Mapping of sea ice concentration using the NASA NIMBUS 5 ESMR microwave radiometer data 1972–1977
Abstract. The Electrically Scanning Microwave Radiometer (ESMR) instrument on board the NIMBUS 5 satellite was a one channel microwave radiometer measuring the 19.35 GHz horizontally polarised brightness temperature (TB) from Dec. 11, 1972 to May. 16. 1977. The original tape archive data in swath projection have recently been made available online by NASA Goddard Earth Sciences Data and Information Services Center (GES DISC). Even though ESMR was a predecessor of modern multi frequency radiometers, there are still parts of modern processing methodology which can be applied to the data to derive the sea ice extent globally.
Here we have reprocessed the entire data set using a modern processing methodology, that includes implementation of pre-processing filtering, dynamical tie-points, and a radiative transfer model (RTM) together with numerical weather prediction (NWP) for atmospheric correction. We present the one channel sea ice concentration (SIC) algorithm and the model for computing temporally and spatially varying SIC uncertainty estimates. Post-processing steps include re-sampling to daily grids, land-spill-over correction, application of climatological masks, setting of processing flags and estimation of sea ice extent, monthly means and estimation of trends. This sea ice dataset derived from NIMBUS 5 ESMR extends the sea ice record with an important reference from the mid 1970s. To make a consistent analysis of the sea ice development through time easier, the same grid and landmask as for EUMETSAT's OSI-SAF SMMR based sea ice CDR have been used for our ESMR dataset. SIC uncertainties have been included for further ease of comparison to other datasets and time periods.
We find that our sea ice extent in the Arctic and Antarctic in the 1970s is generally higher than those available from the National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (DAAC) derived from the same ESMR dataset, with mean differences of 240.000 and 590.000 km2, respectively. The largest differences reach up to 2 million km2, when comparing monthly sea ice extents. Such large differences cannot be explained by the different grids and landmasks of the datasets alone, and must therefore also result from the difference in data filtering and algorithms, such as the dynamical tie-points and atmospheric correction.
The new ESMR SIC data set has been released as part of the ESA Climate Change Initiative Programme (ESA CCI) and is publicly available at: http://dx.doi.org/10.5285/34a15b96f1134d9e95b9e486d74e49cf (Tonboe et al., 2023).
