in an increased sea surface roughness relative to summer months, given the same wind speed at 10 m height typically used in CO₂ transfer algorithms. The correlation between the measured radar backscatter and CO₂ fluxes confirms accordingly that atmospheric stratification affects CO₂ fluxes due to its impact on sea surface roughness and near surface wind and leads to higher CO2 fluxes during winter months.