Sea Ice and Atmospheric Parameter Retrieval From Satellite Microwave Radiometers: Synergy of AMSR2 and SMOS Compared With the CIMR Candidate Mission

Research on improving the prediction skill of climate models requires refining the quality of observational data used for initializing and tuning the models. This is especially true in the polar regions where uncertainties about the interactions between sea ice, ocean, and atmosphere are driving ongoing monitoring efforts. The Copernicus Imaging Microwave Radiometer (CIMR) is an European Space Agency (ESA) candidate mission which promises to offer high resolution, low uncertainty observation capabilities at the 1.4, 6.9, 10.65, 18.7, and 36.5 GHz frequencies. To assess the potential impact of CIMR for sea ice parameter retrieval, a comparison is made between retrievals based on present AMSR2 observations and a retrieval using future CIMR equivalent observations over a data set of validated sea ice concentration (SIC) values. An optimal estimation retrieval method (OEM) is used which can use input from different channel combinations to retrieve seven geophysical parameters (sea ice concentration, multi-year ice fraction, ice surface temperature, columnar water vapor, liquid water path, over ocean wind speed, and sea surface temperature). An advantage of CIMR over existing radiometers is that it would provide higher spatial resolution observations at the lower frequency channels (6.9, 10.65, and 18.7 GHz) which are less sensitive to atmospheric influence. This enables the passive microwave based retrieval of SIC and other surface parameters with higher resolution and lower uncertainty than is currently possible. An information content analysis expands the comparison between AMSR2 and CIMR to all retrievable surface and atmospheric parameters. This analysis quantifies the contributions to the observed signal and highlights the differences between different input channel combinations. The higher resolution of the low frequency CIMR channels allow for unprecedented detail to be achieved in Arctic passive microwave sea ice retrievals. The presence of 1.4 GHz channels on board CIMR opens up the possibility for thin sea ice thickness (SIT) retrieval. A combination of collocated AMSR2 and SMOS observations is used to simulate a full CIMR suite of measurements, and the OEM is modified to include SIT as a retrieval parameter. The output from different retrieval configurations is compared with an operational SIT product. The CIMR instrument can provide increased accuracy for SIC retrieval at very high resolutions with a combination of the 18.7 and 36.5 GHz channels while also maintaining sensitivity for atmospheric water vapor retrieval. In combination with the 1.4 GHz channels, SIT can be added as an eighth retrieval parameter with performance on par with existing operational products. Plain Language Summary In order to improve exiting climate models a number of challenges have to be overcome which have to do with interactions between sea ice, the open ocean, and the atmosphere. One way to address these challenges is to use higher quality satellite observations of these variables for tuning and running these models. The Copernicus Imaging Microwave Radiometer is an European Space Agency candidate mission which promises to offer high resolution and accurate observations at the 1.4, 6.9, 10.65, 18.7, and 36.5 GHz frequencies. At present one of the highest resolution sea ice concentration products is based on the 89 GHz channels of the AMSR2 satellite instrument. This frequency however is not ideal for observing surface parameters as it is sensitive to the presence of clouds. The lower frequency channels of the CIMR instrument are less influenced by the atmosphere. As such these frequencies would be much better suited for observing multiple sea ice parameters like concentration, different ice types, the sea ice thickness, and surface temperature as well as ocean parameters such as sea surface temperature, sea surface salinity, and ocean wind speed. Based on this we RESEARCH ARTICLE 10.1029/2019JC015749 Key Points: • A comparison of current and new (CIMR) passive microwave missions is made for sea ice and atmospheric parameter retrieval in the Arctic • The 18.7 + 36.5 GHz channel combination achieves 4% uncertainty for sea ice concentration retrieval at a resolution of 5 km • The CIMR 1.4 GHz channels allow for retrieval of thin sea ice thickness data alongside other sea ice and atmospheric parameters Correspondence to: R. C. Scarlat, rauls@iup.physik. uni-bremen.de Citation: Scarlat, R. C. , Spreen, G. , Heygster, G. , Huntemann, M. , Patilea, C. , Pedersen, L. T. , & Saldo, R. (2020). Sea ice and atmospheric parameter retrieval from satellite microwave radiometers: Synergy of AMSR2 and SMOS compared with the CIMR candidate mission. Journal of Geophysical Research: Oceans, 125, e2019JC015749. https://doi.org/10. 1029/2019JC015749 Received 10 OCT 2019 Accepted 13 JAN 2020 Accepted article online 10 FEB 2020 ©2020. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. SCARLAT ET AL. 1 of 23 Journal of Geophysical Research: Oceans 10.1029/2019JC015749 perform a performance comparison between the existing AMSR2 89 GHz and different CIMR channel combinations. The analysis is done using a common retrieval method for up to seven different parameters (sea ice concentration, ice types, ice surface temperature, sea surface temperature, ocean wind speed, total water vapor, and cloud liquid water) and which allows for using single or multifrequency channels as input. The focus is on the retrieval of sea ice concentration, but the other atmospheric and surface parameters are discussed as well. The CIMR 6.9 GHz channel offers the best sea ice concentration performance with a resolution of 15 km and an accuracy around 2%. The CIMR channel combination of 18.7 and 36.5 GHz offers good sea ice concentration accuracy at 4% but at a much higher resolution of 5 km, which is comparable with he present day AMSR2 89 GHz high resolution product. Using the CIMR channel combination also provides good sensitivity for retrieving the atmospheric parameters cloud liquid water and water vapor content. By using the full set of CIMR channels, including the two 1.4 GHz ones, sea ice thickness values below 30 cm can be retrieved alongside the other seven parameters.