Herein, the impact of a hyperspectral infrared sounder on a geostationary satellite (GeoHSS) in a regional numerical weather prediction system is investigated during the Baiu seasons in 2017, 2018, and 2020, including events of heavy rainfall. The reanalysis-based observing system simulation experiment (OSSE) technique uses ERA5 as the pseudo-truth atmospheric profile. Temperature and relative humidity pseudo-observations are generated using one-dimensional variational retrieval scheme based on the spectral characteristics of the GeoHSS. Verification against radiosondes shows improvements at various altitudes and forecast times (FTs). Although wind pseudo-observations are not assimilated, winds are also impacted through assimilation cycles and forecasts. Furthermore, the precipitation forecasts show an improving trend with a notable impact to extend the forecasts' lead time. Case studies show impacts on precipitation primarily during longer FTs, accompanied by improved prediction of depressions on the Baiu front and upper-level troughs. These are due to large-scale impacts from the pseudo-observations with a comprehensive coverage over clear-sky areas, propagating to precipitation areas through the assimilation cycle and forecasts. However, the prediction of an event of small-scale localized heavy rainfall is insufficient even at short forecast ranges due to a limited spatial resolution. Experiments show that extracting information in the lower atmosphere is critical and that the impact on upper-level environments is sensitive to using observations in cloudy areas.

In 2019, serious disasters were caused by local strong winds associated with Typhoon Faxai and wide torrential rains with Typhoon Hagibis. Although both tropical cyclones (TCs) followed similar tracks and underwent extratropical transition after the recurvature (ETR), their storm sizes and structures were distinct: Faxai was a small axisymmetric TC, whereas Hagibis was a large asymmetric TC. The purpose of this study is to clarify the effect of storm size on a TC that undergoes ET and its associated synoptic environment. Hagibis causes a larger amount of precipitation more widely than Faxai. A large amount of diabatic heating closely associated with the precipitation leads to low potential vorticity (PV) production downstream of Hagibis in the upper troposphere and the enhancement of the ridge. By contrast, the diabatic heating is relatively small, and the production of low PV area is indistinct downstream of Faxai. Besides the case studies, large (LA) and small (SM) TCs that undergo ETR (LA-ETR and SM-ETR TCs, respectively) in the western North Pacific from 2016 to 2020 are statistically compared using cyclone phase space and composite analyses with the best track and Japanese 55-year Reanalysis datasets. As observed in the case studies, the LA-ETR TCs are characterized by a larger amount of diabatic heating and a more enhancement of the downstream ridge than the SM-ETR TCs. The LA-ETR TCs change into asymmetric structures more drastically than the SM-ETR TCs while moving northward along the westerly jet with increasing the amplitude of the north–south meander. By contrast, the amplitude of the north–south meander of the westerly jet does not increase around the SM-ETR TCs. Therefore, the larger the storm size is, the larger the amplitude of the north–south meander of the westerly jet is, resulting in a more drastic asymmetric structural change of the TC.

Here, I create a dataset of fronts in mid- and high latitudes by applying an objective front detection method to the JRA-55 reanalysis and try climate classification based on dynamic climatology from temperate to polar regions. Additionally, I describe the interannual variations and long-term trends in the frontal zone. The unique feature of this study lies in the methods used for frontal data creation. This includes adding the geopotential height condition at 500-hPa to the conventional thermal-based objective method with equivalent potential temperature, including incorporating latitude-dependent parameters. The former increased the similarity between fronts created by the objective method and manually counted fronts on surface weather maps, while the latter enabled an examination of climate classification based on dynamic climatology by increasing the frontal frequency at high latitudes. The areas where climatic zones can be clearly defined are limited to the east of the great mountains in the mid-latitudes and the region where the Siberia-Canada Arctic frontal zone exists due to the obscuration or unclear seasonal movement of the frontal zones in other areas. The interannual variability in frontal zones is generally consistent with the characteristics of the regional climate variability associated with the El Niño Southern Oscillation, Pacific Decadal Oscillation, and Arctic Oscillation, as reported by previous studies. This study also reveals significant trends in some frontal zones since 1979, such as the northward shift in the eastern part of the North Pacific polar frontal zone during boreal autumn and winter and the decreasing frontal frequency on the northern coast of Norway in the European Arctic frontal zone from boreal winter to summer, including around the Beaufort Sea in the Siberia-Canada Arctic frontal zone in boreal summer.