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第305回 大気海洋物理学・気候力学セミナー のおしらせ

日　時： 1月18日(木) 9:30 - 12:00
Date ： Thu., 18 Jan. 9:30 - 12:00
場　所： 低温科学研究所　3階　講堂
Place ： ILTS, Auditorium

Speaker:Nguyen Thi Hanh (Course in Atmosphere-Ocean and Climate Dynamics/D3)
Title：Stratospheric age of air in NIES-TM and AGCM
発表者：豊田 威信 (北大低温研/助教)
Speaker:Takenobu Toyota (Institute of Low Temperature Science/Assistant Professor)
題目：季節海氷域の海氷レオロジーに関する一考察
Title：An examination of the sea ice rheology for seasonal ice zones based on ice drift and thickness observations

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Stratospheric age of air in NIES-TM and AGCM
（Nguyen Thi Hanh） 発表要旨:


The stratospheric response to climate forcing is often 
unpredictable due to interaction between radiation, 
dynamics and chemistry. Stratospheric change such as 
the decadal increase of water vapor in turn drives global
scale surface warming (Solomon et al. 2010). The difficulty 
in studying the stratospheric change is understood by the 
fact that long-term trend of the strength of Brewer-Dobson 
circulation (BDC) remains inconsistent between diagnoses 
in climate models and estimates from tracer observations 
(Engelet al. 2017). In the present study, we focus on the 
strength of the BDC commonly quantified by the age of air, 
the stratospheric transit time since the entry of air from 
the troposphere. Due to multiple circulation pathways, any 
air parcel is composed of air elements that have different 
age, which is expressed by the age spectrum (Kida 1983; 
Hall and Plumb 1994; Waugh and Hall 2002). The age spectra 
are estimated by applying the Boundary Impulse Response method 
(Haine et al. 2008; Li et al. 2012) to the transport fields of 
National Institute for Environmental Studies Transport Model
(NIES TM) driven by JRA-25 and Center for Climate System
Research/National Institute for Environmental Studies/Frontier 
Research Center for Global Change AGCM (AGCM) nudged to 
ERA-Interim. The age spectra and mean age of stratospheric air 
are discussed by comparing the results simulated by both models. 
The comparison of mean age is also attempted to observational 
estimates from satellite SF6 data (Haenel et al.2015) and 
cryogenic air samplings in March 2015 at Biak, Indonesia
(Sugawara et al. 2017).


季節海氷域の海氷レオロジーに関する一考察
（豊田 威信、Takenobu Toyota） 発表要旨:


現在北極海では季節海氷域の割合が増加して海氷の形態が大きく
変わりつつある。しかしながら、IPCC報告書に用いられている
気候モデルはいずれも平均氷厚の急激な減少を予測できていない
という問題を抱えている。海氷域が大気や海洋に及ぼす影響を
明らかにするためには気候モデルにおいて海氷分布が正しく再現
されるのが望ましいのであるが、海氷の諸過程の中でも力学過程、
特に変形による氷厚発達過程は再現性が良くなく重要な課題と
されている。そこで本研究では海氷レオロジーに着目し、季節海氷域
の例としてオホーツク海の海氷域を対象として現用の海氷モデルで
広く用いられるHibler (1979)のレオロジーの検証を行った。
このレオロジーは領域や水平分解能によらず現在多くの数値海氷
モデルで用いられているが元は北極多年氷域を対象に開発された
ものであり、異なる形態の海氷が存在する季節海氷域で適用可能
なのかどうかは今なお検証が必要である。また、このレオロジーで
設定されている降伏曲線の意味はあまり深く吟味されてこなかった。
そこで、ここではRothrock (1975)の考え方に倣って海氷の変形場
による仕事率という観点から、このレオロジーの意味する事を考察
しながら検証を行った。用いたデータはAMSRから求めた海氷漂流速度
と密接度、紋別流氷レーダーから求めた海流漂流速度、それに
PALSARから求めた氷厚分布である。解析の結果、概ね肯定的な結果が
得られたがオホーツク海では変形の度合いが北極海よりも大きく、
海氷変形過程のパラメタリゼーションという観点からは、特にグリッド
間隔に関して注意深い取り扱いが必要であることが示された。

The validity of the sea ice rheological model formulated 
by Hibler (1979), which is widely used in present numerical 
sea ice models, is examined for the Sea of Okhotsk as an 
example of the seasonal ice zone (SIZ), based on satellite-
derived sea ice velocity, concentration and thickness. 
Our focus was the formulation of the yield curve, the shape 
of which can be estimated from ice drift pattern based on the 
energy equation of deformation, while the strength of the ice 
cover that determines its magnitude was evaluated using ice 
concentration and thickness data. Ice drift was obtained with 
a grid spacing of 37.5 km from the AMSR-E 89 GHz brightness 
temperature using a maximum cross-correlation method. The ice 
thickness was obtained with a spatial resolution of 100 m from 
a regression of the PALSAR backscatter coefficients with ice 
thickness. To assess scale dependence, the ice drift data 
derived from a coastal radar covering a 70-km range in the
 southernmost Sea of Okhotsk were similarly analyzed. The results 
obtained were mostly consistent with Hibler’s formulation that 
was based on the Arctic Ocean on both scales with no dependence 
on a time scale, and justify the treatment of sea ice as a plastic 
material, with an elliptical shaped yield curve to some extent. 
However, it also highlights the difficulty in parameterizing sub-
grid scale ridging in the model because grid scale ice velocities 
reduce the deformation magnitude by half due to the large variation 
of the deformation field in the SIZ.

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