Publications
Following publications have been announced by our department Ocean Surface Dynamics. For further information please contact the marked co-authors of the publications:
Støle-Hentschel, S., Carrasco, R., Nieto-Borge, J.C., Seemann, J., & Toledo, Y. (2024): Improved estimation of the directional wave spectrum from marine radar images by employing a directional modulation transfer function (MTF). Ocean Engineering, Vol 307, 118126, doi:10.1016/j.oceaneng.2024.118126
Abstract:
This study focuses on improving the accuracy of the energy distribution in directional wave spectra from temporal sequences of marine radar images. Wave spectra are obtained by converting the image spectrum to a wave spectrum by employing a modulation transfer function (MTF). While the imaging mechanism in the plane of the radar beam has received much attention, the effect of the combination of different beam positions has been largely neglected. Herein, we demonstrate that any MTF can only be valid for a limited azimuthal coverage. However, the proposed directional MTF is well suited for small windows for which it corrects the relative projection between the average radar beam position and the wavenumber vector. Spectra of different sea states are compared, using the derived MTF and the established MTF. The analysis shows a significant improvement in the energy distribution in the directional wave spectra when the directional spreading is high. It is suggested that future work should aim to remove the necessary restriction to small windows by synthesizing the full directional spectrum from multiple small windows in different directions. In this way, marine radars are suitable to better reproduce sea states with extreme directional spreading, including multi-modal seas.
McCann, D.L., Martin, A.C.H., de Macedo, K.A.C., Carrasco Alvarez, R., Horstmann, J., Marié, L., Márquez-Martínez, J., Portabella, M., Meta, A., Gommenginger, C., Martin-Iglesias, P., & Casal, T. (2024): A new airborne system for simultaneous high-resolution ocean vector current and wind mapping: first demonstration of the SeaSTAR mission concept in the macrotidal Iroise Sea. Ocean Sci., 20, 1109–1122, doi:10.5194/os-20-1109-2024
Abstract:
Coastal seas, shelf seas and marginal ice zones are dominated by small-scale ocean surface dynamic processes that play a vital role in the transport and exchange of climate-relevant properties such as carbon, heat, water and nutrients between land, ocean, ice and atmosphere. Mounting evidence indicates that ocean scales below 10 km have far-ranging impacts on air–sea interactions, lateral ocean dispersion, vertical stratification, ocean carbon cycling and marine productivity – governing exchanges across key interfaces of the Earth system, the global ocean, and atmosphere circulation and climate. Yet, these processes remain poorly observed at the fine spatial and temporal scales necessary to resolve them. The Ocean Surface Current Airborne Radar (OSCAR) is a new airborne instrument with the capacity to inform these questions by mapping vectorial fields of total ocean surface currents and winds at high resolution over a wide swath. Developed for the European Space Agency (ESA), OSCAR is the airborne demonstrator of the satellite mission concept SeaSTAR, which aims to map total surface current and ocean wind vectors with unprecedented accuracy, spatial resolution and temporal revisit across all coastal seas, shelf seas and marginal ice zones. Like SeaSTAR, OSCAR is an active microwave synthetic aperture radar along-track interferometer (SAR-ATI) with optimal three-azimuth sensing enabled by unique highly squinted beams. In May 2022, OSCAR was flown over the Iroise Sea, France, in its first scientific campaign as part of the ESA-funded SEASTARex project. The campaign successfully demonstrated the capabilities of OSCAR to produce high-resolution 2D images of total surface current vectors and near-surface ocean vector winds, simultaneously, in a highly dynamic, macrotidal coastal environment. OSCAR current and wind vectors show excellent agreement with ground-based X-band-radar-derived surface currents, numerical model outputs and NovaSAR-1 satellite SAR imagery, with root mean square differences from the X-band radar better than 0.2 m s−1 for currents at 200 m resolution. These results are the first demonstration of simultaneous retrieval of total current and wind vectors from a high-squint three-look SAR-ATI instrument and the first geophysical validation of the OSCAR and SeaSTAR observing principle. OSCAR presents a remarkable new ocean observing capability to support the study of small-scale ocean dynamics and air–sea interactions across the Earth’s coastal, shelf and polar seas.
Hans, A.C., Brandt, P., Gasparin, F., Claus, M., Cravatte, S., Horstmann, J., & Reverdin, G. (2024): Observed diurnal cycles of near-surface shear and stratification in the equatorial Atlantic and their wind dependence. Journal of Geophysical Research: Oceans, 129, doi:10.1029/2023JC020870
Abstract:
The diurnal cycles of near-surface velocity and temperature, also known as diurnal jet and diurnal warm layer (DWL), are ubiquitous in the tropical oceans, affecting the heat and momentum budget of the ocean surface layer, air-sea interactions, and vertical mixing. Here, we analyze the presence and descent of near-surface diurnal shear and stratification in the upper 20 m of the equatorial Atlantic as a function of wind speed using ocean current velocity and hydrographic data taken during two trans-Atlantic cruises along the equator in October 2019 and May 2022, data from three types of surface drifters, and data from Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) moorings along the equator. The observations during two seasons with similar mean wind speeds but varying surface heat fluxes reveal similar diurnal jets with an amplitude of about 0.11 m s−1 and similar DWLs when averaging along the equator. We find that higher wind speeds lead to earlier diurnal peaks, deeper penetration depths, and faster descent rates of DWL and diurnal jet. While the diurnal amplitude of stratification is maximum for minimal wind speeds, the diurnal amplitude of shear is maximum at 6 m depth for moderate wind speeds of about 5 m s−1. The inferred wind dependence of the descent rates of DWL and diurnal jet is consistent with the earlier onset of deep-cycle turbulence for higher wind speeds. The DWL and the diurnal jet not only trigger deep-cycle turbulence but are also observed to modify the wind power input and thus the amount of energy available for mixing.
Plain Language Summary:
During daytime, solar radiation leads to the formation of a thin warm layer at the ocean surface which can trap heat and wind-forced momentum. Both heat and momentum are transported in the deeper ocean during the evening and night by turbulent mixing. The associated diurnal variation of temperature, current velocity, and their vertical gradients, stratification and velocity shear, are thus relevant for understanding ocean-atmosphere interactions. This study investigates how the diurnal variation in stratification and velocity shear is influenced by the wind speed. For that, basin-scale observations of velocity and temperature, which were collected in the equatorial Atlantic during two trans-Atlantic equatorial cruises and by instruments installed at long-term moorings along the equator, are analyzed. These observations reveal that the wind speed influences the amplitude, the timing, and the vertical structure of the diurnal variation in stratification and velocity shear. Wind speed also influences how deep and how fast this variation propagates from the surface downward. The study concludes that the diurnal variation of stratification and velocity shear impacts first the input of mechanical energy from the atmosphere into the ocean and second the process of turbulent mixing below the night-time mixed layer.
