적도 해수면 온도 변동성에 의해 조절되는 성층권 돌연승온 및 그에 따른 예측성

학위논문 (박사) -- 서울대학교 대학원 : 자연과학대학 지구환경과학부, 2021. 2. 손석우.This study examines that the role of the El Niño/Southern Oscillation (ENSO) in the stratospheric sudden warming (SSW) events in both the Northern Hemisphere (NH) and the Southern Hemisphere (SH). The NH SSW events occur favorably only during El Niño winters, not La Niña winters, which is a contrast result from the literature. The SSW frequency during La Niña winters is quite dependent on the choice of SSW definitions, and this has been changed on the decadal timescale. The nonstationary relationship during La Niña winters is not the result of satellite data assimilation or interannual-to-decadal variability, such as Pacific decadal oscillation. It could result from SSW or ENSO internal variability. The SH SSW events, which are detected only twice in the historical records from 1958, can also be modulated by ENSO. Especially, the second SH SSW event in 2019 is under dramatic interannual conditions such as a record-breaking Indian ocean dipole (IOD), easterly phase of quasi-biennial oscillation (QBO), and central Pacific El Niño. Among those interannual variabilities, ENSO is the strongest driver for the 2019 SH SSW event in the cold-SST model experiments. In these experiments, we reduce or remove the anomalous sea surface temperature or atmospheric initial conditions from the observational data, especially over the IOD, QBO, and ENSO locations. The central Pacific El Niño amplifies the tropospheric wave activity with enhanced South Pacific high anomalies, explained by the teleconnection modulation. The above results in both the NH and SH SSW events suggest that the ENSO is one of the key drivers to modulate the stratospheric extreme. This relationship is applied to the subseasonal-to-seasonal (S2S) prediction of SSW events in the NH, aiming to improve the stratospheric and tropospheric prediction skills. However, this study with Global/Regional Integrated Model systems cannot find any significant improvement in the prediction skills with respect to the different ENSO phases. It is not found in other interannual variabilities, such as QBO or SSW type. As a result, this study finds that the key role of ENSO in both the NH and SH SSW event. The possibility of SSW events in both hemispheres is increased during El Niño winters. However, this relationship does not hold for the SSW prediction in the S2S timescale.본 연구에서는 남반구 및 북반구에서 발생한 성층권 돌연승온 사례가 엘니뇨 남방진동에 어떤 영향을 받는지 살펴보았다. 먼저 북반구 돌연승온의 경우 기존 연구와는 달리 엘니뇨 겨울철에만 돌연승온 발생빈도가 증가하는 것을 확인할 수 있었다. 기존 연구 결과는 특정 돌연승온 정의에 대해서만 라니냐 겨울철에도 돌연승온이 증가했기 때문인 것으로 확인되었다. 또한 라니냐 겨울철 시기의 돌연승온의 경우 정의에 대한 민감도뿐만 아니라 시기에 대해서도 민감함을 보였다. 특히, 약 20년 주기의 장기변동성을 보이는 등 돌연승온과 일관된 관계를 보이지 못하였다. 남반구 돌연승온은 역대 2번만 관측될 정도로 매우 드문 현상에 해당한다. 특히 2019년 9월 가장 강한 돌연승온이 확인되면서, 해당 돌연승온이 왜 발생했는지를 확인하고자 하였다. 계절내-계절 시간규모 예측 모형에서 성층권 초기장 및 해수면 온도를 바꿔주어 주요 인자에 대해 논의하였다. 결과적으로 적도 태평양 해수면 온도(혹은 엘니뇨/남방진동)를 낮췄을 때 유의미한 돌연승온 예측성능 차이를 보였다. 추가적으로 북반구 돌연승온 사례 예측성능 검정을 통해 돌연승온 예측성능을 검정하고, 이러한 예측성능이 준2년주기진동 혹은 엘니뇨/남방진동 등 경년변동성에 영향을 받는지 확인하였다. 경년변동성 차이에 따른 유의미한 예측성능 차이는 확인할 수 없었으며, 이는 엘니뇨/남방진동 위상 차이는 돌연승온 발생에 영향을 주지만 예측성능에는 영향을 못 미쳤음을 시사한다.1. Introduction 1 2. Data, methods, and model 8 2.1. Data 8 2.2. SSW, ENSO, PDO, and QBO detections 10 2.3. GRIMs description 18 2.4. Experimental design 20 2.5. Evaluation metrics 24 2.6. S2S operational models 29 3. Role of ENSO in the NH SSW event 30 3.1. SSW frequency for ENSO phase 30 3.2. Sensitivity test for the ENSO-SSW relationship 42 3.3. Decadal changes in the ENSO-SSW relationship 44 4. Role of ENSO in the SH SSW event 53 4.1. 2019 SH SSW event 53 4.2. Key driver of the 2019 SH SSW event 59 5. Role of ENSO in the NH SSW predictability 71 5.1. Model mean biases of GRIMs 71 5.2. SSW prediction 75 6. Summary and discussion 97 References 102 국문초록 112 감사의 글 114Docto

파라메트릭 롤 평가를 위한 단계별 해석기법 개발 및 실선 적용

학위논문 (박사)-- 서울대학교 대학원 : 조선해양공학과, 2013. 2. 김용환.A multi-level approach for the quantitative evaluation of parametric roll is developed and its application results to modern commercial ships are presented and reviewed in this thesis. Despite many previous studies, it is still a challenging issue to quantify the vulnerability of a ship to parametric roll occurrence on sea-going vessels. Therefore, occurrence mechanism and physical and stochastic characteristics of parametric roll are investigated in depth and this thesis presents a new numerical form for quantitatively analyzing the susceptibility of a ship to parametric roll in random sea ways based on the study. Due to the non-ergodic characteristics of parametric roll motion, numerous direct time-domain simulations are needed to obtain a stable long-term distribution of parametric rolls. To avoid such heavy computational demand and to accelerate numerical simulations, a 1.5-DOF computational model for parametric roll prediction is developed for both regular- and irregular-wave excitations, adopting the approximated GZ curve. In this model, the concept of transfer functions is introduced for the mean and the first harmonic component of GM, which is introduced to approximate GM fluctuation. The simulation results obtained by using this model are compared to those of a three-dimensional weakly nonlinear simulation program. The sensitivities of the simulation results to the initial value, time window and number of simulations are investigated by applying Monte-Carlo simulation, and their proper values are proposed. This thesis also introduces several numerical approaches to analyze parametric roll from simple Mathieu equation to advanced numerical tools based on IRF method and Rankine panel method. Using advantages of various approaches including 1.5-DOF GM-GZ approximation method, a multi-level approach is developed to evaluate the vulnerability of a ship to parametric roll quantitatively. It consists of 1st level check by Mathieu equation, 2nd level check for regular wave, 3rd level check for irregular wave and operational guidance by IRF method. Most advanced tool based on Rankine panel method is used to verify each numerical tool. This multi-level approach is applied to several modern commercial ships including 4 post-Panamax container ships, 3 PCTCs (Pure Car and Truck Carrier), 3 Passenger ships, VLCC and S175 based on North Atlantic wave data. Based on the simulation results, the vulnerability of each ship to parametric roll is evaluated, and the influences of still water GM, roll damping coefficient and ship speed are reviewed and discussed. It is confirmed that each level provides consistent results and the results give very useful information of the vulnerability to parametric roll for each ship type and ship length. As a powerful and effective countermeasure against parametric roll, operational guidance is very important to support crews decision making in harsh environmental condition. In this thesis, the procedure for operational guidance development based on IRF method is proposed and it is applied to 8,000 TEU container ship. All the application results are provided and summarized with easy instruction to help ship crews efficient decision making and to avoid severe parametric roll. The contribution of this study on ship design, dynamic stability criteria are discussed and future works to reinforce the current approach and numerical scheme are proposedChapter 1. Introduction 1 1.1 Background 1 1.2 State of the arts 6 1.3 Objective and work of scope 10 1.4 Outline of thesis 11 Chapter 2. Characteristics of Parametric Roll 14 2.1 Occurrence mechanism of parametric roll 14 2.1.1 Basic mechanism 14 2.1.2 Ship motion and wave effects 18 2.2 Physical characteristic 18 2.2.1 Initiation stage 19 2.2.2 Development and steady state 21 2.2.3 Roll damping effect 22 2.2.4 GM fluctuation 24 2.2.5 GZ fluctuation 36 2.3 Stochastic characteristics 38 2.3.1 Non-ergocity 38 2.3.2 Occurrence in irregular seas 45 2.3.3 Probability density function 50 Chapter 3. Theoretical Background 55 3.1 1-DOF Mathieu equation 55 3.2 1.5- DOF GM-GZ approximation 58 3.2.1 GZ approximation 59 3.2.2 Roll motion equation 65 3.3 Impulse response function approach 66 3.4 3D Rankine panel method 70 3.5 Comparison of different approaches 73 Chapter 4. Multi-level Approach for Evaluation of Parametric Roll 76 4.1 General 76 4.2 Environmental conditions. 77 4.3 Development of multi-level approaches 82 4.3.1 Multi-level approach 82 4.3.2 1st level (resonance check) 84 4.3.3 2nd level (regular wave check) 86 4.3.4 3rd level (Irregular wave check) 86 4.3.5 Operational guidance 87 4.4 Comparison with ABS Guide 89 Chapter 5. Application to Real Ships 91 5.1 Sample ship selection 91 5.2 Multi-level vulnerability check 92 5.2.1 RAO of and 92 5.2.1 1st level (resonance check) 96 5.2.2 2nd level (regular wave check) 98 5.2.3 3rd level (Irregular wave check) 101 5.4 Operational Guidance for 8,800 TEU Container ships 104 5.4.1 GM=2.5 m 107 5.4.2 GM=1.5 m 108 5.4.3 GM=0.83 m 109 5.4.4 Review and summary of operational guidance 111 Chapter 6. Conclusions 116 Chapter 7. Discussions 120 Bibliography 122Docto

성층권 돌연승온 예측성

학위논문 (석사)-- 서울대학교 대학원 : 지구환경과학부, 2016. 2. 손석우.The predictability of sudden stratospheric warming (SSW) events, such as those that occurred on January 5, 2004 and January 7, 2013, are examined by integrating an operational weather prediction model. The model is initialized from 25 days before the onset of a SSW event to 20 days after, and integrated for 30 days. The prediction skill is then evaluated by computing both mean squared error and anomaly correlation coefficient with respect to the reanalysis data. The 2004 SSW event, which was characterized as a vortex displacement event, was preconditioned by the vertical propagation of a wavenumber-one wave in the lower stratosphere, while the 2013 SSW event, a vortex split event, was driven by the amplification of wavenumber-one followed by wavenumber-two waves. These two different events were reliably predicted with a maximum lead-time of nine days and seven days, respectively. However, their nature is quite different. For the 2004 displacement SSW event, zonal-mean errors accounted for a great part of the forecast errors, whereas the forecast errors of the 2013 split SSW event were highly influenced by the zonally asymmetric components. Comparatively a large magnitude of zonally asymmetric errors in the split SSW were mainly caused by an underestimated amplification of zonal-wavenumber two waves in the lower stratosphere. This result indicates that the predictability of SSW events may be dependent on the morphology of the polar vortex. To confirm this finding, the five most recent displacement and split SSW events were further investigated in this paper. On average, the model shows a better prediction skill for the displacement SSW events than the split SSW events with about one-day longer lead-time. Moreover, all displacement events show a smaller ratio of zonally asymmetric errors to zonal-mean errors than the split events. In addition, underestimated-vertical propagation of wavenumber two waves explains a large zonally asymmetric errors in all split cases.1. Introduction 1 2. Data and model configuration 5 3. Methods 6 3.1 Sudden stratospheric warming 6 3.2 Evaluation metrics 8 3.3 Eliassen-Palm flux analysis 11 4. Results 11 4.1 A case study 13 4.2 Multi-case study 25 5. Conclusions and discussions 30 Reference 33 초록 37Maste

Impact of Sudden Stratospheric Warming on the Surface Air Temperature in East Asia

The sudden stratospheric warming (SSW), which is characterized by an abrupt increase of polar stratospheric temperature by several tens of degrees in a week, has been known to affect tropospheric weather and climate on sub-seasonal time scale in the boreal winter. Such downward coupling has been often examined in North Atlantic and Europe, but rarely examined in East Asia. In this study, by applying the two definitions of SSW to the reanalysis data, the possible impacts of the SSW events on the surface air temperature (SAT) and tropospheric circulation in East Asia are analyzed. It is found that Eurasian continent, including Siberia and the Northeast Asia, tends to experience anomalously cold SAT for up to sixty days after the SSW events. The resulting SAT anomalies largely resemble those associated with negative Artic Oscillation. However, over East Asia, SSW-related SAT change is weak and not statistically significant. Only during the extreme SSW events when the downward coupling between the stratosphere and troposphere is strong, East Asia exhibits significantly cold SAT anomalies. This relationship is presented by grouping SSW events into those followed by cold SAT anomalies over East Asia and those by warm anomalies for varying threshold values of the SSW events.N

Decadal Changes in the Relationship between Arctic Oscillation and Surface Air Temperature over Korea

The relationship between the Arctic Oscillation (AO) and surface air temperature (SAT) over Korea is re-examined using the long-term observation and reanalysis datasets for the period of December 1958 to February 2020. Over the entire period, Korean SAT is positively correlated with the AO index with a statistically significant correlation coefficient, greater than 0.4, only in the boreal winter. It is found that this correlation is not static but changes on the decadal time scale. While the 15-year moving correlations are as high as 0.6 in 1980s and 1990s, they are smaller than 0.3 in the other decades. It is revealed that this decadal variation is partly due to the AO structure change over the North Pacific. In the period of 1980s-1990s, the AO-related sea level pressure fluctuation is strong and well defined over the western North Pacific and the related temperature advection effectively changes the winter SAT over Korea. hi the other periods, the AO-related circulation anomaly is either weak or mostly confined within the central North Pacific. This result suggests that Korean SAT-AO index relationship, which becomes insignificant in recent decades is highly dependent on mean flow change in the North Pacific.N

Long-term Trend Analysis of Extreme Temperatures in East Asia Using Quantile Regression

This study explores the long‐term trends of extreme temperatures of 270 observation stations in East Asia (China, Japan, and Korea) for 1961‐2013. The 5th percentile of daily minimum temperatures (TN05%) and 95th percentile of daily maximum temperatures (TX95%), derived from the quantile regression, are particularly examined in term of their linear and nonlinear trends. The warming trends of TN05% are typically stronger than those of TX95% with more significant trends in winter than in summer for most stations. In both seasons, warming trends of TN05% tend to amplify with latitudes. The nonlinear trends, quantified by the 2nd‐order polynomial fitting, exhibit different structures with seasons. While summer TN05% and TX95% were accelerated in time, winter TN05% underwent weakening of warming since the 2000s. These results suggest that extreme temperature trends in East Asia are not homogeneous in time and space.N

Assessment of stratospheric prediction skill of the GloSea5 hindcast experiment

This study explores the 6-month lead prediction skill of stratospheric temperature and circulations in the Global Seasonal forecasting model version 5 (GloSea5) hindcast experiment over the period of 1996 similar to 2009. Both the tropical and extratropical circulations are considered by analyzing the Quasi-Biennial Oscillation (QBO) and Northern Hemisphere Polar Vortex (NHPV). Their prediction skills are quantitatively evaluated by computing the Anomaly Correlation Coefficient (ACC) and Mean Squared Skill Score (MSSS), and compared with those of El Nino-Southern Oscillation (ENSO) and Arctic Oscillation (AO). Stratospheric temperature is generally better predicted than tropospheric temperature. Such improved prediction skill, however, rapidly disappears in a month, and a reliable prediction skill is observed only in the tropics, indicating a higher prediction skill in the tropics than in the extratropics. Consistent with this finding, QBO is well predicted more than 6 months in advance. Its prediction skill is significant in all seasons although a relatively low prediction skill appears in the spring when QBO phase transition often takes place. This seasonality is qualitatively similar to the spring barrier of ENSO prediction skill. In contrast, NHPV exhibits no prediction skill beyond one month as in AO prediction skill. In terms of MSSS, both QBO and NHPV are better predicted than their counterparts in the troposphere, i.e., ENSO and AO, indicating that the GloSea5 has a higher prediction skill in the stratosphere than in the troposphere.N

Prediction skill of GloSea5 model for stratospheric polar vortex intensification events

본 연구에서는 한국기상청의 장기예측시스템 현업모형인 GloSea5의 성층권 극소용돌이 강화사례에 대한 예측성 을 진단 및 검증하였다. 진단에 사용된 통계량은 이상상관계수(ACC, Anomaly Correlation Coefficient)와 평균제곱근 예측성(MSSS, Mean Squared Skill Score)으로, 1991-2010년간 발생한 14개 극소용돌이 강화사례에 대한 GloSea5의 예 측성한계는 ACC를 기준으로 13.6일, MSSS를 기준으로 18.5일로 나타났다. 모형의 평균제곱오차(MSE, Mean Squared Error)의 각 성분을 정량적으로 비교분석한 결과, 예측성을 저하시키는 가장 큰 요인은 맴돌이(에디)오차로, 그 중 에디의 위상오차가 전체 예측오차의 큰 부분을 차지하는 것으로 나타났다. 또한 극소용돌이 현상이 수평적으로 큰 규모를 가지는 만큼 동서파수 1의 에디와 관련한 오차가 더 작은 규모의 에디에 비해 가장 크게 예측오차에 기여하는 것으로 나타났다. 한편, 분석한 사례들에 대하여 GloSea5의 대류권 순환에 대한 예측성은 성층권 예측성과는 큰 관련이 없는 것으로 나타났다. 이는 단순히 GloSea5 모형이 성층권-대류권 접합과정을 잘 모의하지 못하기 때문에 나타난 결과로 유추할 수 있다. 하지만, 극소용돌이 강화에 의한 영향에 비해 대류권에서 내부변동성의 절대적인 크기가 종종 크게 나 타난다는 점을 감안하면, 모형에서 성층권-대류권 접합을 잘 모의하고 있더라도 극소용돌이 강화 자체만의 영향이 뚜렷 하게 나타나지 않았을 가능성 또한 간과하면 안 될 것이다. This study evaluates the prediction skills of stratospheric polar vortex intensification events (VIEs) in Global Seasonal Forecasting System (GloSea5) model, an operational subseasonal-to-seasonal (S2S) prediction model of Korea Meteorological Administration (KIVIA). The results show that the prediction limits of VIEs, diagnosed with anomaly correlation coefficient (ACC) and mean squared skill score (MSSS), are 13.6 days and 18.5 days, respectively. These prediction limits are mainly determined by the eddy error, especially the large-scale eddy phase error from the eddies with the zonal wavenumber 1. This might imply that better prediction skills for VIEs can be obtained by improving the model performance in simulating the phase of planetary scale eddy. The stratospheric prediction skills, on the other hand, tend to not affect the tropospheric prediction skills in the analyzed cases. This result may indicate that stratosphere-troposphere dynamic coupling associated with VIEs might not be well predicted by GloSea5 model. However, it is possible that the coupling process, even if well predicted by the model, cannot be recognized by monotonic analyses, because intrinsic modes in the troposphere often have larger variability compared to the stratospheric impact.N

Linear and Nonlinear Trends of Extreme Temperatures in Korea

This study explores the long-term trends of surface air temperatures in 11 KMAstations over the period of 1960~2012. Both linear and nonlinear trends are examined for the95th, 50th, and 5th percentiles of daily maximum (Tmax) and minimum temperatures (Tmin) byusing quantile regression method. It is found that in most stations linear trends of Tmax and Tminare generally stronger in winter than in summer, and warming trend of the 5th percentile temperature(cold extreme) is stronger than that of the 95th percentile temperature (warm extreme)in both seasons. The nonlinear trends, which are evaluated by the second order polynomial fitting,show a strong nonlinearity in winter. Specifically, winter temperatures have increaseduntil 2000s but slightly decreased afterward in all percentiles. This contrasts with the 95th and50th percentiles of summer Tmin that show a decreasing trend until 1980s then an increasingtrend. While this result is consistent with a seasonal dependence of the recent global warminghiatus, most of the nonlinear trends are statistically insignificant, making a quantitative attributionof nonlinear temperature trends challenging.N

Subseasonal-to-seasonal (S2S) prediction skills of GloSea5 model: Part 1. Geopotential height in the northern hemisphere extratropics

This study explores the Subseasonal-to-Seasonal (S2S) prediction skills of the Northern Hemisphere mid-latitude geopotential height in the Global Seasonal forecasting model version 5 (GloSea5) hindcast experiment. The prediction skills are quantitatively verified for the period of 1991 similar to 2010 by computing the Anomaly Correlation Coefficient (ACC) and Mean Square Skill Score (MSSS). GloSea5 model shows a higher prediction skill in winter than in summer at most levels regardless of verification methods. Quantitatively, the prediction limit diagnosed with ACC skill of 500 hPa geopotential height, averaged over 30 degrees N similar to 90 degrees N, is 11.0 days in winter, but only 9.1 days in summer. These prediction limits are primarily set by the planetary-scale eddy phase errors. The stratospheric prediction skills are typically higher than the tropospheric skills except in the summer upper-stratosphere where prediction skills are substantially lower than upper-troposphere. The lack of the summer upper-stratospheric prediction skill is caused by zonal mean error, perhaps strongly related to model mean bias in the stratosphere.N