Abstract

Sea Ice is one of the essential climate variables as defined by the World Meteorological Organization and one of its key properties is it’s thickness, the Sea Ice Thickness (SIT). While Altimeter products dominate the thick ice regime and slowly changing thicker ice cover, the retrieval of thin sea ice thickness is mainly performed by thermal imager (e.g. MODIS [Mäkynen et al., 2013]) and passive microwave observations. Several variants for the retrieval of the thickness of thin sea ice were proposed for space borne L-band microwave radiometers such as SMOS by ESA and SMAP by NASA. The algorithm used here in this ATBD is based on the work of [Huntemann et al., 2014] and [Paţilea et al., 2019]. It works originally on TB intensity and polarization difference in the 40-50° incidence angle range. In this ATBD the algorithm is modified to use directly instrument provided TBs in horizontal and vertical polarization, including their uncertainties. Another modification compared to [Huntemann et al., 2014, Paţilea et al., 2019] is the removal of a forced upper limit of 50 cm of retrievable ice thickness. Higher ice thicknesses, even though retrieved, do come with higher uncertainties. With this modification it is more similar to the ESA SMOS algorithm [Tian-Kunze et al., 2014], which use SMOS TB closer to nadir and thus is not directly applicable to CIMR.. The evaluation is done using SMOS brightness temperatures against the ESA CCI Round robin data package [Pedersen et al., 2021] and the ESA SMOS product. In addition an evaluation on simulated CIMR test scenes is performed.