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Takahiro Toyoda, Nariaki Hirose, L. Shogo Urakawa, Hiroyuki Tsujino, Hideyuki Nakano, Norihisa Usui, Yosuke Fujii, Kei Sakamoto, and Goro Yamanaka

1. Introduction Sea ice is an important component of the polar ocean climate system that greatly affects air–sea heat exchange. For example, the high albedos of sea ice and of snow on top of the ice, compared with the open ocean, reduce absorption of shortwave radiation by the ocean–sea ice system. Latent heat release to the atmosphere is also greatly reduced when the ocean surface is covered by sea ice with a large dry surface area, and the sensible heat flux and upward longwave radiation may

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Teresa Valkonen, Timo Vihma, and Martin Doble

1. Introduction The Antarctic sea ice zone covers approximately 19 × 10 6 km 2 in winter and 3.5 × 10 6 km 2 in summer ( Parkinson 2004 ). In situ observations of the atmosphere over this vast area have been rare, restricted to ship observations (mostly summertime; Andreas 1985 ; Wendler et al. 2005 ), wind and temperature measurements from drifting buoys ( Kottmeier and Sellman 1996 ), and detailed boundary layer observations from two drifting ice stations, in 1992 ( Andreas et al. 2000

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Hiroshi Sumata, Frank Kauker, Michael Karcher, and Rüdiger Gerdes

1. Introduction Sea ice plays an important role in the Arctic climate system. It reflects larger amounts of solar radiation than the open ocean and it substantially modulates the exchange of heat, freshwater, and momentum between the ocean and the atmosphere (e.g., Wadhams 2002 ; McPhee 2008 ; Thomas and Dieckmann 2009 ). Well-adjusted sea ice models are thus necessary for climate studies ( Budikova 2009 ; Overland 2016 ), and for the further development of climate models ( Notz 2015

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K. Andrea Scott, Mark Buehner, Alain Caya, and Tom Carrieres

1. Introduction An accurate estimate of the sea ice state is critical for providing information to ensure safe ship navigation in ice-infested waters, for improved numerical weather prediction (NWP) near ice-covered regions, and for climate studies. If the Arctic continues to warm as projected, there will be an increased need for accurate sea ice information because of an increase in ship traffic for transport and natural resource extraction in ice-covered regions. The recent results of the

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Hiroshi Sumata, Frank Kauker, Michael Karcher, and Rüdiger Gerdes

1. Introduction Sea ice is one of the most distinctive features of the Arctic climate system. Although sea ice forms on a thin solid layer of a few meters thick, it substantially modulates heat, freshwater, and momentum exchanges between the atmosphere and the ocean ( Wadhams 2002 ; McPhee 2008 ; Thomas and Dieckmann 2009 ). Improvement of dynamic and thermodynamic processes in sea ice models thus constitutes an important part of climate modeling and a prerequisite for meaningful predictions

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Keith M. Hines, David H. Bromwich, Lesheng Bai, Cecilia M. Bitz, Jordan G. Powers, and Kevin W. Manning

1. Introduction Sea ice, which provides a layer of thermal insulation between the ocean and atmosphere and reflects most of the incident solar insolation, is central to polar climate studies (e.g., Vihma 2014 ). During the twentieth century, Southern Hemisphere sea ice was characterized by large seasonal variations in areal coverage of relatively thin ice surrounding the Antarctic continent, while much of the Northern Hemisphere’s sea ice was thicker multiyear ice in the Arctic Ocean that was

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Thomas W. Collow, Wanqiu Wang, Arun Kumar, and Jinlun Zhang

1. Introduction According to the Fifth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC), annual Arctic sea ice extent (SIE) is very likely (90%–100% confident) to have decreased at a rate of 0.45 to 0.51 million km 2 decade −1 during the 1979–2012 period ( Vaughan et al. 2013 ), leading to projections of a summer ice free Arctic by the 2030s ( Wang and Overland 2012 ). Sea ice loss can be attributed to both anthropogenic influences and natural variability ( Kay et

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Yang Liu, Laurens Bogaardt, Jisk Attema, and Wilco Hazeleger

1. Introduction As one of the most noticeable frontiers with visible changes due to global warming, the Arctic has received more and more attention in recent decades. This is accompanied with increased commercial and scientific activities as a result of sea ice melting. This drives a demand for reliable operational sea ice forecasts, especially for shipping companies and related stakeholders ( Gascard et al. 2017 ; Stephenson and Pincus 2018 ). Therefore, it is of crucial importance to improve

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Denis Sergeev, Ian A. Renfrew, and Thomas Spengler

1. Introduction High-latitude mesoscale cyclones, and their intense subcategory polar lows (PLs), usually occur concomitantly with marine cold air outbreaks (CAOs) (e.g., see Kolstad 2006 ). The Svalbard Archipelago is the major orographic obstacle for these CAOs in the Norwegian and Barents Seas and climatological studies (e.g., Condron et al. 2006 ; Michel et al. 2018 ) report a maximum of mesoscale cyclone activity in its vicinity. With a retreating Arctic sea ice ( Cavalieri and

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T. C. Johns, E. W. Blockley, and J. K. Ridley

1. Introduction Numerical weather prediction (NWP) models conventionally consist of an atmospheric model (usually with an embedded or coupled land surface model) driven with specified surface boundary conditions over the ocean and sea ice. Persisted sea surface temperatures (SSTs) and sea ice fractional coverage through the forecast, or persisted anomalies with respect to a seasonally varying climatology, are typically used as the boundary conditions for ocean and sea ice surfaces. Over recent

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