Dual-polarized ratio algorithm for retrieving Arctic sea ice concentration from passive microwave brightness temperature
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ORIGINAL ARTICLE
Dual-polarized ratio algorithm for retrieving Arctic sea ice concentration from passive microwave brightness temperature Shugang Zhang • Jinping Zhao • Karen Frey Jie Su
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Received: 23 February 2012 / Revised: 13 December 2012 / Accepted: 23 December 2012 / Published online: 24 January 2013 Ó The Oceanographic Society of Japan and Springer Japan 2013
Abstract We present a new algorithm for retrieving sea ice concentration from the AMSR-E data, the dual-polarized ratio (DPR) algorithm. The DPR algorithm is developed using vertically and horizontally polarized brightness temperatures at the same channel of 36.5 GHz. It depends on the ratio of dual-polarized emissivity, a, which is determined empirically at about 0.92 by remotely sensed brightness temperature in winter and used for the other seasons as well. The ice concentration retrieved by the DPR is compared with those by the NT2 and ABA algorithms. Since the main difference among these algorithms takes place in marginal ice zones, 17 marginal ice zones are chosen. The retrieved ice concentrations in these zones are examined by the ice concentration obtained by the MODIS data. The mean error, root-mean-square error and mean absolute error of the DPR algorithm are relatively better than those from the other two algorithms. The results of this study illustrate that the DPR algorithm is a more accurate algorithm for retrieving sea ice concentration from the AMSR-E brightness temperature, and can be used for operational purposes. Keywords Arctic Ice concentration AMSR-E Brightness temperature Dual-polarized ratio algorithm
S. Zhang (&) J. Zhao J. Su Ocean University of China, Qingdao, People’s Republic of China e-mail: [email protected] J. Zhao e-mail: [email protected] K. Frey Clark University, Worcester, USA
1 Introduction Polar regions play an important role in the global climate system and the energy balance. Furthermore, Arctic sea ice is a key environmental variable and is a sensitive indicator of global warming (Lemke 1987; Comiso et al. 2003). The sea ice albedo of more than 0.8 for the predominantly snow-covered seasonal ice or multi-year ice is much higher than that of the open ocean of about 0.1 (Grenfell 1983; Comiso et al. 2003), which results in a sharp contrast in surface energy flux between the marginal ice zone and the open ocean. Sea ice is an effective insulator that restricts the exchanges of energy and momentum between the ocean and the atmosphere. During wintertime, the heat flux through open water is two orders of a magnitude higher than that through thick ice (Maykut 1978). Sea ice also alters oceanic structure and circulation during ice growing or melting stages. For example, the cold and dense water formed from sea ice growth in the Arctic leads and polynyas helps to maintain the halocline in the Arctic Ocean (Aagaard et al. 1981; Cavalieri and Martin 1994). So it is important to monitor sea ice extent and its physical property in different seasons and regions to reveal the energy exchange between ocean an
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