Mapping sea ice concentration using Nimbus-5 ESMR and local dynamical tie points

Abstract. As part of the European Space Agency's Climate Change Initiative the one channel US satellite microwave radiometer Nimbus-5 ESMR (N5ESMR) level 1 data have been reprocessed to estimate global sea ice concentration from 11 December 1972 to 16 May 1977. The full data set is available in the CEDA Archive: DOI: 10.5285/8978580336864f6d8282656d58771b32 at a grid resolution of 25 x 25 km² and a daily timestep (Tellefsen et al., 2025). A new methodology using locally and seasonally variable algorithm coefficients called tie points has been used to calculate the sea ice concentration in both first-year ice and multi-year ice in the Arctic and the radiometrically distinct ice types A and B in Antarctica. Validation of sea ice concentration using Arctic sea ice charts from the US National Ice Center shows an overestimation of open-water SIC of up to 20 % in some places and an underestimation of SIC in sea ice covered regions near the ice edge. Validation also shows that local dynamical tie points (LDTP) improve the mapping of sea ice concentration for different types of ice, while estimates of the extent of sea ice are identical to the previous processing of the same data in Kolbe et al. (2024). A new set of quality control (QC) filters has been developed that discards far fewer data points (57.7 % reduction) than the filters in the previous processing. The data set therefore closes significant gaps in the sea ice concentration and sea ice extent record compared to the earlier data record. Of the 1.6 billion data points recorded by the satellite, 23.0 % have been discarded. 1136 days during the 1616-day period from 1972 to 1977 are covered (at least partly), which gives estimates of the mean monthly sea ice minimum and maximum extent in the Arctic and Antarctica during this period, except for some dropouts in 1973 and 1975.