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 1972–16 May 1977. The full data set is available in the CEDA Archive: https://doi.org/10.5285/8978580336864f6d8282656d58771b32 at a grid resolution of 25×25km2 and a daily timestep. A new methodology using locally and seasonally variable algorithm coefficients called tie points has been used to calculate the sea ice concentration in radiometrically distinct ice types in the Arctic and 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 % where open ocean meets land and near the ice edge and an underestimation of SIC in sea ice covered regions near the ice edge, especially in the Greenland Sea. 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. 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. However, 1136 d during the 1616 d period from December 1972–May 1977 are covered (at least partially). This coverage gives estimates of the mean monthly sea ice minimum and maximum extent in the Arctic and the Antarctic during this period, except for March in 1973 which is the month with maximum sea ice extent (SIE) in the Arctic and the minimum in the Antarctic.