A parametric inversion scheme for the retrieval of two‐dimensional (2‐D) ocean wave spectra from look cross spectra acquired by spaceborne synthetic aperture radar (SAR) is presented. The scheme uses SAR observations to adjust numerical wave model spectra. The Partition Rescaling and Shift Algorithm (PARSA) is based on a maximum a posteriori approach in which an optimal estimate of a 2‐D wave spectrum is calculated given a measured SAR look cross spectrum (SLCS) and additional prior knowledge. The method is based on explicit models for measurement errors as well as on uncertainties in the SAR imaging model and the model wave spectra used as prior information. Parameters of the SAR imaging model are estimated as part of the retrieval. Uncertainties in the prior wave spectrum are expressed in terms of transformation variables, which are defined for each wave system in the spectrum, describing rotations and rescaling of wave numbers and energy as well as changes of directional spreading. Technically, the PARSA wave spectra retrieval is based on the minimization of a cost function. A Levenberg‐Marquardt method is used to find a numerical solution. The scheme is tested using both simulated SLCS and ERS‐2 SAR data. It is demonstrated that the algorithm makes use of the phase information contained in SLCS, which is of particular importance for multimodal sea states. Statistics are presented for a global data set of 11,000 ERS‐2 SAR wave mode SLCS acquired in southern winter 1996.

Coastal wind speed gradients relevant for offshore windfarming are analysed based on synthetic aperture radar (SAR) data. The study concentrates on situations with offshore wind directions in the German Bight using SAR scenes from the European satellites Sentinel-1A and Sentinel-1B. High resolution wind fields at 10 m height are derived from the satellite data set and respective horizontal wind speed gradients are investigated up to about 170 km offshore. The wind speed gradients are classified according to their general shape with about 60% of the cases showing an overall increase of wind speeds with growing distance from the coast. About half of the remaining cases show an overall wind speed decrease and the other half a decrease with a subsequent increase at larger distances from the coast. An empirical model is fitted to the horizontal wind speed gradients, which has three main parameters, namely, the wind speed over land, the equilibrium wind speed over sea far offshore, and a characteristic adjustment length scale. For the cases with overall wind speed increase, a mean absolute difference of about 2.6 m/s is found between wind speeds over land and wind speeds far offshore. The mean normalised wind speed increase with respect to the land conditions is estimated as 40%. In terms of wind power density at 10 m height this corresponds to an absolute average growth by 0.3 kW/m² and a normalised increase by 160%. The distance over which the wind speed grows to 95% of the maximum wind speed shows large variations with maximum above 170 km and a mean of 67 km. The impact of the atmospheric boundary layer stability on horizontal wind speed gradients is investigated using additional information on air and sea temperature differences. The absolute SAR-derived wind speed increase offshore is usually higher in unstable situations and the respective adjustment distance is shorter. Furthermore, we have found atypical cases with a wind speed decrease offshore to be often connected to stable atmospheric conditions. A particular low-level jet (LLJ) situation is analysed in more detail using vertical wind speed profiles from a wind LIDAR system.

Temperature extremes not only directly affect the marine environment and ecosystems but also indirectly influence hydrodynamics and marine life. In this study, the role of heat wave events in the occurrence and persistence of thermal stratification was analysed by simulating the water temperature of the North Sea from 2011 to 2018 using a fully coupled hydrodynamic and wave model within the framework of the Geesthacht Coupled cOAstal model SysTem (GCOAST). The model results were assessed against reprocessed satellite data and in situ observations from field campaigns and fixed Marine Environmental Monitoring Network (MARNET) stations. To quantify the degree of stratification, the potential energy anomaly throughout the water column was calculated. The air temperatures and potential energy anomalies in the North Sea (excluding the Norwegian Trench and the area south of 54^∘ N) were linearly correlated. Different from the northern North Sea, where the water column is stratified in the warm season each year, the southern North Sea is seasonally stratified in years when a heat wave occurs. The influences of heat waves on the occurrence of summer stratification in the southern North Sea are mainly in the form of two aspects, i.e. a rapid rise in sea surface temperature at the early stage of the heat wave period and a higher water temperature during summer than the multiyear mean. Another factor that enhances the thermal stratification in summer is the memory of the water column to cold spells earlier in the year. Differences between the seasonally stratified northern North Sea and the heat wave-induced stratified southern North Sea were ultimately attributed to changes in water depth.

The increasing number of wind farms installed in the North Sea has an impact on the downstream wind speed. This has been hypothesized as well for sea state properties. Wave effects can be expected in particular in fetch-limited conditions with offshore wind directions. With systematic flights deploying an airborne laser scanner, these impacts are shown directly for the first time. The flights were conducted perpendicular to the main wind direction upstream and downstream of the cluster of the offshore wind parks Amrumbank West, Nordsee Ost, and Meerwind Süd/Ost. The flight legs covered the area potentially influenced by the wind parks and the undisturbed area next to the wind parks. The analysis of the spectral energy distribution shows a re-distribution of the wave energy in the downstream area with enhanced energy at smaller wavelengths. The effect is still clearly visible at a distance of 55 km. As the sea surface constitutes the link between the atmosphere and the ocean, it is very likely that wind parks modify the properties of the water column as well.