過去のセミナー

最新のセミナー案内は こちら をご覧ください。

2018年6月21日 David Thompson博士,Yohai Kaspi博士セミナー

場 所:東京大学先端科学技術研究センター3号館 中2階セミナールーム
日 時:6月21日(木)14:00~17:15
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講演者:Prof. David J. Thompson (Colorado State University)
題 目:A key constraint on the depth of the extratropical troposphere
要 旨:In this talk, I will argue that the depth of extratropical troposphere is primarily constrained by the same physics that govern the temperature of tropical anvil clouds. That is: The extratropical tropopause is constrained by the rapid decrease with height of radiative cooling by water vapor in clear sky regions, and thus by the thermodynamic constraints placed on saturation water vapor pressure. The results suggest that the climate feedbacks associated with high cloud in the tropics are also important in the extratropics, thus roughly doubling the surface area affected by the positive high cloud feedback. They also imply that the extratropical tropopause should stay at roughly the same temperature and thus lift under climate change.
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講演者:Prof. Yohai Kaspi (Weizmann Institute of Science)
題 目:Dynamics of Jupiter’s atmosphere in light of the Juno mission
要 旨:The Juno spacecraft has been in orbit around Jupiter since July 2016. In this talk we will review the mission and the preliminary results from its first year. Particularly we will focus on the Juno gravity experiment, which has revealed the depth and vertical structure of Jupiter’s jet-streams and interior flows.
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2018年6月19日 ミニワークショップ ― atmospheric dynamics and climate variability

場 所:東京大学先端科学技術研究センター3号館 中2階セミナールーム
日 時:6月19日(火)10:00~17:00
(visitorの講演のみ掲載)
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講演者:Prof. David J. Thompson (Colorado State University)
題 目:Periodicity in the extratropical circulation
要 旨:Various measures of extratropical storm activity exhibit periodicity on time scales of 20–25 days. The periodicity is most robust in the Southern Hemisphere, where it is readily apparent in hemispheric averages of eddy activity and precipitation. In this talk, I will explore the dynamics of the periodicity and its projection onto synoptic spatial scales in both the Northern and Southern Hemispheres.
The ~20-25 day periodicity in extratropical storm activity derives from out-of-phase anomalies in eddy activity that form in the wake of decaying wave packets as they propagate to the east. The out-of-phase anomalies in eddy activity give rise to periodicity not only on hemispheric scales, but also on synoptic scales when the circulation is sampled along a path following the flow to the east. Experiments run on a simple model suggest that the projection of the periodicity onto synoptic scales derives from 1) two-way interactions between the heat fluxes and baroclinicity in the lower troposphere and 2) the contrasting eastward speeds of the source of the periodicity in the lower troposphere and wave packets in the upper troposphere.
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講演者:Prof. Yohai Kaspi (Weizmann Institute of Science)
題 目:Dynamics of Jupiter’s atmosphere in light of the Juno mission
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2018年5月29日 NORPAN セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:5月29日(火)15:00~18:10
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講演者:Mr. Leonidas Tsopouridis (University of Bergen)
題 目:Air-sea interaction processes in the Gulf Stream region
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講演者:Dr. Lea Svendsen (University of Bergen)
題 目:Pacific decadal variability and Arctic temperature trends in wind stress pacemaker experiments
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2018年4月19, 24, 27日 Yohai Kaspi 准教授セミナー

【セミナー1】

日 時:2018年4月19日(木) 17:15-18:45
場 所:東京大学先端科学技術研究センター3号館 M2階 セミナー室
題 目:Dynamics of eddy-driven jets and macroturbulent scales in the atmosphere
要 旨
East-west atmospheric jet streams dominate the dynamics of Earth and the other planets. We study the dynamics of eddy-driven jets using a high-resolution idealized atmospheric GCM, where we systematically vary Earth’s rotation rate. This allows for a clear separation between the subtropical and eddy-driven jets, which in Earth's atmosphere, due to Earth’s size and rotation rate, are often merged together. We find that due to asymmetries in baroclinicity across the width of the jet, eddy-driven jets in general tend to migrate poleward. The migrating jets allow a latitude-by-latitude analysis of the macroturbulent scales controlling the jets. It is found that over a wide range of eddy scales and latitudes the jet width scales with the Rhines scale. This is analyzed through partitioning of the barotropic energy spectrum to the role of eddy-mean and eddy-eddy interactions, in transferring energy up- and down-scale. We find that this can explain the observed energy spectrum in Earth's atmosphere. Finally, the role of eddy-eddy interactions is analyzed through comparison to simulations where they are omitted. It is found that eddy-eddy interactions are not a prerequisite for formation of jets, and in fact limit the meridional extent of multiple eddy-driven jets.

【セミナー2】

日 時:2018年4月24日(火) 10:15-11:45
場 所:東京大学先端科学技術研究センター3号館 207号室
題 目:Mechanisms controlling the spatial structure of midlatitude storm tracks and their dependence on climate change
要 旨
Midlatitude storm tracks dominate the transport of momentum, heat and water-vapor in the extratropics. Therefore, identifying the mechanisms governing their temporal and spatial structure is vital for understanding weather and climate. In this talk, we analyze the spatial structure of midlatitude storm tracks by tracking transient cyclonic eddies in an idealized GCM with several levels of complexity and in CMIP5 simulations. The localized atmospheric response is decomposed in terms of a time-zonal mean background flow, a stationary wave and a transient eddy field. The Lagrangian tracks are used to construct cyclone composites and perform a spatially varying PV budget. Three distinct mechanisms that contribute to the spatial structure emerge: transient nonlinear advection, latent heat release and stationary advection. The downstream evolution of the PV composites shows the different role of these mechanisms as the storm tracks evolve downstream. We demonstrate that the poleward motion of individual cyclones increases with increasing global mean temperatures, and by this provide an alternate quantitative explanation to the poleward shift of storm tracks under climate change.

【セミナー3】

日 時:2018年4月27日(金) 16:30-18:00
場 所:東京大学先端科学技術研究センター3号館 307号室
題 目:The relation between jet characteristics and a midwinter minimum in eddy activity over the Pacific and Atlantic basins
要 旨
The existence of a midwinter minimum in eddy activity over the Pacific has been a key puzzle ever since its identification by Nakamura (1992). Here we show that a similar phenomenon to the well known Pacific midwinter minimum can be found over the Atlantic in years when the jet is on average more equatorward in winter. The reduction in eddy activity over the Atlantic is smaller than the Pacific case and persists for a shorter duration. It is suggested that this difference is due to the Atlantic jet being overall more eddy-driven and climatologically farther poleward than the Pacific jet. To further evaluate the relation between the character of the jet and eddy activity, an idealized GCM that is designed to resemble different climatological jet characteristics is used. We find that differences in jet characteristics between a purely eddy-driven jet (as often occurs in the Atlantic and during the Pacific shoulder seasons), and a mixed eddy-driven-subtropical jet (as often occurs in midwinter in the Pacific) play an important role in the relation between jet strength and eddy activity. It is therefore suggested that the seasonal difference in jet characteristics plays an important role in the existence of the Pacific midwinter minimum. This is demonstrated both in reanalysis data and in idealized GCM simulations.

2017年11月29日 NORPAN Workshop

場 所:東京大学先端科学技術研究センター3号館307号室
日 時:11月29日(火)13:00~16:30
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講演者:Dr. Thomas Spengler (University of Bergen)
題 目:Unifying perspectives on atmosphere-ocean interactions during cyclone development
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講演者:Ms. Kristine Haualand (University of Bergen)
題 目:Diabatic effects on baroclinic development
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講演者:Dr. Christian Weijenborg (University of Bergen)
題 目:Isentropic slope as a measure of baroclinic instability and its role in cyclone intensification
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講演者:Mr. Leonidas Tsopouridis (University of Bergen)
題 目:Influence of the Gulf Stream sea surface temperature front on the evolution of storms
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講演者:Dr. Fumiaki Ogawa (University of Bergen)
題 目:Difference between mean and instantaneous wind direction associated with air-sea fluxes
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2017年11月19日~22日 John M. Wallace名誉教授ら 来訪

場 所:東京大学先端科学技術研究センター3号館207号室、M202号室、3号館南棟ENEOSホール
日 時:11月19日(日)~11月22日(水)
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3日間にわたる内部でのセミナーに加え,21日午後には公開講演会を開催しました。
詳細は こちら をご覧ください。
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2016年9月13日 Yvan Orsolini博士セミナー

場 所:東京大学先端科学技術研究センター3号館307号室
日 時:9月13日(火)16:00~17:30
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講演者:Dr. Yvan Orsolini (University of Bergen)
題 目:Coupling of the troposphere-stratosphere-mesosphere-thermosphere during SSW
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2016年7月22日 Seok-Woo Son教授セミナー

場 所:東京大学先端科学技術研究センター3号館 中2階セミナールーム
日 時:7月22日(金)16:30~17:00
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講演者:Prof. Seok-Woo Son (Seoul National University)
題 目:The role of stratosphere-troposphere dynamic coupling on the subseasonal-to-seasonal prediction

要 旨
It has been widely recognized that stratosphere can affect tropospheric circulation on various time scales. Such influences include downward coupling associated with the Antarctic ozone (O3), equatorial Quasi-Biennial Oscillation (QBO), and stratospheric sudden warming (SSW).
In this presentation, their implications to the subseasonal-to-seasonal (S2S) prediction are discussed with a particular emphasis on the possible impacts of the QBO on the boreal-winter Madden-Julian Oscillation (MJO) and the associated teleconnection pattern in the operational S2S prediction models.
The predictability of SSW and its influence on the extratropical surface weather and climate are also briefly discussed.
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2016年7月8日 Thomas Spengler教授セミナー

場 所:東京大学先端科学技術研究センター3号館 307号室
日 時:7月8日(金)16:00~17:30
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講演者:Prof. Thomas Spengler (University of Bergen)
題 目:Upper Tropospheric Jet Axis Detection: Winter 2013/2014 and Northern Hemispheric Variability
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2016年6月29日 Shang-Ping Xie教授セミナー

場 所:東京大学先端科学技術研究センター3号館 中2階セミナールーム
日 時:6月29日(金)15:00~16:30
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講演者:Prof. Shang-Ping Xie (Scripps Institution of Oceanography)
題 目:Convective jump, post-El Niño Pacific-Japan pattern, and the conundrum of SST-convection relationship over the Northwest Pacific

要 旨
The abrupt onset of deep convection in late July over the tropical Northwest Pacific is associated with dramatic changes in East Asian climate, including the termination of the Meiyu-Baiu rainy period in southeastern China-Japan and the start of the rainy season for northeastern China (Ueda et al. 1995). What triggers this Ueda Jump has been a mystery as local SSTs exceed 28oC at least a month earlier. We show that the gradual moistening of the mid-troposphere leads to a threshold behavior in the Ueda Jump. Specifically, the entrainment in the stable dry layer above the planetary boundary layer causes convective plumes lose buoyancy until the gradual moistening of the dry layer eventually allows the entraining plume to retain positive buoyancy. Once breaking through the dry layer, the convection can reach all the way to the tropopause, forcing a Pacific-Japan pattern to transform East Asian climate.
This summer is being closely watched because the Pacific-Japan pattern tends to develop following a major El Nino, resulting in an active Meiyu-Baiu season in East Asia. On one hand, atmospheric GCMs forced with observed SST can generally simulate the interannual Pacific-Japan pattern but on the other hand, they distort the local SST-rainfall correlation. We will tackle this conundrum in the seminar.
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2016年2月10日 木下武也博士・神代剛氏セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:2月10日(水)14:30~18:30
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講演者:木下武也博士(情報通信研究機構)
題 目:波と背景場の相互作用及び波の伝播を記述する方程式系について

要 旨
緯度高度断面における波と平均場の相互作用を記述する変形オイラー平均 (TEM) 系 は、これまで多くの研究に用いられてきた。その結果、成層圏の ブリュワ・ドブソン循環はプラネタリー波・傾圧波・大気重力波、中間圏 の夏極から冬極に流れる子午面循環は主に大気重力波によって駆動されて いることがわかっている。しかしながら上記の循環を含む空気塊の経度 方向の流れや、子午面循環の経度依存性 については、まだ解明されて いないことが多く、TEM系を3次元に拡張する研究が行われてきた。 一方でTEM系は、地表面付近の物質輸送や不安定波に伴う残差流を正確に 記述できない問題が指摘されている。温位面上の質量重みつき東西平均 (MIM) 系は、上記課題を克服した方程式系であり、最近では寒気流出に着目 した解析が行われている。しかしながら、MIM系を3次元に拡張した研究は、 特に波活動を記述するfluxについては行われていない。そこで本発表では、 TEM系及びTEM系を3次元に拡張した研究を紹介するとともに、MIM系の 波活動度fluxにおいて波の伝播を記述するfluxの3次元化について議論する。

講演者:神代剛氏(気象研究所)
題 目:雲タイプ別にみた夏季北太平洋層状性下層雲量の数年規模変動
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2015年11月24日 Yang Zhang博士セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:11月24日(火)17:00~18:30
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講演者:Dr. Yang Zhang (Nanjing University)
題 目:Roles of barotropic and baroclinic eddy feedbacks in the midlatitude eddy-driven jet response to lower-tropospheric thermal forcing

要 旨
Both observations and climate model simulations have shown that the eddy-driven jet exhibits significant meridional shift in response to the lower-boundary thermal forcing, such as the recent Arctic amplified warming induced by the sea ice loss, extra-tropical sea surface temperature anomalies and El Nino-like oceanic warming. Understanding the dynamical mechanisms of the atmospheric response to such lower- boundary thermal forcing is central for the prediction of the mid-latitude climate and evaluation on the climate change sensitivity. In this talk, the mechanism through which lower level thermal forcing affecting the jet shift is investigated by using an idealized dynamical model. Further more, through the Finite Amplitude Wave Activity diagnostics and by overriding the barotropic wind in the potential vorticity advection, the relative roles of barotropic and baroclinic processes in the eddy feedbacks are quantified and explicitly compared. Unlike the conventional baroclinic viewpoint, our study suggests the important role of barotropic feedback process in the total atmospheric response to the lower boundary thermal forcing. The mechanism revealed in this study also provides a dynamical interpretation for the change in Rossby wave breaking frequency during El Nino or under climate warming. ------------

2015年11月26日 Yang Zhang博士・Harry Hendon博士セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:11月26日(木)15:00~18:30
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講演者:Dr. Yang Zhang (Nanjing University)
題 目:The Annular Modes: dynamics, diagnostics and model simulations

要 旨
The annular modes (with its former name Artic Oscillation, Antarctic Oscillation) are the leading modes of Northern/Southern Hemisphere extratropical variability on intraseasonal time scales, which is often described as a nearly zonally symmetric latitudinal displacement of the midlatitude jet. The dominance and persistence of the annular modes are suggested due to the positive eddy-mean flow feedbacks. Understanding the dynamics of the Annular Modes is important for the extratropical intraseasonal and decadal predictability. In this talk, I will first review the barotropic and baroclinic mechanisms that have been proposed for the positive eddy-mean flow feedbacks, and introduce a new mechanism through which low-frequency and synoptic eddies working symbiotically extending the persistence of the jet shift. This talk further introduces a new method to quantify the relative contributions of the barotropic and baroclinic eddy feedbacks to the Annular Modes persistence. Through a “hybrid Eulerian-Lagrangian Finite Amplitude Wave Activity diagnostic” using the ERA-40 and ERA- Interim reanalysis data, the different roles of barotropic and baroclinic eddy feedbacks in the annular modes are identified. The new diagnostics are also applied to assess the Annular Mode simulations in the CMIP5 models. The abilities of the state of the art climate models in simulating the eddy feedbacks in the annular modes are discussed.
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講演者:Dr. Harry Hendon (Bureau of Meteorology in Melbourne Australia)
題 目:Weakened Predictability of El Nino in the Early 21st Century

要 旨
Predictive skill for El Niño in the tropical eastern Pacific across a range of forecast systems and models declined sharply in the early 21st century relative to what was achieved in the last two decades of the 20th century despite ongoing improvements of forecast systems and ocean observations used to initialize the forecasts. This decline coincided with a reduction of El Niño variability together with a shift in Pacific climate to a stronger Walker circulation at the end of the 20th century that has previously been associated with the hiatus in global warming. We show, using seasonal forecast sensitivity experiments with the Australian Bureau of Meteorology coupled model seasonal prediction system POAMA2.4, that the shift to a stronger Walker circulation weakened the ocean-atmosphere feedbacks that amplify El Niño in the eastern Pacific, thus resulting in weaker variability that is less predictable. Anticipating future decadal changes in El Niño predictability is an outstanding challenge because prediction of natural decadal variations of Pacific climate such as occurred at the end of the 20th century has not been demonstrated and no consensus has yet emerged about impacts of anthropogenic climate change on El Niño variability, hence predictability, in a future climate.
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2015年6月22日 Stuecker博士セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:6月22日(水)16:00~
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講演者:Dr. Malte Stuecker (ハワイ大学)
題 目:ENSO/Annual Cycle interactions and their impact on Indo-Pacific climate

要 旨
Nonlinear interactions between the El Niño-Southern Oscillation (ENSO) and the Western Pacific warm pool annual cycle generate an atmospheric combination mode (C-mode) of atmospheric circulation variability. We demonstrate that C-mode dynamics are responsible for the development of an anomalous low-level North-West Pacific anticyclone (NWP-AC) during El Niño events. The NWP-AC is embedded in a large-scale meridionally anti-symmetric Indo-Pacific atmospheric circulation response and has been shown to exhibit large impacts on the Asian Monsoon system.
In contrast to previous studies, we find the role of air-sea coupling in the Indian Ocean and North-West Pacific only of secondary importance for the NWP-AC genesis. Moreover, the NWP-AC is clearly marked in the frequency domain by near-annual combination tones, which have been overlooked in previous Indo-Pacific climate studies.
Furthermore, we hypothesize a positive feedback loop involving the anomalous low-level NWP-AC through El Niño and C-mode interactions: the development of the NWP-AC as a result of the C-mode acts to rapidly terminate El Niño events. The subsequent phase shift from retreating El Niño conditions towards a developing La Niña phase terminates the low-level cyclonic circulation response in the Central Pacific and thus indirectly enhances the NWP-AC and allows it to persist until boreal summer. Anomalous local circulation features in the Indo-Pacific (such as the NWP-AC) can be considered a superposition of the quasi-symmetric linear ENSO response and the meridionally anti-symmetric annual cycle modulated ENSO response (C-mode).
We emphasize that it is not adequate to assess ENSO impacts by considering only interannual timescales. C-mode dynamics are an essential (extended) part of ENSO and result in a wide range of deterministic high-frequency variability. A general framework for this frequency cascade will be discussed.
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2015年4月22日 Orsolini博士ほかセミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:4月22日(水)14:00~
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講演者:Dr. Thomas Spengler (ベルゲン大学)
題 目:Climatological analysis of the slope of isentropic surfaces and its tendencies over the North Atlantic

要 旨
The maintenance of baroclinicity along the mid- and high-latitude storm tracks is a matter of ongoing debate. Using an isentropic framework, a novel diagnostic based on the tendency equation for the slope of isentropic surfaces ? a measure of the potential for baroclinic development ? is presented. The tendency comprises contributions from dynamic processes, latent heat release, radiation, and sub-gridscale turbulence, which incorporates the effect of sensible heat fluxes. A climatology of these tendencies over the North Atlantic is compiled for the winters 2009 and 2010.
It is found that adiabatic tilting flattens the isentropic surfaces, reflecting the action of growing baroclinic cyclones. This tendency is climatologically balanced by the generation of slope by diabatic processes. In the lower troposphere, the most intense diabatic increase of slope is found along the oceanic frontal zone associated with the Gulf Stream and at higher latitudes in the Labrador Sea, the Nordic Seas and the Barents Sea. Latent heat release and sensible heat fluxes both contribute substantially in these regions. A quantitative analysis of cold air outbreaks emphasises their important role for restoring the slope in the lower troposphere over the Gulf Stream region and off the sea-ice edge at high latitudes. In the upper troposphere, latent heat release due to cloud microphysical processes is the dominant mechanism maintaining the slope.
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講演者:Dr. Annick Terpstra (ベルゲン大学)
題 目:The influence of surface fluxes on forward shear polar low development in idealized simulations

要 旨
To gain insight into the influence of surface fluxes on the dynamical mechanisms of polar low formation we utilise an idealised numerical baroclinic channel model with boundary layer parameterisations. Growth of the perturbation is initiated by a low level warm-cored cyclone.

Firstly, we explore the role of latent heating during polar low formation. The experimental design resembles a typical forward-shear moist-baroclinic environment at high-latitudes. Cyclogenesis is triggered by a weak, low-level thermal perturbation in hydrostatic and geostrophic balance. Within the experimental setup, significant disturbance growth is possible in absence of upper level forcing, surface fluxes, and radiation. The relative importance of the generation of eddy available potential energy by diabatic versus baroclinic processes is used to differentiate between the dynamical processes contributing to disturbance growth. The experiments indicate that sufficient latent heat release in the north-eastern quadrant of the cyclone is crucial for rapid disturbance intensification, where environmental relative humidity, baroclinicity and static stability all play a role in the potential of latent heat release to occur. The relative shallow perturbation depth at high-latitudes improves the effectiveness of latent heat release on cyclone amplification.

Secondly, we explore the role of surface fluxes during polar low formation. In addition to the previous experimental design we included parameterisations for surface friction and surface turbulence fluxes. Furthermore we imitate a cold air outbreak by introducing a uniform temperature difference between the sea-surface temperature and low-level air temperature. Evaluation of the spatial distribution of the generation of eddy available potential energy indicates that surface sensible heat fluxes are decreasing the eddy available potential energy. Futhermore, simulations including surface turbulence fluxes exhibit the development of a warm-core, which in addition decreases the eddy available potential energy.
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講演者:Dr. Yvan Orsolini (ベルゲン大学)
題 目:Impact of springtime snow over the Himalayan-Tibetan Plateau on the onset of the Indian summer monsoon in coupled forecasts

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2015年3月26日 Hsu博士セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
日 時:3月26日(木)16:30~18:30
講演者:Dr. Huang-Hsiung Hsu (中央研究院)
題 目:SSTa, SICa, and Extreme Circulation during the 2013–14 Boreal Winter

要 旨
Unprecedented atmospheric circulations with extreme weather and climate were observed in the extratropical Northern Hemisphere during the winter of 2013-2014. The anomalous circulations were the manifestation of the Pacific pattern or the North Pacific Oscillation/Western Pacific teleconnection pattern but with extremely large amplitude. Simulation results suggest that the anomalous atmospheric circulations were constructively induced by anomalous sea surface temperature (SST) in the tropical Pacific and extratropical North Pacific, as well as the low sea ice concentration (SIC) in the Arctic. Natural variability played a major role in inducing the anomaly pattern, whereas the anomalously warm SST and low sea ice concentration in the Bering Sea contributed to the intensity. If the anthropogenic warming has a significant impact on causing the synchronization of the aforementioned anomalies in SST and SIC and this trend continues, severe winters similar to that in 2013–2014 may occur more frequently in the near future.
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2014年11月21日 Ping Chang教授セミナー

日 時:11月21日(金)16:30~18:00
場 所:東京大学先端科学技術研究センター3号館207号室
講演者Prof. Ping Chang (Department of Oceanography, Texas A&M University; 東大先端研フェロー,学振招聘研究者)
題 目:Ocean Mesoscale Eddy-Atmosphere (OMEA) Interaction

要 旨
Like storms and weather in the atmosphere, mesoscale eddies are ubiquitous features in the world oceans, and carry more than half of the kinetic energy of the global ocean circulation. It has been widely recognized that mesoscale ocean eddies play a major role in the mixing and transport of water properties, affecting the generation and maintenance of strong ocean currents and fronts, as well as the physical and biogeochemical properties of water mass. However, the extent to which ocean mesoscale eddies can directly influence the atmosphere, and thereby impact weather and climate, is far from being fully understood. The central theme of this talk is to demonstrate that the ocean and atmosphere interact strongly at frontal- and meso-scales, and this interaction can be critically important for understanding, simulating and predicting weather and climate extremes, such as tropical cyclones and extratropical winter storms. High-resolution modeling and observational evidence will be presented to showcase how ocean mesoscale eddies can exert both local and remote influences on near-surface wind and rainfall patterns and how ocean mesoscale eddy-atmosphere (OMEA) feedback can potentially affect energetics and structures of strong ocean currents and fronts. Current generation global climate models and observing systems are inadequate in fully resolving the complex nature of OMEA interaction. Improving our capability to better observe and model OMEA interaction remains one of the grand challenges for oceanographers and climate scientists.
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2015年1月23日 東大先端研セミナー

場 所:東京大学先端科学技術研究センター3号館207号室
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日 時:1月23日(金)10:00~11:30
講演者:Dr. Lin Wang (中国大気科学院大気物理研究所)
題 目:The extreme European cold spell in 2011/12 winter: observed features and possible causes
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日 時:1月23日(金)12:30~14:00
講演者:Prof. Ping Chang (Texas A&M University)
題 目:Towards Understanding of Extratropical Climate Variability: Role of Atmospheric Synoptic Storms and Frontal-and Meso-scale Ocean-Atmosphere Interaction

要 旨
Western boundary current regimes, such as the Kuroshio and Gulf Stream Extension regions, are critical regions for atmospheric synoptic storm development and strong frontal-and meso-scale ocean-atmosphere interaction. In this talk, we will attempt to explore potential contributions from both these processes to large-scale extratropical climate variability.
In the first part of the talk, we will present results of an observationally-based analysis on the relationship between extreme flux events over Kuroshio Extension region (KER) and Gulf Stream region (GSR) associated with North Pacific and Atlantic cold-air outbreaks and patterns of large-scale climate variability in these regions.
In the second part of the talk, we will present results of high-resolution climate model simulations designed to explore and understand how frontal-and meso-scale ocean-atmosphere interaction can affect large-scale atmospheric and oceanic circulations.
Collectively, the findings of these studies point to the importance of frontal-and meso-scale ocean-atmosphere dynamics in extratropical climate variability, and thus the need of improving their representation in climate models.
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2014年11月21日 Ping Chang教授セミナー

日 時:11月21日(金)16:30~18:00
場 所:東京大学先端科学技術研究センター3号館207号室
講演者Prof. Ping Chang (Department of Oceanography, Texas A&M University; 東大先端研フェロー,学振招聘研究者)
題 目:Ocean Mesoscale Eddy-Atmosphere (OMEA) Interaction

要 旨
Like storms and weather in the atmosphere, mesoscale eddies are ubiquitous features in the world oceans, and carry more than half of the kinetic energy of the global ocean circulation. It has been widely recognized that mesoscale ocean eddies play a major role in the mixing and transport of water properties, affecting the generation and maintenance of strong ocean currents and fronts, as well as the physical and biogeochemical properties of water mass. However, the extent to which ocean mesoscale eddies can directly influence the atmosphere, and thereby impact weather and climate, is far from being fully understood. The central theme of this talk is to demonstrate that the ocean and atmosphere interact strongly at frontal- and meso-scales, and this interaction can be critically important for understanding, simulating and predicting weather and climate extremes, such as tropical cyclones and extratropical winter storms. High-resolution modeling and observational evidence will be presented to showcase how ocean mesoscale eddies can exert both local and remote influences on near-surface wind and rainfall patterns and how ocean mesoscale eddy-atmosphere (OMEA) feedback can potentially affect energetics and structures of strong ocean currents and fronts. Current generation global climate models and observing systems are inadequate in fully resolving the complex nature of OMEA interaction. Improving our capability to better observe and model OMEA interaction remains one of the grand challenges for oceanographers and climate scientists.
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2014年10月10日 堀田大介博士セミナー

日 時:10月10日(金)16:45~18:15
場 所:東京大学先端科学技術研究センター3号館207号室
講演者:堀田大介(気象庁数値予報課技術専門官)
題 目:アンサンブル・データ同化に基づく観測の予報感度(EFSO)を用いた「先回り的品質管理」(Proactive QC)

要 旨
 2011年8月より3年間、米国メリーランド大学の応用数学プログラムに留学し、数値天気予報、特にデータ同化に関する研究を行って博士号を取得した。本セミナーでは博士論文の成果のうち、アンサンブル・データ同化を用いた感度解析に関連する話題を2つ紹介する。セミナーでは冒頭で数値予報とデータ同化に関する簡単な解説を行なう予定なので、以下に述べた研究の概要が理解できなくとも心配しないで頂きたい。

 1つ目は観測データの品質管理への応用である。大気はカオス的力学系であり、初期値に誤差があればそれが如何に微小であってもモデルの積分時間が長くなるとともに予報が現実から乖離してしまう。数値予報システムにおいては、短時間の予報を観測により修正し新たな初期値を作成することにより予報の現実からの乖離を防いでいる。この処理をデータ同化という。しかし、実際の観測値には品質の悪いものも含まれており、これらを同化してしまうとかえって予報が極端に悪化することがある。現業システムでは、個々の観測値を他の観測値やモデルの予報と比較し、不整合がある場合にはこれを排除する処理(品質管理(Quality Control; QC)という)を行い「悪い観測」の同化を防いでいるが、完全に防ぐことはできない。一つ一つの観測について、それが実際に予報を改善するかをどうかを基準に品質管理を行なうことができれば理想的であるが、これまでそれを実現可能な計算コストで行う方法がなかった。一方、近年になって、一つ一つの観測が予報に与えるインパクトを、アンサンブル・データ同化を用いることにより低い計算コストで診断できる手法(Ensemble Forecast Sensitivity to Observations; EFSO)が開発され(Kalnayet al. 2012; Tellus A)、NCEPの現業システムにてその有用性が確認された(Ota et al. 2013; Tellus A)。そこで本研究では、EFSOを用いて予報に悪影響を与えうる観測データを事前に同定しこれを同化しないことで予報の改善をはかる新しい品質管理手法「先回り的品質管理」(Proactive QC) を提案し、これを低解像度版のNCEP現業数値予報システムに実装して実験を行った。実験の結果、34日間136事例中20事例において、予報を悪化させたであろう「悪い」観測がEFSOにより検出された。さらに、検出された20事例について「悪い」観測を排除して同化・予報実験を行ったところ、18事例で予報が改善されること、またそのうち7事例については局所的な予報誤差が30%以上も小さくなることが確認された。本手法は、予報をオンライン的に改善できるのみならず、検出された「悪い」観測に関する知見を蓄積することでオフライン品質管理の改善にも資することが期待できる。また、再解析は実時間の現業システムに比べ時間的制約が厳しくないことから、この手法の応用には特に適している。

 2つ目は観測誤差共分散行列の最適化への応用である。データ同化では予報と観測を「最適な」重みにより加重平均することで解析値を得る。この「最適な重み」(カルマンゲイン行列)は予報と観測それぞれの先験的な不確かさを表わす背景誤差共分散行列(B)および観測誤差共分散行列(R)をもとに算出される。これらの共分散行列はデータ同化システムへ外部から与えられるパラメータであり、チューニングの対象となる。Rのチューニング手法の一つとして、Rの各要素に微小な変分を与えた場合の予報誤差の増減をアジョイント法により見積もる方法が提案されている(Daescu 2008; MWR)が、アジョイントモデルが必要となることから開発や計算にかかるコストが大きいという問題がある。本研究では、アンサンブルを用いた、アジョイント不要で計算コストの低いアルゴリズム(Ensemble Forecast Sensitivity to R; EFSRと呼ぶ)を考案し、簡単な低次元モデルを用いた系でその有効性を確認したのち、これをNCEPの現業システムに実装した。実験の結果、EFSRの診断結果に基づきRをチューニングすることで実際に観測による予報へのインパクトを改善できることが示された。
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Title: Proactive QC based on Ensemble Forecast Sensitivity to Observations(EFSO) and Ensemble Forecast Sensitivity to observation covariance matrix R (EFSR)

Abstract:
Despite recent major improvements in numerical weather prediction (NWP) systems, operational NWP forecasts occasionally suffer from an abrupt drop in forecast skill, a phenomenon called “ orecast skill dropout." Recent studies have shown that the "dropouts" occur not because of the model's deficiencies but by the use of flawed observations that the operational quality control (QC) system failed to filter out. Thus, to minimize the occurrences of forecast skill dropouts, we need to detect and remove such flawed observations.A diagnostic technique called Ensemble Forecast Sensitivity to Observations (EFSO) enables us to quantify how much each observation has improved or degraded the forecast. A recent study (Ota et al., 2013, Tellus A) has shown that it is possible to detect flawed observations that caused regional forecast skill dropouts by using EFSO with 24-hour lead time and that the forecast can be improved by not assimilating the detected observations.

Inspired by their success, in the first part of this study, we propose a new QC method, which we call Proactive QC (PQC), in which flawed observations are detected 6 hours after the analysis by EFSO and then the analysis and forecast are repeated without using the detected observations. This new QC technique is implemented and tested on a lower-resolution version of NCEP's operational global NWP system. The results we obtained are extremely promising; we have found that we can detect regional forecast skill dropouts and the flawed observations after only 6 hours from the analysis and that the rejection of the identified flawed observations indeed improves 24-hour forecasts.

In the second part, we show that the same approximation used in the derivation of EFSO can be used to formulate the forecast sensitivity to observation error covariance matrix R, which we call EFSR. We implement the EFSR diagnostics in both an idealized system and the quasi-operational NWP system and show that it can be used to tune the R matrix so that the utility of observations is improved. Proactive QC, if implemented into the operational system, will allow us to build a database of defective observations. We believe that we can help instrument and algorithm developers to identify and fix potential flaws in their algorithms by providing such database along with relevant metadata. Another important application of this study is to use EFSO and EFSR to accelerate the development of optimal assimilation methods for new observing systems. Current approach based on Observing System Experiment (OSE) has difficulties in obtaining statistically significant signals from new observing systems in the presence of the already abundant observations that are available in the "control" system. EFSO should address this issue by finding forecast impact of each observation and allowing the comparison of the impacts of different observation processing algorithms. Another non-trivial issue in designing assimilation methods for new observing systems is how to optimally specify the R matrix for them; our EFSR diagnostics can be used in this respect.
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