SAR monitoring of young sea ice in the Marginal Ice Zone

Young sea ice types, namely grease-pancake ice (GPI), and thin ice floes as well, mainly compose the marginal ice zone (MIZ) during the freezing period. Such types of sea ice are considered the primary source of the sea ice fringing the Antarctica continent during winter season through the process called the "pancake cycle", and are massively produced in the Arctic as a result of the sea ice extent and volume decline. The properties of such sea ice types are exceedingly difficult to measure autonomously, due to their size, transient nature, and the hostile environment where they form and grow. However, some internal properties of the ice cover, such as the thickness, can in principle be inferred by measuring the modifications induced by the presence of the sea ice cover on the propagation of gravity waves coming from the open ocean. In this regard, the available spaceborne SAR imagery can be operated as a synoptic wave buoy and measure the attenuation and the shortening of incoming ocean waves inside GPI fields. The SAR inversion procedure is based on knowledge of the open sea wind-generated waves spectrum. It is essential that such waves come from the open ocean before traveling inside the GPI field. The mapping between ocean wave spectrum and SAR image spectrum is provided by the closed nonlinear integral transformation by Hasselmann and Hasselmann (Hasselmann and Hasselmann 1991) and later developments, which account for the SAR image cross-spectrum (Engen and Johnsen 1995). The wind wave spectrum is thus changed according to the rheological properties and to the thickness of the ice layer crossed in order to achieve the best fit between the observed SAR image spectrum and the simulated one. Finally, swell waves are estimated directly from the observed SAR image spectrum regardless of their provenience. The ability of different viscous layer models to describe the attenuation of gravity waves propagating in GPI covered ocean has been already investigated (De Santi et al. 2018). In particular, the Keller's model (Keller, 1998), the two-layer viscous model (De Carolis and Desiderio 2002) and the close-packing model (De Santi and Olla 2017) have been extensively validated by using wave attenuation data collected during two different field campaigns (Weddell Sea, Antarctica, April 2000; Western Arctic Ocean, autumn 2015). For thin GPI, good retrievals of ice thickness have been obtained by considering the ice layer as the only source affecting the wave dynamics, so that the wind input can be disregarded. Validation of these viscous layer models is now extended by inverting different ERS and Sentinel 1 SAR images acquired in the Arctic.