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An intense rainband associated with Typhoon 1326 (Wipha) induced a fatal debris flow on Izu Oshima, Japan, on October 15 - 16, 2013. This rainband formed along a local front between the southeasterly humid warm air around the typhoon and the northeasterly cold air from the Kanto Plain. In this paper, the Japan Meteorological Agency Nonhydrostatic Model was optimized for the "K computer", and ultra-high-resolution (500 - 250 m grid spacing) numerical simulations of the rainband with a large domain were conducted. Two of main factors that affect a numerical weather prediction (NWP) model, (1) grid spacing and (2) planetary boundary layer (PBL) schemes [Mellor-Yamada-Nakanishi-Niino (MYNN) and Deardorff (DD)], were investigated. Experiments with DD (Exps_DD: grid spacings of 2 km, 500 m, and 250 m) showed better reproducibility of the rainband position than experiments with MYNN (Exps_MYNN: grid spacings of 5 km, 2 km, and 500 m). Exps_DD simulated distinct convective-scale up/downdraft pairs on the southeast/northwest sides of the front, whereas those of Exps_MYNN were not clear. Exps_DD yielded stronger cold pools near the surface than did Exps_MYNN. These differences in the boundary layer structures likely had a large impact on the position of the front and the associated rainband. Exps_DD with the 500-m grid spacing showed the best precipitation performance according to the Fractions Skill Score. To check other factors which influence precipitation forecast, model domain sizes, lateral boundary conditions in nesting simulations, and terrain representations were investigated. In the small domain experiments, the rain-planband shapes were very different from the observations. In the experiment using a nesting procedure, the deterioration of the forecast performance was acceptably reduced. The model with fine terrains reproduced the intense rain over the island. These results demonstrate that the ultra-high-resolution NWP model with a large domain has the possibility to improve predictions of heavy rain. [ABSTRACT FROM AUTHOR]

Disasters caused by heavy rainfall associated with quasi-stationary line-shaped mesoscale convective systems (MCSs) frequently occur in Japan. Thus, highly accurate quantitative precipitation forecast (QPF) information that contributes to decision-making by municipalities to issue evacuation orders is necessary. To this end, we developed a blending forecasting system (BFS) for predicting heavy rainfall associated with MCSs. The BFS blends 1-h observed rainfall and forecasts of extrapolation-based nowcasting (EXT) in the first hour and numerical weather prediction (NWP) in the second hour, predicting 3-h accumulated rainfall (P3h) and its return period (RP) of up to 2 h ahead with a higher horizontal resolution (1 km) and higher-frequency updates (every 10 min) compared to the current operational systems. A blending technique with a spatial maximum filter for tolerating forecast displacement errors (BLEDE) was applied to the predicted rainfall of EXT and NWP. To improve the accuracy of the NWP, vertical profiles of water vapor obtained with two water vapor lidars (WVLs) were assimilated into the NWP. This combination predicted rare heavy rainfall with an RP of more than 10 years in the same city where flooding occurred for a heavy rainfall event associated with quasi-stationary line-shaped MCSs in southern Kyushu on 10 July 2021. The BFS yielded such forecast information 40 min earlier than the existing warning information, indicating the potential for providing a longer lead time for evacuation. The improvement in forecast accuracy was due to both BLEDE and WVL data assimilation (WVL-DA); however, the contribution of BLEDE was more than five times that of WVL-DA in terms of predicting the P3h for the threshold of 80 mm. Additionally, the sensitivity of the predicted rainfall to the background error covariance matrix in WVL-DA is also discussed. [ABSTRACT FROM AUTHOR]

This study examined the roles of heat fluxes from the Kuroshio Current in enhancing a frontal convective rainband associated with an extratropical cyclone that brought record-breaking heavy rainfall to Miyake Island, Japan, in January 2017. A simulation of the rainband (control experiment) and sensitivity experiments without the sensible and latent heat fluxes from the Kuroshio Current were conducted using a regional cloud-resolving model. The rainband developed along a non-classic front (outer front), which formed to the north of a warm front associated with the cyclone. The control experiment reproduced the intensity and migration of the rainband well. As the rainband developed, the heat fluxes from the Kuroshio Current became evident around the cold conveyor belt of the cyclone to the south of the rainband. The latent heat fluxes were ~ 2.3 times larger than the sensible heat fluxes. Comparisons between the control and sensitivity experiments demonstrated that the heat fluxes, especially the latent heat fluxes, enhanced the rainband. The sensible heat fluxes slightly intensified convective instability in the lower troposphere, whereas the latent heat fluxes significantly increased the near-surface moisture content and the convective instability. A frontal updraft along the outer front released the increased convective instability, which intensified the moisture convergence, condensation, and updraft, enhancing the rainband. The findings show that the heat fluxes from the Kuroshio Current, especially the latent heat fluxes, enhanced the heavy rainfallproducing rainband by increasing the moisture content and the convective instability. [ABSTRACT FROM AUTHOR]

In order to explore large-scale atmospheric factors causing heavy rainfall events that occurred widely in western Japan, a composite analysis of atmospheric fields during the past heavy rainfall events in the region is conducted using the Japanese 55-year Reanalysis. During heavy rainfall events, atmospheric fields are characterized by an upper-tropospheric trough over the Korean Peninsula (KP), an upper-tropospheric ridge to the east of Japan, a surface high-pressure system to the southeast of Japan, and southwesterly moisture flux. The composite analysis indicates that a clear wave train due to quasi-stationary Rossby wave packet propagation (RWPP) along the polar front jet over Siberia tends to occur just before extreme events. Further analysis considering various time scale variabilities in the atmosphere reveals that surface high-pressure anomalies to the southeast of Japan are dominated by variability with a 25 - 90 day period, whereas variability with an 8 - 25 day period dominates lower-pressure anomalies over the East China Sea (ECS) in relation to the development of the upper-tropospheric trough around the KP. We also investigate atmospheric fields during an extreme heavy rainfall event that occurred in early July 2018 (HR18). Atmospheric features during HR18 are generally similar to those of the other heavy rainfall events. However, a remarkable RWPP occurred along the subtropical jet (STJ) in late June 2018 and intensified a surface high-pressure system to the southeast of Japan. Additionally, a low-pressure system with an 8 - 25 day period to the south of Japan developed in association with wave breaking induced by the remarkable RWPP along the STJ and propagated northwestward toward the ECS and then to Japan. The simultaneous development of high- and low-pressure systems contributed to the extreme southerly moisture flux into western Japan. HR18 is also characterized by a sharp upper-tropospheric trough over the KP that is dominated by high frequency variability with a period < 8 days. [ABSTRACT FROM AUTHOR]