7. Discussion and conclusion
7.2. Conclusion
The track forecast error of the WRF model for TCs over the western North Pacific was analyzed. A cluster analysis was applied based on TC location through the k-means clustering algorithm, and five clusters were classified. Since there were robust differences in the track error between clusters with dissimilar transition directions, we grouped the clusters into two groups: group 1 (C1 and C2) and group 2 (C3 and C4). The two groups had apparent differences in track errors. A higher number of large error cases were included in group 1 TCs, while more small error cases were included in group 2 TCs. To understand this regional difference in track error, we analyzed the optimal steering vectors. The steering vector difference between model and analysis data, which indicates the environmental wind error, was larger for group 1 TCs, especially at the late forecast lead time. This result corresponded to the rapidly increasing track error of group 1 TCs after 48 hours.
The pattern correlation of 500 hPa geopotential height, which is highly related to environmental wind, was also lower for group 1 TCs. This showed that the large track error for group 1 TCs arises from the unrealistic representation of environmental fields affected by the subtropical high or mid-latitude trough. The residuals, which contained the features not considered in the environmental wind, were also larger for group 1 TCs. The significant forecast error for group 1 TCs at the late forecast time can be attributed to the fact that the environmental wind error is larger for group 1 TCs. In addition, the more complicated processes of mid-latitude TCs are not adequately simulated.
The spectral nudging, which is not often used in TC forecast, is applied on real-time TC forecast. Case studies for Typhoons Neoguri and Vongfong showed that spectral nudging benefitted track forecasting by improving the simulation of large-scale wind and the subtropical high. To optimize the impact of spectral nudging on intensity forecast, the sensitivity of nudging options was tested. Sensitivity experiments showed that spectral nudging was optimized when the cut-off wavelength was increased and the nudging coefficient decreased. The spectral nudging generally improved the track forecast, especially as the forecast lead time increased. Spectral nudging improved the forecast of the large-scale fields which play an important role in steering TCs. Furthermore, our results showed that the effect of nudging depended on the locations of TCs.
The most effective method (Selective and Optimized Spectral Nudging, SOSN) was suggested to utilize the spectral nudging on real-time TC forecast.
12–4 km moving nesting experiments were conducted to understand the effect of increasing the model resolution on the five-day forecasts of TCs over the WNP. The 12 km single domain experiments did not properly reproduce the MWS, and strong wind speeds faster than 60 m s−1 were not captured using the 12 km horizontal grid spacing. However, the simulations of TC intensification improved in 12–4 km moving nesting experiments. The RMSEs of MWS and MSLP were reduced, and the wind-pressure relationship was improved in the 2DM experiment. The 1DM experiment tended to simulate rightward-biased TC tracks compared to the actual TC track in the track forecast. The tendency to deflect the TC track rightward decreased for intense TCs
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in the 2DM experiment, and was associated with an improved intensity forecast for the intense TCs. The reduction in cross-track bias was especially significant for the intense TCs located at lower latitudes. The TCs moving toward mid-latitudes seemed to be more affected by the prediction of the surrounding environmental field. The TCs in lower latitudes were more sensitive to the intensity forecast because the accuracy of the environmental field prediction was relatively high, and intensity forecasts were better than those for the mid- latitude TCs.
To understand the effect of air-sea interaction on TC forecast, the ocean model coupling and the initialization with high-resolution ocean reanalysis data were applied. For typhoon SOULIK, the initialization with HYCOM reanalysis reduced the sudden unrealistic intensification during the early forecast time. The cold wake was realistically simulated by the coupled model, and the ocean feedback improved the simulation of decaying process for the late forecast time. Therefore, the intensity forecast was improved in coastal regions.
An integrated TC forecast system based on the WRF model with the spectral nudging and moving nesting method was established for the real-time forecast of TCs over the western North Pacific. The integrated TC forecast system had the smallest track and intensity error among the experiments. The integrated TC forecast system had smaller or similar track error compared to GFS, which has advantages in the track forecasts. Its intensity error was comparable to that of HWRF, which has an advanced intensity forecast.
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Acknowledgement
학위과정을 마치면서 그 동안 큰 도움이 되었던 분들에게 이 글을 통하여 감사를 전합니다.
우선 학부 2학년 때부터 박사졸업까지 저를 지도해 주신 차동현 교수님께 가장 먼저 감사하다는 말씀 을 드립니다. 자상한 모습으로 차근차근 연구를 진행할 수 있도록 아낌없는 조언을 해 주신 덕분에 박 사학위를 마칠 수 있었습니다. 또한 연구에서 뿐만이 아니라 인생에서도 큰 배움이 있었습니다. 항상 긍정적인 모습을 보여주시는 교수님을 보면서 포기가 빨랐던 저는 포기하지 않고 도전하는 자세를 배 워 나갔습니다. 교수님께서 주신 가르침은 평생 잊지 않고 앞으로 자랑스러운 제자가 되도록 노력하 겠습니다.
또한 논문심사위원장으로 멋지고 날카로운 조언들로 논문의 완성도를 높여 주신 이명인 교수님, 수 업을 너무 잘하셔서 대기과학에 흥미를 느끼게 해주시고 연구의 의미를 다시 한번 생각하게 해 주신 강사라 교수님, 박사의 의미와 자세를 알려주시고 세심한 조언들을 해 주신 진천실 박사님, 코로나로 인해 직접 뵙진 못했지만 태풍에 대해 많은 가르침을 주신 문유민 박사님께 깊은 감사를 드립니다.
수많은 시간을 함께하고 항상 즐거운 연구실 생활을 하게 해준 연구실 멤버들에게도 감사의 인사를 전합니다. 우선 대운동장 회담으로부터 시작해 연구실 동기가 되고 이젠 가장 편한 친구들이 된 김가 영, 김진은에게 고맙다는 말을 전합니다. 연구실 초창기에 들어와서 힘든 서버관리와 행정일을 맡아 연구에 집중할 수 있게 해준 명우형, 사고뭉치 후배였지만 이젠 어엿한 예비 박사가 되어 조언을 해준 민규, 졸업준비 할 때 선배라고 서버 양보 많이 해준 동혁이, 부사수로 크고 작은 일들을 싫은 내색없 이 도와준 진영이에게 감사드립니다. 또한 맛있는 거 많이 사주시는 박창용 박사님, 코딩 많이 도와준 길형, 스몰토크로 다져진 커피 정회원 멤버 태형이형과 해린, 벌써 랩장이 되어버린 경민, 쓸데없는 소 리 해도 잘 받아주는 지원, 산책마니아 태호, 하드카운터 석우형, 사이보그 우진, 그리고 연구실의 미 래가 될 은지, 태훈, 하은 모두 즐겁게 연구실 생활해줘서 감사합니다.
그리고 피씨방에 살던 취준생이였지만 이젠 멋진 사회인이 된 민욱, 성훈, 준수, 병민 덕분에 즐겁게 대학생활 했습니다. 오티조 선후배로 만났지만 이젠 친구가 된 광영, 민혁, 대순, 원중, 한웅, 수연, 나영 외에도 모든 21조 후배들에게도 감사합니다. 제주도 갈때마다 반겨주는, 이름 거론하기 너무 많은 진목재 친구들과 승헌, 민해 모두 고맙고, 대학원 생활동안 많이 놀아준 규철, 승진이형도 감사합니다.
대학 첫 룸메였던 철구, 유일한 학교 밖 친구 현아 또한 감사합니다. 연구실 선배가 없던 저에게 연구실 선배 역할을 대신 해준 은교형 대현이형도 감사합니다.
마지막으로 제가 어떤 결정을 해도 항상 믿고 응원해주시는 부모님께 진심으로 감사드립니다. 앞으로 더 자랑스러운 아들이 되겠습니다. 거의 아들처럼 챙겨주는 누나와 멋진 매형, 하늘에서도 저를 자랑스러워하실 할아버지, 집에 갈때마다 항상 반겨 주시는 할머니에게도 큰 감사를 드리며 이 논문을 바칩니다.