Ice Concentration Retrieval from the Analysis of Microwaves: Evaluation of a New Methodology Optimized for the Copernicus Imaging Microwave Radiometer

Catherine Prigent,Victor Pellet,Filipe Aires,Carlos Jimenez,Lise Kilic

doi:10.3390/rs12101594

Abstract

A new methodology has been described in Kilic et al. (Ice Concentration Retrieval from the Analysis of Microwaves: A New Methodology Designed for the Copernicus Imaging Microwave Radiometer, Remote Sensing 2020, 12, 1060, Part 1 of this study) to estimate Sea Ice Concentration (SIC) from satellite passive microwave observations between 6 and 36 GHz. The Ice Concentration Retrieval from the Analysis of Microwaves (IceCREAM) algorithm is based on an optimal estimation, with a simple radiative transfer model derived from satellite observations at 0% and 100% SIC. Observations at low and high frequencies have different spatial resolutions, and a scheme is developed to benefit from the low errors of the low frequencies and the high spatial resolutions of the high frequencies. This effort is specifically designed for the Copernicus Imaging Microwave Radiometer (CIMR) project, equipped with a large deployable antenna to provide a spatial resolution of ∼5 km at 18 and 36 GHz, and ∼15 km at 6 and 10 GHz. The algorithm is tested with Advanced Microwave Scanning Radiometer 2 (AMSR2) observations, for a clear scene over the north polar region, with collocated Moderate Resolution Imaging Spectroradiometer (MODIS) estimates and the Ocean Sea Ice—Satellite Application Facilities (OSI SAF) operational products. Several algorithm options are tested, and the study case shows that both high spatial resolution and low errors are obtained with the IceCREAM method. It is also tested for the full polar regions, winter and summer, under clear and cloudy conditions. Our method is globally applicable, without fine-tuning or further weather filtering. The systematic use of all channels from 6 to 36 GHz makes it robust to changes in ice surface conditions and to weather interactions.

Highlights

Since 1979, Arctic sea ice extent has decreased year on year, with a September sea ice reduction of ∼12% per decade (e.g., [1,2])
The observations are considered at the spatial resolution of the lower frequency channel used in the algorithm
Note that in current Sea Ice Concentration (SIC) operational algorithms (OSI SAF for instance), each observation is used at its original resolution and the spatial resolution issue is accounted for by an additional term in the error budget [13]

Summary

Introduction

Since 1979, Arctic sea ice extent has decreased year on year, with a September sea ice reduction of ∼12% per decade (e.g., [1,2]). The Sea Ice Concentration (SIC) has been retrieved from satellite microwave radiometer data for ∼40 years, and the daily estimates of the global sea ice extent from these observations are one of the longest continuous climate records (e.g., [8]). Current microwave SIC algorithms rely on empirical methods, using the difference in radiometric signatures between the open ocean and the sea ice, based on the fact that the ocean emissivity is significantly lower than the sea ice emissivity, between 6 and 90 GHz. The retrieval algorithms are derived from coincident data sets of satellite observations and in situ measurements (such as the Round Robin Data Package (RRDP) [9]) with fully ice covered sites (100% SIC) and purely open ocean areas (0% SIC), called tie points. An evaluation of an ensemble of SIC algorithms showed that the algorithm using 6.9 GHz observations had the lowest error [14], because that frequency is less affected by the atmosphere and by the snow cover than the higher frequencies

Methods

Results

Conclusion