Waves (Continuation)

In the context of a changing sea ice and wave climate, the build-up of ocean waves in ice-free parts of the Arctic seas and their impact on both the coastline and marginal ice zone gains growing attention. First indications for the possibility of wave exposure having an increasing tendency over the last few decades have emerged (Thomson et al.(2016)). Related sea state trends are governed by different factors including the timing and amount of ice retreat in summertime and the frequency and strength of wind forcing. In general, those trends are based on satellite climatology and model runs. Current efforts are undertaken to improve the representation of ice in sea state models, among others by means of observations, e.g. Zieger et al.(2015).

In recent years, several strong swell wave events in Arctic waters were recorded by in-situ measurements during ship campaigns (Collins et al.(2015)). Consistency checks with models were performed to interpret observations in Ardhuin et al.(2016). Moreover, case studies on swell wave events in the MIZ were conducted using SAR imagery from satellites (Schulz-Stellenfleth and Lehner(2002), Ardhuin et al.(2015), Gebhardt et al.(2016)).

During the ONR Sea State DRI Project a campaign was performed by the research vessel Sikuliaq and the interaction of sea state and sea ice in the MIZ was investigated. Several wave events including swell penetrating into the sea ice as well as wave build up in off ice wind fetch conditions were measured and at the same time imaged by Sentinel-1 SAR imagery .For the cruise report see Thomson(2015). Further information on the cruise support by TS-X satellite images can be found in Lehner and Gemmrich(2016).

Different sources of wavelength variability, both ice and non-ice related, were identified and the results were cross-checked with a model hindcast from ECMWF (European Centre for Medium-Range Weather Forecasts).

Thomson, J., Fan, Y., Stammerjohn, S., Stopa, J., Rogers, W. E., Girard-Ardhuin, F., Ardhuin, F., Shen, H., Perrie, W., Shen, H., Ackley, S., Babanin, A., Liu, Q., Guest, P., Maksym, T., Wadhams, P., Fairall, C., Persson, O., Doble, M., Graber, H., Lund, B., Squire, V., Gemmrich, J., Lehner, S., Holt, B., Meylan, M., Brozena, J., and Bidlot, J.-R.: Emerging trends in the sea state of the Beaufort and Chukchi seas, Ocean Modelling, 105, 1-12, http://dx.doi.org/10.1016/j.ocemod.2016.02.009, 2016.

Zieger, S., Babanin, A. V., Rogers, W. E., and Young, I. R.: Observation-based source terms in the third-generation wave model WAVEWATCH, Ocean Modelling, 96, 2-25, http://dx.doi.org/10.1016/j.ocemod.2015.07.014, 2015.

Pleskachevsky, A., Rosenthal, W., and Lehner, S.: Meteo-marine parameters for highly variable environment in coastal regions from satellite radar images, ISPRS Journal of Photogrammetry and Remote Sensing, http://dx.doi.org/10.1016/j.isprsjprs.2016.02.001, 2016

Gebhardt, C., Bidlot, J.-R., Gemmrich, J., Pleskachevsky. A., Lehner., S., and Rosenthal., W.: Wave observation in the marginal ice zone with the TerraSAR-X satellite, Ocean Dynamics, 66, 839-852, doi:10.1007/s10236-016-0957-8, 2016

Gemmrich, J., Pleskachevsky, A., Lehner, S., and Rogers, E.: Surface waves in arctic seas, observed from TerraSAR-X, 2015 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), Milan, 2015, pp. 3615-3617, doi: 10.1109/IGARSS.2015.7326604, IEEE, 2015

Measurements of wave heights in marginal ice zones are limited to very few in situ data. Here we revisit the linear and quasilinear theories of Synthetic Aperture Radar imaging of waves in the particular case of waves in sea ice.  Instead of only working with spectra, we have developed an iterative nonlinear algorithm to estimate phase-resolved deterministic maps of wave-induced orbital velocities, from which elevation spectra can be derived. Application of this algorithm to Sentinel 1A wave mode images in the Southern Ocean shows that  it produces reasonable results for swells in all directions except when they propagate at a few degrees off the range direction. The estimate of wave parameters is expected to work best when the shortest wave components, those which cause a pixel displacement of the order of the dominant wavelength in azimuth, can be neglected. Otherwise short waves produce a blurring of the image, increasing exponentially with the azimuthal wavenumber and reducing the estimated wave amplitude. Given the expected spatial attenuation of waves in ice-covered regions,  our deterministic method should apply  beyond a few tens of kilometers in the ice, without any correction for short wave effects. In situ data collected around the ice edge as part of the 2015 SeaState DRI cruise in the Beaufort confirm the progressive image blurring caused by such short waves, and the apparent reduction in the wave modulation. When short waves propagate from the open ocean towards the ice, this blurring can produce an unrealistic apparent increase of wave height, from the open ocean up to a few tens of kilometers inside the ice. The method is tested on two Interferometric Wide swath (IW) mode images from Sentinel 1A, and was applied to thousands of wave mode images from S1A and S1B. Automatic application requires a careful screening for ice features that could otherwise be interpreted as wave energy.