Typhoons are one of the most destructive weather systems with potentially serious impacts on human life, economics, social systems, and the environment. In particular, typhoons Jebi (2018), Faxai (2019), and Hagibis (2019), which made landfall in Japan, caused serious natural disasters in various areas of Japan and resulted in the record-breaking amount of insurance claims paid due to strong winds, torrential rainfalls, high waves, and storm surge. It is of great scientific interest that the causes and processes are clarified particularly on strong winds of Jebi and Faxai and heavy rainfall of Hagibis.
Regarding the prediction of typhoon intensity in 2018-2019, rapid intensification and weakening of typhoons remain challenging scientific topics. The interactions between typhoon Trami (2018) and the ocean and between typhoon Jongdari (2018) and the upper-tropospheric cold-core low are interesting topics in understanding the effect on the typhoon track predictions. Furthermore, the effect of global warming on these meteorological events in the future are also topics of great interest in addition to the effect on climatological typhoon activity in the western North Pacific.
In this special edition jointly coordinated with Journal of the Meteorological Society of Japan, we publish papers on all aspects of typhoons in 2018-2019, such as formation, movement, intensification, weakening, structure change, strong wind, heavy rainfall, high wave, storm surge, and the interaction with terrain, ocean, or midlatitude systems. We also publish papers on studies on the linkage between typhoons and climate processes that include both natural and anthropogenic origin and on the effect of global warming on meteorological events associated with typhoons in 2018-2019.

View full abstract

One of the remarkable environmental characteristics of tropical cyclone (TC) Hagibis (2019) was the positive sea surface temperature (SST) anomaly observed in the western North Pacific Ocean. In this study, an ensemble-based sensitivity experiment was conducted with a nonhydrostatic model, focusing on the impact of SST on TC motion. The TC with the analyzed SST (warm run) moved faster near mainland Japan than with the lowered SST (cold run), as the TC in the warm run was embedded earlier in the mid-latitude westerly jet located to the north than that in the cold run. The TC displacement was consistent with the large decrease of geopotential height at 500-hPa (Z500) in the north of TC Hagibis during the warm run. Further investigation showed that the approach to the westerly jet presumably induced the low local inertial stability as well as the southwesterly vertical wind shear enhancing the upward mass flux in the north of the TC. They led the enhanced upper-tropospheric northward outflow from the TC energized by the warm SST, and it resulted in the decrease of the Z500 in the north. This study suggests that warm SST can affect TC tracks through interaction with mid-latitude westerly jets.

View full abstract

Impacts of historical warming on extremely heavy rainfall induced by Typhoon Hagibis (2019) are investigated using a storyline event attribution approach with the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM). Control experiments based on JMA mesoscale analysis data well reproduce the typhoon's track, intensity, and heavy precipitation. First, two non-warming experiments are conducted: One excludes both 40-year atmospheric and oceanic temperature trends from 1980 to 2019, and the other excludes the oceanic trend only. A comparison between control and non-warming experiments indicates that historical warming strengthens typhoons and increases the amount of total precipitation by 10.9% over central Japan. The difference between CTL and non-warming experiments without both atmospheric and oceanic temperature trends is larger than that without just the oceanic trend (7.3%). Additional sensitivity experiments without Japan's topography indicate that topography enhances not only total precipitation but also the changes in total precipitation due to historical warming. Through the storyline event attribution approach, it is concluded that historical warming intensifies strength of Typhoon Hagibis (2019) and enhances the extremely heavy precipitation induced by the typhoon.

View full abstract

Numerical experiments on Typhoon Trami (2018) using a regional 1-km-mesh three-dimensional atmosphere–ocean coupled model in current and pseudo-global warming (PGW) climates were conducted to investigate future changes of a slow-moving intense typhoon under the warming climate. Over the warmer sea in the PGW climate, the maximum near-surface wind speed rapidly increased around the large eye of the simulated Trami. The stronger winds in the PGW simulation versus the current simulation caused a 1.5-fold larger decrease of sea surface temperature (SST) in the storm core-region. In the PGW climate, near-surface air temperature increased by 3.1°C. A large SST decrease due to ocean upwelling caused downward heat fluxes from the atmosphere to the ocean. The magnitude of the SST decrease depended strongly on initial ocean conditions. Consideration of the SST decrease induced by an intense typhoon, and a slow-moving storm in particular, indicated that such a typhoon would not always become more intense under the warmer climate conditions. An atmosphere–ocean coupled model should facilitate making more reliable projections of typhoon intensities in a warming climate.

View full abstract

Typhoon Hagibis (2019) caused widespread flooding and damage over eastern Japan. The associated rainfall maxima were primarily observed on the windward mountain slopes along with the west of the leading edge of a low-level front. Concomitantly, a significant positive value in sea surface temperature anomalies (SSTAs) was observed in association with an ocean eddy over the Oyashio region, together with anomalous warmth over the entire western North Pacific. The present study examines the role of the SSTAs in the rainfall distribution associated with Hagibis, to deepen our understanding of the influence of the midlatitude ocean on tropical cyclones and associated rainfall. Our sensitivity experiments demonstrate that the observed warm SSTAs had the potential to displace the rainfall caused by Hagibis inland and thereby acted to increase precipitation along the Pacific coast of northeastern Japan. Our results suggest that midlatitude SSTAs on ocean-eddy scales can also influence the synoptic-scale atmospheric front and associated heavy rainfall.

View full abstract

In the 2019 tropical cyclone season in the western North Pacific, Typhoons FAXAI and HAGIBIS made landfall in Japan while keeping the intensity, resulting in serious disasters. This study addresses the influences of an increasing trend and variations in the upper ocean heat content above 26°C (tropical cyclone heat potential: TCHP) from January 1982 to June 2020 on FAXAI and HAGIBIS. TCHP underneath FAXAI and HAGIBIS in 2019 was higher than the climatological mean except for a part of mature phase of HAGIBIS due to HAGIBIS-induced sea surface cooling. TCHP significantly increased with the interannual oceanic variations (IOVs) in the subtropical (15°N-20°N, 140°E-150°E) and midlatitude (30°N-35°N, 130°E-140°E) areas where FAXAI and HAGIBIS intensified or kept the intensity. From an empirical orthogonal function (EOF) analysis of TCHP, we demonstrate that the leading three EOF modes of TCHP explain approximately 76.8% of total variance, but the increase in TCHP along the tracks of FAXAI and HAGIBIS particularly in the early intensification of HAGIBIS cannot be explained only by the IOVs included in the leading three EOF modes but rather by the warming trend irrespective of the IOVs.

View full abstract

The predictability of Typhoon Hagibis in October 2019 is examined with ensemble forecasts from four major operational numerical weather prediction centers. From six to four days before the landfall, the forecast from the Japan Meteorological Agency was the best among the four centers. However, the error increased sharply three days before the landfall. Consistent with the westward track error, a northwestward bias is found in the environmental winds. The ensemble sensitivity analysis for the landing region indicates a large sensitivity to a ridge located to the southeast of the typhoon. The member with the largest track error has perturbations that act to weaken the ridge. A low-pressure disturbance to the southeast of the ridge is found to migrate westward faster than the member with the smallest track error. Therefore, the typhoon is advected westward by the easterlies associated with the low. These results indicate a significant influence of the tropical disturbance on the predictability of Hagibis.

View full abstract

Torrential rain associated with Typhoon Hagibis (2019) caused extensive destruction across Japan. To project future changes of the record-breaking rainfall, numerical experiments using a regional 1-km-mesh three-dimensional atmosphere–ocean coupled model were conducted in current (CNTL) and pseudo-global warming (PGW) climates. The water vapor mixing ratio in the lower troposphere increased by 23% in response to a 3.34 K increase in sea surface temperature (SST) in the PGW climate. The abundant moisture supply by the westward winds of the typhoon caused strong precipitation from its rainbands for a long period, resulting in 90% increase in total precipitation in eastern Japan before landfall. However, the strong PGW typhoon caused high SST-cooling. Mean precipitation in eastern Japan during the typhoon passage increased by 22% when the SST-cooling east of Kanto was strengthened from 0.11 K to 0.72 K from the CNTL to PGW simulations; the increase was above 29% when the SST-cooling was lowered.

Since Typhoon Hagibis accelerated as it traveled northward, the magnitude of the SST-cooling and weakening of the typhoon were suppressed. Consequently, strong precipitation in the inner-core of the strong PGW typhoon caused 30% increase in precipitation in the areas on the Pacific side of northern Japan.

View full abstract

In October of 2019, Typhoon Hagibis brought abundant rainfall to eastern Japan that caused flooding of the Chikuma River in the northern region of Nagano prefecture. This study simulated the effects of changes in the elevation of the model terrain every 100 or 300 m with a regional meteorological model to understand the cause of the heavy precipitation that accompanied the typhoon in Nagano prefecture and the influence of the heights of mountains on the amount of rainfall. The model reproduced the typhoon track and spatiotemporal distribution of heavy precipitation. Mountains in the northern region of Nagano Prefecture contributed to the heavy precipitation, which increased at an approximately constant rate of 4.4 mm per 100 m increase of elevation. However, the rate of increase was especially large at elevations of 900-1200 m. The correlation of precipitation with topographic height was not as strong in the south as in the north, but the rate of variation was also anomalously high at elevations of 900-1200 m. These elevations roughly corresponded to the level of free convection or to elevations between the level of free convection and the lifted condensation level around the typhoon track.

View full abstract