Challenges in sea ice retrieval methods using satellite microwave radiometers

Dr Natalia IvanovaSea ice concentration has been retrieved in polar regions with satellite microwave radiometers for over 30 years. Sea ice information providers, including EUMETSAT's OSISAF, ArcticROOS/Nansen Center, Copernicus OSI-TAC, NSIDC, University of Bremen and others, produce daily updates of the Arctic Sea ice area and extent. The sea ice project of European Space Agency's Climate Change Initiative (ESA CCI) has performed a detailed inter-comparison of 30 different sea ice retrieval algorithms and systematically evaluated their performance at high and low sea ice concentrations.

Image descriptionLead by Nansen Center scientist Dr. Natalia Ivanova the paper Inter-comparison and evaluation of sea ice algorithms: towards further identification of challenges and optimal approach using passive microwave observations in The Cryosphere presents the key results of an extensive algorithm inter-comparison and evaluation experiment. The skills of 30 sea ice algorithms were evaluated systematically over low and high sea ice concentrations. Evaluation criteria included standard deviation relative to independent validation data, performance in the presence of thin ice and melt ponds, and sensitivity to error sources with seasonal to inter-annual variations and potential climatic trends, such as atmospheric water vapour and water-surface roughening by wind. A selection of 13 algorithms is shown in the article to demonstrate the results. The figure demonstrates improvement of selected algorithms’ performance when atmospheric correction is applied.

Based on the findings, a hybrid approach is suggested to retrieve sea ice concentration globally for climate monitoring purposes. This approach consists of a combination of two algorithms plus dynamic tie point’s implementation and atmospheric correction of input brightness temperatures. The method minimizes inter-sensor calibration discrepancies and sensitivity to the mentioned error sources. 

Paper sitation: Ivanova, N., Pedersen, L.T., Tonboe, R.T., Kern, S., Heygster, G., Lavergne, T., Sørensen, A., Saldo, R., Dybkjær, G., Brucker, L., and Shokr, M.: Inter-comparison and evaluation of sea ice algorithms: towards further identification of challenges and optimal approach using passive microwave observations, The Cryosphere, 9, 1797-1817, doi:10.5194/tc-9-1797-2015, 2015.

 

FIgure caption: Histograms for SSM/I sea ice concentration (SIC) obtained by the OSISAF algorithm over open water (SIC = 0%) in the Northern Hemisphere in 2008 (entire year) without correction (upper panel, left) and with radiative transfer model (RTM) correction (upper panel, right). The histograms contain 21 bins of 2% SIC. Bottom panel: decrease in standard deviations for 10 SIC algorithms due to the atmospheric correction of the measured brightness temperatures.

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