Editorial for the special edition on Research on Extreme Weather Events that Occurred around East Asia in 2017 – 2021

Heavy rainfall disasters caused by mesoscale convective systems occur annually in East Asia. It is crucial to gain a thorough understanding of the physical processes and dynamic mechanisms behind these extreme rainfall events. This calls for the integration of various research methods, including observational studies, statistical data analysis, and weather forecasting with data assimilation. In this special edition, coordinated with Journal of the Meteorological Society of Japan (JMSJ) and Scientific Online Letters on the Atmosphere (SOLA), we have published six articles in JMSJ and nine articles in SOLA covering extreme events in East Asia from 2017 to 2021. Here is an overview of the six papers in JMSJ.

Sato et al. (2022) examined the relationship between winter heavy snowfall amount and meteorological and oceanographic conditions around western Hokkaido coastal regions. Through case studies of heavy snowfall events in 2020 and 2021, and statistical analysis using reanalysis datasets dating back to 1979, they discovered that major factors contributing to heavy snowfall include cold air temperature combined with strong wind speed anomalies over warm surface-layer oceans, which result in increased upward turbulent heat flux. Hatsuzuka et al. (2022) evaluated the performance of “immediate very-short-range forecast of precipitation” (VSRF) provided by the Japan Meteorological Agency (JMA), specifically for senjo-kousuitai events in Kyushu in 2019 and 2020. Their study demonstrated that the VSRH product is effective in forecasting areas with heavy rainfall of more than 80 mm over 3-hour accumulation period, up to 2-hours ahead. Tsuji and Takayabu (2023) conducted a unique analysis focusing on the hierarchical structure of the synoptic and sub-synoptic water vapor environment preceding a heavy rainfall event over Kyushu in 2020. Using moisture budget analysis with the MAUL condition (Takemi and Unuma 2020), their study revealed that the increase in precipitable water was due to moisture advection in the free troposphere nine hours before the heavy rainfall event. Following this moistening, wind convergence processes in both the boundary layer and the free troposphere became dominant, leading to intense precipitation. The MAUL condition was used to visualize the locations where significant moistening in the free troposphere occurred. Three papers demonstrated the high performance of recent numerical simulation or data assimilation techniques in investigating the predictability and dynamics of extreme events. Ito et al. (2024) conducted numerical simulations of tornadoes in a mini-supercell associated with Typhoon Tapah in 2019, revealing the mechanisms that determine the location of tornado occurrence within the mini-supercell. Kato et al. (2024) and Toyooka et al. (2024) explored the impacts of data assimilation on the precipitation forecasts. Kato et al. (2024) quantified the contribution of new observation (water vapor lidar) and a novel blending technique using a spatial maximum filter to tolerate forecast displacement errors for a senjo-kousuitai event in 2021. Toyooka et al. (2024) investigated three different methods, backward trajectory, ensemble-based linear sensitivity analysis, and observation system experiment with non-linear ensemble forecasting, to identify the most sensitive observation regions for improving torrential rainfall forecasts. The results from three methods showed good agreement.

To mitigate the damages caused by extreme weather events, it is essential to consolidate our research achievements and further enhance the predictability of these events.

References

•  Hatsuzuka,  D.,  R.  Kato,  S.  Shimizu, and  K.  Shimose, 2022: Verification of forecasted three-hour accumulated precipitation associated with “senjo-kousuitai” from very-short-range forecasting operated by the JMA. J. Meteor. Soc. Japan, 100, 995–1005.
•  Ito,  J.,  H.  Niino, and  E.  Tochimoto, 2024: Numerical simulation of tornadoes in a mini-supercell associated with typhoon tapah on 22 September 2019. J. Meteor. Soc. Japan, 102, 185–208.
•  Kato,  R.,  S.  Shimizu,  K.  Shimose,  K.  Hirano,  K.  Shiraishi,  S.  Yoshida,  T.  Sakai, and  T.  Nagai, 2024: Improvement of two-hour-ahead QPF using blending technique with spatial maximum filter for tolerating forecast displacement errors and water vapor lidar assimilation. J. Meteor. Soc. Japan, 102, 445–464.
•  Sato,  K.,  T.  Kameda, and  T.  Shirakawa, 2022: Heavy snowfall at Iwamizawa influenced by the Tsushima warm current. J. Meteor. Soc. Japan, 100, 873–891.
•  Takemi,  T., and  T.  Unuma, 2020: Environmental factors for the development of heavy rainfall in the eastern part of Japan during typhoon Hagibis. SOLA, 16, 30–36.
•  Toyooka,  D.,  T.  Kawabata, and  H. L.  Tanaka, 2024: Windward region sensitivity and its effects on heavy rainfall prediction investigated with ensemble systems. J. Meteor. Soc. Japan, 102, 162–183.
•  Tsuji,  H., and  Y.  Takayabu, 2023: A hierarchical structure of the heavy rainfall event over Kyushu in July 2020. J. Meteor. Soc. Japan, 101, 271–287.