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第325回 大気海洋物理学・気候力学セミナー のおしらせ

日　時： 8月1日(木) 9:30 - 12:00
Date ： Thu., 1 Aug. 9:30 − 12:00
場　所： 環境科学院D201室
Place ： Env. Sci. Bldg. D201

Speaker:Pat Wongpan (Institute of Low Temperature Science / Postdoctoral Researcher)
Title：Using under-ice spectra to determine fast ice algal biomass in Saroma-ko lagoon, Hokkaido, Japan and Representation of sub-ice platelet layer in a one-dimensional thermodynamic sea ice model

発表者：平沢 雅弘（大気海洋物理学・気候力学コース/D2）
Speaker:Masahiro Hirasawa (Course in Atmosphere-Ocean and Climate Dynamics / Doctoral Course Student D2)
題目：日本海を通過する寒冷前線の多重構造とその形成メカニズム
Title：The Structure and Formation Mechanism of Multiple cold fronts Passing the Sea of Japan

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Using under-ice spectra to determine fast ice algal biomass in Saroma-ko lagoon, 
Hokkaido, Japan and Representation of sub-ice platelet layer in a one-dimensional 
thermodynamic sea ice model 
Pat Wongpan 発表要旨：


　
　Fast ice is a key component of coastal ecosystems in polar regions, providing a 
habitat for ice algal communities. The first year to multiyear ice ratio in the Arctic is 
increasing and sea ice is thinning towards the predicted summer ice-free Arctic in 
this century. To date the estimation of algal biomass by satellites has only applied 
to the unfrozen ocean. This study examines the relationships between the normalized 
difference indices calculated from under-ice hyperspectral measurements, and ice 
algal biomass for land-fast first year ice in Saroma-ko lagoon, Hokkaido, Japan where 
sea ice is thin (~0.5 m). We analyze physical properties of snow and ice supporting 
our 27 paired in situ optical and biological measurements along transect lines across 
multiple scales covering over 250 m × 250 m in February 2019. Our new observation-based 
algorithms can be applied to non-invasively estimate land-fast ice algal biomass which 
will fill the gap of monitoring algal biomass under the ice cover during winter and winter-spring 
transition for thin first year ice. Together with the ocean color remote sensing, our algorithms 
will help improve the understanding of the temporal and spatial variability of algal biomass 
using moorings and underwater vehicles focusing with the thin Arctic sea ice scenario.

　Fast ice grows due to conductive heat losses to the atmosphere. Near continental 
ice shelves, where Ice Shelf Water (ISW) exists at the ocean surface, fast ice also 
thickens because of interaction with the supercooled water column. This ISW is a 
result of the ice shelf–ocean interaction and it creates specific sea ice forms: the porous, 
friable sub-ice platelet layer and incorporated platelet ice. However, the large-scale 
distribution and seasonality of platelet ice are not well documented, which is where 
model representations may help to progress the understanding of their role in the 
functioning of the Southern Ocean. 

　In this second part, we introduce a representation of platelet ice processes by 
analysing mushy-layer physics emerging from a one-dimensional sea ice model 
(the one-dimensional Louvain-la-Neuve Sea Ice Model: LIM1D). We evaluate the 
approach by forcing LIM1D with meteorological observations and prescribed oceanic 
heat flux based on observations from an over-winter study in 2009 on the land-fast 
sea ice of McMurdo Sound, Antarctica. We also evaluate the response of the simulated 
sub-ice platelet layer to oceanic heat flux, by comparison with observation-based retrievals 
from a ~20 km transect in November 2009. Sub-ice platelet layers several meters thick 
are observed and simulated. Analysis of model results suggests that the high liquid fraction 
of the sub-ice platelet layer implies a low thermal conductivity and a high specific heat. 
If the imposed heat loss to the ocean becomes large, the conductive heat flux through 
the sub-ice platelet layer decreases, which thermodynamically decouples the ocean from 
the sea ice. Sensitivity experiments revealed that deep snow intensifies this effect. 

　In summary, a realistic sub-ice platelet layer emerges in a one-dimensional thermodynamic 
sea ice model when suitably forced.  Key factors to formation are sufficient heat loss to 
underlying water and thermal insulation provided by the ice and snow. The sub-ice platelet 
layer is stabilized by its liquid content, yielding low thermal conductivity and high heat capacity. 
Ultimately, the model will not only help to understand the coupled heat and salt transfer of the 
sub-ice platelet layer and how this controls the biogeochemical specifics of Antarctic fast ice, 
but also allow upscaling of these processes to the scale of the Southern Ocean. 



日本海を通過する寒冷前線の多重構造とその形成メカニズム
平沢 雅弘 (Masahiro Hirasawa) 発表要旨：



　日本海上で観測された多重構造を持つ寒冷前線の事例について、数値シミュレー
ションによる解析を行い、その形成メカニズムを明らかにした。
　寒冷前線の特徴（通過時の気温降下、急激な風向変化）を複数隣接して持つも
のは多重寒冷前線と呼ばれ、合体時には降水やシア―の強化を経て、激しい大気
現象をもたらす。多重寒冷前線に関する研究は、ヨーロッパ周辺海域やアメリカ
大陸を通過するものについて観測や統計的手法を中心として行われており、西風
時にロッキー山脈やシエラネバダ山脈の風下に生じる風下トラフは、通過する寒
冷前線が多重寒冷前線となる要因の一つである。一方、冬季の日本海上も温帯低
気圧とともに多くの前線が通過し、その周辺地形からも地形の力学的・熱的効果
によって多重寒冷前線が生じ得る。しかし、この地域においてこれらの効果に着
目した研究は日本海寒帯気団収束帯（JPCZ）やメソ低気圧に関するものが多く、
前線に与える地形の効果に関する研究は行われていない。
　本研究では、日本海上で観測された多重構造を持つ寒冷前線の事例について、
非静力学領域気象モデルWRFを用いて再現実験を行った。その結果、寒冷前線通
過前の日本海上において、南西寄りの暖気移流時に、朝鮮半島のテベク山脈風下
側で、おろし風とハイドロリックジャンプによる混合に伴いメソ前線が発生し、
中国大陸で生じた寒冷前線に接近される形で、日本海上で多重寒冷前線となって
いたことがわかった。また、このメソ前線に関連して生じる内部重力波は、モデ
ル内で高度9km付近に高層雲を形成し、この雲はメソ前線とともに朝鮮半島を離
岸していくが、気象衛星ひまわりでも同様な雲の挙動が確認でき、これらのメカ
ニズムが現実でも起こっていたと考えられる。
　さらに、朝鮮半島の標高を0m（地表面状態は陸）にした感度実験を行うと、寒
冷前線は多重構造を持たなかった。これらから、朝鮮半島の力学効果が今回の多
重寒冷前線の発生要因であったと考えられる。  

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