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The Cryosphere An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Dec 2018

Research article | 07 Dec 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal The Cryosphere (TC).

Comparison of ERA5 and ERA-Interim near surface air temperature and precipitation over Arctic sea ice: Effects on sea ice thermodynamics and evolution

Caixin Wang1,2, Robert M. Graham2, Keguang Wang1, Sebastian Gerland2, and Mats A. Granskog2 Caixin Wang et al.
  • 1Norwegian Meteo rological Institute, Tromsø, 9293, Norway
  • 2Norwegian Polar Institute, Fram Centre, Tromsø, 9296, P.O. Box 6606 Langnes, Norway

Abstract. Rapid changes are occurring in the Arctic, including a reduction in sea ice thickness and coverage and a shift towards younger and thinner sea ice. Snow and sea ice models are often used to study these ongoing changes in the Arctic, and are typically forced by atmospheric reanalyses in absence of observations. ERA5 is a new global reanalysis that will replace the widely used ERA-Interim (ERA-I). In this study, we compare the 2 m air temperature (T2M) and precipitation between ERA I and ERA5, and evaluate these products using buoy observations from Arctic sea ice. We further assess how biases in reanalyses influence the snow and sea ice evolution in the Arctic, when used to force a thermodynamic sea ice model. We find that both reanalyses have a warm bias over Arctic sea ice in relation to the buoy observations. The warm bias is smaller in the warm season, and larger in the cold season, especially when the T2M is lower than −25 °C. Interestingly, the warm bias in the new ERA5 is on average 2.1 °C (daily mean) larger than ERA-I during the cold season. While ERA-I is drier than most modern reanalyses in the Arctic, the total precipitation along the buoy trajectories is often lower in ERA5 than in ERA-I. Nonetheless, the snowfall products are broadly similar for both ERA I and ERA5. ERA-I had substantial anomalous Arctic rainfall, which is greatly reduced in ERA5. Simulations with a freezing degree days (FDD) model and a 1D thermodynamic sea ice model demonstrate that the warm bias in ERA5 acts to reduce thermodynamic ice growth. However, the lower precipitation in ERA5 results in a thinner snow pack that allows more heat loss to the atmosphere. Thus, the larger warm bias and lower precipitation in ERA5, compared with ERA I, compensate in terms of the effect on winter ice growth. Ultimately, we find slightly thicker ice at the end of growth season when using ERA5 forcing, compared with ERA-I. Thus differences in the precipitation fields of the two reanalyses have a larger influence on the sea ice evolution than the T2M.

Caixin Wang et al.
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Caixin Wang et al.
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Short summary
Warm bias and lower precipitation over Arctic sea ice were found in ERA5. The warm bias was larger in cold season, especially when 2 m air temperature (T2M) was <−25 °C, and smaller in warm season, compared with ERA-Interim (ERA-I). Substantial anomalous Arctic rainfall in ERA-I was greatly reduced in ERA5. Using ERA5 and ERA-I forcing a 1D sea ice model, one can get a slightly thick ice by the end of April. Differences in precipitation fields had a larger effect on sea ice evolution than the T2M.
Warm bias and lower precipitation over Arctic sea ice were found in ERA5. The warm bias was...