Abstract
Multiyear ice (MYI) concentration can be retrieved from passive or active microwave remote sensing observations. One of the algorithms that combines both observations is the Environmental Canada Ice Concentration Extractor (ECICE). However, factors such as ridging, snow wetness and metamorphism can cause significant changes in brightness temperature and backscatter, leading to misidentification of FYI as MYI, hence increasing the estimated MYI concentrations suddenly. This study introduces a correction scheme to restore the MYI concentrations under these conditions. The correction utilizes ice drift records to constrain the MYI changes and uses two thresholds of passive microwave radiometric parameters to account for snow wetness and metamorphism. The correction is applied to MYI concentration retrievals from ECICE with inputs from QuikSCAT and AMSR-E observations, acquired over the Arctic region in a series of winter seasons (October to May) from 2002 to 2009. Qualitative comparison with the Radarsat-1 SAR images and quantitative comparison against results from previous studies show that the correction works well by removing the anomalous high MYI concentrations. On average, the correction reduces the estimated MYI area by 5.2 × 105 km2 (14.3%) except for the April–May time frame, when the reduction is larger as the warmer weather prompts the condition of the anomalous snow radiometric signature. Due to the long-lasting (i.e., from one to several weeks) effect of the warm spells on FYI, the correction could be important in climatological research and operational applications.
Highlights
For pixels that are located outside the new domain, multiyear ice (MYI) concentrations of the second day are corrected without checking any radiometric indicator of snow wetness and metamorphism
Two thresholds, ∆C M and ∆Tb0, are used in the correction procedure. ∆C M indicates the sudden increases of MYI concentrations beyond which the increase can probably be anomalous, whereas ∆Tb0 is the value above which the sudden decreases in Tb37h is considered to be caused by snow metamorphism
The correction scheme reverses this wrong trend and produces a decrease of the MYI area which fairly well agrees with the outflow through the Fram Strait, which makes up about 90% of the Arctic sea ice export
Summary
Arctic sea ice extent has decreased by over 4% per decade from 1979 to 2010 [1], while the declining rate for the multiyear ice (MYI), ice that survives at least one summer melt, is much larger, 9%–15% per decade in the past three decades [2,3,4,5,6,7]. In a validation study of ECICE [30] using combined input from passive and active microwave observations, it was found that during warm spells in spring, caused by large cyclones that brought warm air to the region, the algorithm identifies FYI as MYI. The authors attributed this anomaly to a melt-refreeze cycle over the snow-covered FYI surface. This leads to a sudden and wrong increase of the retrieved MYI concentration from ECICE.
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