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Information Center for Mathematical Science

세미나

Information Center for Mathematical Science

세미나

2018 서울대학교 수리과학부 세미나

Following Bonatti-Viana/Avila-Viana, a fiber-bunched SL_n(R) cocycle

2018 서울대학교 수리과학부 세미나

2018 서울대학교 수리과학부 세미나

이 발표에서는 위상학적 자료 분석(Topological Data Analysis)을 어떻게 하고, 그에 대한 통계적 추정을 어떻게 하는지 간단하게 소개한다. 위상학적 자료 분석은 일반적으로는 자료에서 위상학적인 특성을 추출하는 것을 뜻하는데, 여기서는 persistent homology와 군집 나무(cluster tree)의 통계적 추정을 어떻게 하는지 알아본다. persistent homology는 자료를 여러 해상에서 관측하고 지속성 있게 나타나는 위상 특성을 추려낸다. 군집 나무는 자료의 분포 함수의 레벨 집합의 군집이 어떤 계층 구조를 생성하는지 봄으로써 분포 함수를 요약한다. 분포 함수의 persistent homology와 군집 나무는 직접 계산할 수 없으므로 자료로부터 추정하게 된다. 이때 생성되는 오차를 통계적으로 정량화할 수 있는데, 이 발표에서는 persistent homology와 군집 나무의 경우에 어떻게 통계적으로 유효한 신뢰 집합을 얻을 수 있는지 제시하고, 어떻게 실제로 계산할 수 있는지 알아본다.

2018 서울대학교 수리과학부 세미나

In this talk we consider isolated points in the spectrum of linear pencil of operators.

KAIST 수리과학과 세미나

In many applications, the dynamics of gas and plasma can be accurately modeled using kinetic Boltzmann equations. These equations are integro-differential systems posed in a high-dimensional phase space, which is typically comprised of the spatial coordinates and the velocity coordinates. If the system is sufficiently collisional the kinetic equations may be replaced by a fluid approximation that is posed in physical space (i.e., a lower dimensional space than the full phase space). The precise form of the fluid approximation depends on the choice of the moment-closure. In general, finding a suitable robust moment-closure is still an open scientific problem. In this work we consider two specific closure methods: (1) a regularized quadrature-based closure (QMOM) and (2) a nonextensible entropy-based closure (QEXP). In QMOM, the distribution function is approximated by Dirac deltas with variable weights and abscissas. The resulting fluid approximations have differing properties depending on the detailed construction of the Dirac deltas. We develop a high-order discontinuous Galerkin scheme to numerically solve resulting fluid equations. We also develop limiters that guarantee that the inversion problem between moments of the distribution function and the weights and abscissas of the Dirac deltas is well-posed.In QEXP, the true distribution is replaced by a Maxwellian distribution multiplied by a quasi-exponential function. We develop a high-order discontinuous Galerkin scheme to numerically solve resulting fluid equations. We break the numerical update into two parts: (1) an update for the background Maxwellian distribution, and (2) an update for the non-Maxwellian corrections. We again develop limiters to keep the moment-inversion problem well-posed.

연세대학교 대기과학과 세미나

The APEC Climate Center (APCC) is a leading climate information service provider and research institute that serves the 21 APEC member economies as well as the broader international community. The vision of APCC is to contribute to economic growth in the APEC region and support the protection of lives and property, the reduction of economic losses, and enhance economic opportunities. In order to accomplish such vision, APCC produces and provides value-added, reliable, and timely climate prediction to countries in the APEC region, while serving as a key climate information center to distribute climate data, prediction, and related tools. APCC also leads in developing and applying interdisciplinary application techniques, combining climate and other related sectors to serve the social needs and respond to climate change in specific countries and regions. In addition, APCC assists developing economies from the APEC region and beyond in building their capacities to produce reliable climate prediction and to maintain and utilize APCC products created through our climate information application research. In this seminar, I would like to first demonstrate the use of seamless prediction and the development an early warning system by which the provision of preemptive information is made available. Second, I also would like to introduce interdisciplinary research of how agricultural productivity can benefit from climate data and enhanced agrometeorological services.

연세대학교 대기과학과 세미나

The development and availability of high-power ultrashort pulse lasers have attracted many interesting applications such as weather control, lightening guiding, LIDAR, ion-tumor therapy, proton radiography, and fast ignition for inertial confined fusion. As ultrafast lasers propagate through a medium, a highly complex and dynamic physical phenomenon caused by nonlinear interaction between the intense optical field and the propagating medium generates a plasma channel called laser filament. This laser filament has special characteristics such as self-focusing, self-phase modulation and self-healing effect. These properties allow the laser filament to maintain its dimensions for long distances (much longer than the Rayleigh length of an equivalent Gaussian beam), generate broader spectrum, and propagate kilometric distances. This long distance propagation of high intense laser beam allows us to explore remote sensing of explosives/chemical weapons, weather control, lightening guiding and even microwave guiding. In addition to the ultrashort pulse lasers, a recent development of ultra-intense ultrashort pulse lasers up to 4 PW peak power allowed fundamental researches such as direct laser ion acceleration in a relativistic regime. This direct laser proton/ion/electron acceleration opens up various applications of compact ultrashort electron/proton beams and radiations sources for gamma-ray spectroscopy, remote nondestructive inspection, ultrafast bio/nano imaging technology, high-energy particle colliders, proton therapy and radiography. The presentation will cover from the basic concept of laser filament to laser matter interaction in the relativistic regimes and the transition from science fictions to real applications.

연세대학교 대기과학과 세미나

Conventionally in the atmospheric science, atmospheric phenomena have been studied with the three different spatiotemporal regimes of synoptic-, meso- and micro-scales. The atmospheric motions on length scale smaller than a few thousands of kilometers but greater than a few kilometers, and on time scales shorter than several days but longer than an hour are categorized as the mesoscale flows. In fact weather highly impacting our everyday lives, such as severe thunderstorms, usually belongs to the mesoscale regime. Thus, mesoscale models (MMs), which explicitly resolve mesoscale flows, usually have been used for everyday weather forecasting. In order to consider the effect of sub-grid scale (SGS) stress and fluxes on resolved mesoscale flows, MM employs an atmospheric boundary layer (ABL) parameterization. With a conventional MM, it is hard to accurately simulate natural atmospheric flows across the different scale regimes, particularly between meso- and micro-scale regimes. In this presentation, speaker will raise the issue of the research gap between meso- and micro-scale regimes in terms of both physical understanding and numerical modeling. To fill the gap, speaker will suggest a new study subject of meso- and micro-meteorology.

연세대학교 대기과학과 세미나

Climate models robustly imply that some significant change in precipitation patterns will occur. Models consistently project that the intensity of individual precipitation events increases by approximately 6%–7% K−1, following the increase in atmospheric water content, but that total precipitation increases by a lesser amount (1%–2% K−1 in the global average in transient runs). Some other aspect of precipitation events must then change to compensate for this difference. The authors develop a new methodology for identifying individual rainstorms and studying their physical characteristics—including starting location, intensity, spatial extent, duration, and trajectory—that allows identifying that compensating mechanism. This technique is applied to precipitation over the contiguous United States from both radar-based data products and high-resolution model runs simulating 80 years of business-as-usual warming. In the model study the dominant compensating mechanism is a reduction of storm size. In summer, rainstorms become more intense but smaller; in winter, rainstorm shrinkage still dominates, but storms also become less numerous and shorter duration. These results imply that flood impacts from climate change will be less severe than would be expected from changes in precipitation intensity alone. However, these projected changes are smaller than model–observation biases, implying that the best means of incorporating them into impact assessments is via “data-driven simulations” that apply model-projected changes to observational data. The authors therefore develop a simulation algorithm that statistically describes model changes in precipitation characteristics and adjusts data accordingly, and they show that, especially for summertime precipitation, it outperforms simulation approaches that do not include spatial information.

서울대학교 지구환경과학부 세미나

Western boundary currents (WBCs) are swift and narrow oceanic currents found in major ocean gyres caused by latitudinal gradients in the effect of Earth’s rotation. The WBCs flow poleward along the western boundary of the ocean basins transporting heat, salt, and nutrients from low to high latitudes. In particular, the subtropical WBCs are associated with the largest supply of heat and moisture into the atmosphere, and coupled to storm tracks in the North Pacific. Importance of these currents are, however, still poorly quantified and international community has been trying to estimate the mass and heat balances and possible changes associated with greenhouse-gas-induced climate change. This talk reviews previous and ongoing efforts to understand the WBCs variability in the North Pacific including the North Equatorial Current, Kuroshio, and Kuroshio Extension.

서울대학교 지구환경과학부 세미나

수치예보 모델이 일기예보에서 차지하는 비중은 과거에 비하여 점차 증가하고 있다. 과거에는 수치예보가 일부 전문가 집단에서만 다루어졌다면, 최근에는 언론을 통하여 대중에게도 수퍼컴퓨터와 이를 이용한 수치예보가 많이 알려지고 있다. 하지만 수치예보의 기본 개념을 명확히 설명하는 경우는 많지 않다. 본 강연에서는 수치예보의 기본을 제시하고 모델 내부의 역학과정과 물리과정을 설명한다. 또한 분광 모델의 개념과 그 특징을 설명하며 장, 단점을 살펴보고자 한다. 수치예보 모델의 사용 분야는 강수, 기온과 같은 날씨 요소의 예측이 큰 비중을 차지하지만, 이 외에도 다양한 대기 중의 현상을 재현하거나 예측하는 데에도 사용된다. 본 강연에서는 안정한 물 동위원소를 모델에 구현하여 고기후 재현, 수분 추적, 방사능 물질 추적에 사용되는 응용 분야를 소개하고자 한다. 안정한 물 동위원소에는 강수와 증발과 같은 대기 현상의 정보가 남아 있으며, 수 천년에서 수 만전 전까지의 자료를 확보할 수 있는 장점이 있기 때문에 이를 이용하여 제한적이나마 대기 상황을 재현할 수 있다. 또한 특정 동위원소 사이의 비율은 공기 덩어리 내부에서 동일하게 유지되기 때문에 이를 통해 공기의 이동을 추적할 수 있다. 이를 이용하여 대기 중 수분을 추적하여 강수 사례 분석이나 기후 변화를 연구 할 수 있으며, 방사능 물질의 이동을 추적하고 지면에 침적되는 정도를 계산하여 환경 영향 평가의 목적으로 활용 될 수 있다.

서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

주요한 온실가스 중 하나인 아산화질소는 농경지 밭토양에서 주로 발생되며 화학비료 시비와 밀접한 관련이 있다. 이런 아산화질소의 발생을 저감하기 위한 관리 방법 중 하나로 바이오차르 토양 투입이 고려되고 있는데, 바이오차르 투입에 따른 아산화질소 발생 저감 효과는 토양마다 바이오차르 종류마다 다르다고 보고되고 있는 실정이다. 이 발표에서는 바이오차르의 투입에 따른 농경지 토양에서의 아산화질소 발생 변화를 3 개 야외 실험지에서 확인하고 이 현상을 이해하기 위한 메커니즘에 대한 고찰을 진행하고자 한다.

서울대학교 지구환경과학부 세미나

Geochemical monitoring of groundwater in seismically-active regions has been carried out since 1970s. Precursors were well documented, but often criticized for anecdotal or fragmentary signals, and for lacking a clear physico-chemical explanation for these anomalies. Here we report – as potential seismic precursor – oxygen isotopic ratio anomalies of +0.24‰ relative to the local background measured in groundwater, a few months before the Tottori earthquake (M 6.6) in Southwest Japan. Samples were deep groundwater located 5 km west of the epicenter, packed in bottles and distributed as drinking water between September 2015 and July 2017, a time frame which covers the pre- and post-event. Small but substantial increase of 0.07‰ was observed soon after the earthquake. Laboratory crushing experiments of aquifer rock aimed to simulating rock deformation under strain and tensile stresses were carried out. Measured helium degassing from the rock and 18O-shift suggest that the co-seismic oxygen anomalies are directly related to volumetric strain changes. The findings provide a plausible physico-chemical basis to explain geochemical anomalies in water and may be useful in future earthquake prediction research.

서울대학교 지구환경과학부 세미나

It is quite recent that the deep learning algorithms have been widely applied to fields including computer vision, speech recognition, natural language processing, audio recognition, machine translation, etc. As one of first attempts to apply the deep learning algorithms for climate predictions, the ENSO prediction using convolutional neural network (CNN) will be presented in this talk. First, the performance of the CNN model is tested in the perfect model framework, and the forecast experiments confirm the CNN guarantees the skillful ENSO forecast than that using the simple linear model (i.e. multiple linear regression (MLR) model). The correlation skill of the Nino3.4 index using CNN model is 0.69 for 8-month lead forecast, which shows systematically superior than that using the MLR model (i.e. less than 0.4). It is also found that the CNN model generally shows better performance in predicting El Niño whose time-evolution is quite different from that in the typical El Niño. The CNN model is also applied to predict real ENSO cases during 1981-2010. Due to the limitation in the number of samples in the observation, we utilized long-term integrations of the multiple numbers of AOGCM participated in CMIP5 to train the CNN model. The comparison of the forecast skills of the CNN models with the dynamical models participated in multi-model prediction projects (i.e. NMME), the CNN model exhibits comparable forecast skills with the NMME for 8-month lead forecast. However, it is found that the CNN model has ability to predict real ENSO events up to 17-month lead forecasts to some extent. The possible interpretation for this long-term prediction using the CNN model will be partly given in this talk.

서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

국제단위계(SI)는 과학적 측정에 기반한 국제적으로 일관되고 합의된 측정단위의 체계이다. 측정의 결과는 양의 값(value of a quantity)과 측정불확도(measurement uncertainty), 측정 단위(measurement unit)의 세 가지가 모두 갖추어져야 완전한 의미를 나타내게 된다. 과학적인 측정량(measurand)은 최종적으로 국제단위계의 7개의 기본단위, 미터(meter, 기호: m), 킬로그램(kilogram, 기호: kg), 초(second, 기호: s), 암페어(ampere, 기호: A), 켈빈(kelvin, 기호: K), 몰(mole, 기호: mol), 칸델라(candela, 기호: cd)로 표현될 수 있다. 몰(mole)은 7개의 기본단위 중에서 물질의 양(amount of substance)에 대한 단위이다. 현재 몰의 정의는 탄소- 12의 질량을 바탕으로 정의되어 있기 때문에 질량의 정의와 직접 연결되어 있다. 하지만, 현재 질량 단위는 약속으로 정한 인공물인 “국제 킬로그램 원기(IPK)”를 사용하여 정의되어 있기 때문에 변동 가능성을 포함한 여러 가지 문제점을 안고 있으며 몰 역시 이런 문제점들의 영향을 받을 수 있다. 또한, 몰은 물질의 양으로써 구성요소의 수에 대한 개념을 포함하고 있기 때문에 주로 사용되는 분야인 화학과 생물학에서는 몰이 아보가드로 상수와 밀접하게 관련되어 인식되고 있다. 하지만, 몰의 정의에서는 이에 대한 명시적인 기술이 없이 탄소-12의 질량만으로 정의가 이루어져 있음으로 인해 혼란을 줄 수 있다. 따라서, 국제도량형위원회(CIPM), 국제도량형국(BIPM) 등의 국제기구 및 각 나라의 국가측정표준기관(NMI)에서는 국제단위계의 재정의를 진행하였으며 몰도 새롭게 재정의를 하게 되었다. 본 세미나에서는 2019년부터 시행될 새로운 국제단위계에서 몰의 재정의를 위한 방법 중 하나인 X-ray crystal density 실험에 의한 아보가드로 상수의 정확한 결정 연구에 대해 소개한다.

서울대학교 지구환경과학부 세미나
서울대학교 지구환경과학부 세미나

서울대학교 지구환경과학부 세미나

Several atmospheric teleconnetion patterns such as Pacific-Japan pattern, Circumlglobal Teleconnection, and Eurasian pattern, which affect Korean Peninsula in summertime, have been known over recent decades. Several studies about the teleconnection patterns are discussed. Also, these teleconnection patterns can be applied to seasonal prediction even though the patterns are not stationary in a changing climate. Seasonal prediction skills of the intensity of the Northeast Asian summer monsoon are relatively low using state-of-the-art general circulation models. It is introduced that seasonal-mean precipitation in Northeast Asian region are strongly correlated to western North Pacific (WNP) subtropical High variability in summertime. The relationship between Northeast Asian and WNP summer monsoons keeps with a high correlation coefficient over the entire period. Many general circulation models fail to predict the Northeast Asian summer precipitation but can well capture and predict interannual variability of the western North Pacific subtropical High, which dominates climate anomalies in the western North Pacific-East Asian region. Besides, interannual variability of the Northeast Asian summer monsoon is highly correlated to the WNP subtropical High variability. Based on this relationship, we suggest a seasonal prediction model using several coupled general circulation models and canonical correlation analysis for Northeast Asian summer precipitation anomalies in this study. This methodology provides with considerable benefit of seasonal prediction skills for Northeast Asian summer rainfall anomalies.