Verification of Forecast Performance of a Rapid Refresh Wave Model Based on Wind–Wave Interaction Effect

In this study, we constructed a rapid refresh wave forecast model using sea winds from the Korea Local Analysis and Prediction System as input forcing data. The model evaluated the changes in forecast performance considering the influence of input wind–wave interaction, which is an important factor that determines forecast performance. The forecast performance was evaluated by comparing the forecast results of the wave model with the significant wave height, wave period, and wave direction provided by moored buoy observations. During the typhoon season, the model tended to underestimate the conditions, and the root mean square error (RMSE) was reduced by increasing the wind and wave interaction parameter. The best value of the interaction parameter that minimizes the RMSE was determined based on the results of the numerical experiments performed during the typhoon season. The forecast error in the typhoon season was higher than that observed in the analysis results of the non-typhoon season. This can be attributed to the variations of the wave energy caused by the relatively strong typhoon wind field considered in the wave model.

A new approach for location-specific seasonal outlooks of typhoon and super typhoon frequency across the Western North Pacific region

With an average of 26 tropical cyclones (TCs) per year, the western North Pacific (WNP) is the most active TC basin in the world. Considerable exposure lies in the coastal regions of the WNP, which extends from Japan in the north to the Philippines in the south, amplifying TC related impacts, including loss of life and damage to property, infrastructure and environment. This study presents a new location-specific typhoon (TY) and super typhoon (STY) outlook for the WNP basin and subregions, including China, Hong Kong, Japan, Korea, Philippines, Thailand, and Vietnam. Using multivariate Poisson regression and considering up to nine modes of ocean-atmospheric variability and teleconnection patterns that influence WNP TC behaviour, thousands of possible predictor model combinations are compared using an automated variable selection procedure. For each location, skillful TY and STY outlooks are generated up to 6 months before the start of the typhoon season, with rolling monthly updates enabling refinement of predicted TY and STY frequency. This unparalleled lead time allows end-users to make more informed decisions before and during the typhoon season.

A Report of the Questionnaire Survey on Awareness of COVID-19 and Shelters

The 2019 novel coronavirus, or COVID-19, has rapidly spread across the world, and has since become not only a health problem but also a socioeconomic problem. In disaster-prone countries like Japan, there is substantial concern about the occurrence of natural disasters during the COVID-19 pandemic. When a natural disaster occurs, many people evacuate to shelters, and an outbreak of infectious disease often happens in shelters. In this situation, we should consider the management of evacuation shelters under this pandemic situation. Many types of research have investigated infectious diseases in shelters after a natural disaster. However, these are about infectious diseases after a disaster, and not about the occurrence of natural disasters under a pandemic like the ongoing COVID-19 pandemic. Moreover, such researches mainly focus on medical teams or local governments who respond to infectious diseases. Based on the above mentioned information, the purpose of this study is to clarify (i) the change in citizens’ awareness of risk influenced by the spread of COVID-19 and (ii) the measurement and decision necessary for evacuating people under COVID-19. We administered a questionnaire to Japanese citizens from April to May 2020, and found that majority of people do not want to have evacuation life under COVID-19, and this tendency is larger among people who live together with the aged and the care-needed. In addition, this research confirmed the risk awareness structure applicable to both COVID-19 and natural disasters. The results show that many people firmly ask governments to take measures against infectious disease such as using hotels as shelters, securing personal spaces in shelters, storing masks and disinfectors, and so on during an outbreak of an infectious disease. However, it is not enough to take such measures only; it is necessary to provide accurate information and mental support to reduce anxiety in people. Based on these results, we should consider more about the management of shelters under COVID-19 as soon as possible before the arrival of typhoon season.

Plant debris are hotbeds for pathogenic bacteria on recreational sandy beaches

On recreational sandy beaches, there are guidelines for the management of bacterial pollution in coastal waters regarding untreated sewage, urban wastewater, and industrial wastewater. However, terrestrial plant debris on coastal beaches can be abundant especially after floods and whilst it has rarely been considered a concern, the bacterial population associated with this type of pollution from the viewpoint of public health has not been adequately assessed. In this study, microbes associated with plant debris drifting onto Kizaki Beach in Japan were monitored for 8 months throughout the rainy season, summer, typhoon season, and winter. Here we show that faecal-indicator bacteria in the plant debris and sand under the debris were significantly higher than the number of faecal bacteria in the sand after a 2015 typhoon. When we focused on specific pathogenic bacteria, Brevundimonas vesicularis and Pseudomonas alcaligenes were commonly detected only in the plant debris and sand under the debris during the survey period. The prompt removal of plant debris would therefore help create safer beaches.

Assimilation of Himawari-8 imager radiance data with the WRF-3DVAR system for the prediction of Typhoon Soudelor

Himawari-8 is a next-generation geostationary meteorological satellite launched by the Japan Meteorological Agency. It carries the Advanced Himawari Imager (AHI) on board, which can continuously monitor high-impact weather events with high frequency in space and time. The assimilation of AHI radiance data was implemented with the three-dimensional variational data assimilation system (3DVAR) of the Weather Research and Forecasting Model for the analysis and prediction of Typhoon Soudelor (2015) in the Pacific typhoon season. The effective assimilation of AHI radiance data in improving the forecast of the tropical cyclone during its rapid intensification has been realized. The results show that, after assimilating the AHI radiance data under clear-sky conditions, the typhoon position in the background field of the model was effectively corrected compared with the control experiment without AHI radiance data assimilation. It is found that the assimilation of AHI radiance data is able to improve the analyses of the water vapor and wind in a typhoon's inner-core region. The analyses and forecasts of the minimum sea level pressure, the maximum surface wind, and the track of the typhoon are further improved.

Extreme Rainfall in Taiwan: Seasonal Statistics and Trends

Taiwan regularly experiences precipitation extremes of hundreds of millimeters per day, especially between May and September. In this study, Taiwan’s extreme rainfall (ER) is analyzed over a 56-year time period in different seasons and geographic regions, using a recently released, high-resolution gridded rainfall dataset. ER is defined using a seasonally- and geographically-varying 99th percentile threshold to better resolve the characteristics of the most intense rainfall seen in different locations and times of year. The resulting monthly ER rates are largest in typhoon season and smallest in fall, winter, and spring. ER is spatially homogeneous in Mei-Yu and typhoon seasons and concentrated in northern Taiwan in during the rest of the year.A trend analysis revealed a positive trend in island-mean ER for the winter, spring, and typhoon seasons. In winter and spring, these trends are most pronounced in the north. In Mei-Yu season, ER has increased most over the southwestern mountain slopes; and in typhoon season, ER has increased consistently over much of Taiwan. These changes often exceed 1% per year. In many areas, typhoon season accounts for the largest fraction of the observed annual ER trend. TCs produce most of the observed typhoon season ER and ER trend, with nearly half of the typhoon season ER trend being associated with increases in TC frequency and duration around central and northern Taiwan.Certain regional changes in ER characteristics, particularly in areas with low sample size or complex seasonal contributions, merit further investigation in future work.

Evaluation of IMERG Level-3 Products in Depicting the July to October Rainfall over Taiwan: Typhoon Versus Non-Typhoon

This study assesses the performance of satellite precipitation products (SPPs) from the latest version, V06B, Integrated Multi-satellitE Retrievals for Global Precipitation Mission (IMERG) Level-3 (including early, late, and final runs), in depicting the characteristics of typhoon season (July to October) rainfall over Taiwan within the period of 2000–2018. The early and late runs are near-real-time SPPs, while final run is post-real-time SPP adjusted by monthly rain gauge data. The latency of early, late, and final runs is approximately 4 h, 14 h, and 3.5 months, respectively, after the observation. Analyses focus on the seasonal mean, daily variation, and interannual variation of typhoon-related (TC) and non-typhoon-related (non-TC) rainfall. Using local rain-gauge observations as a reference for evaluation, our results show that all IMERG products capture the spatio-temporal variations of TC rainfall better than those of non-TC rainfall. Among SPPs, the final run performs better than the late run, which is slightly better than the early run for most of the features assessed for both TC and non-TC rainfall. Despite these differences, all IMERG products outperform the frequently used Tropical Rainfall Measuring Mission 3B42 v7 (TRMM7) for the illustration of the spatio-temporal characteristics of TC rainfall in Taiwan. In contrast, for the non-TC rainfall, the final run performs notably better relative to TRMM7, while the early and late runs showed only slight improvement. These findings highlight the advantages and disadvantages of using IMERG products for studying or monitoring typhoon season rainfall in Taiwan.