A waterspout was observed on 5 March 2024 over the ocean near Okinawa, Japan, south of the center of an extratropical cyclone. Doppler radar observations revealed a mesocyclone above the waterspout, suggesting that the mesocyclone was responsible for the waterspout formation. A convective system associated with the mesocyclone was likely a mini-supercell because static instability was moderate whereas low-level vertical wind shear was large around Okinawa, and because the echo top of the convective system was not high. Numerical simulations with realistic boundary conditions were performed to investigate the development process of the mesocyclone and waterspout. In the 250-m horizontal resolution simulation, a mini-supercell featuring a low-level mesocyclone and a strong near-surface vortex below it was reasonably reproduced. A vorticity budget analysis indicated that tilting and vertical advection of vorticity contributed to the development of the mesocyclone. The strong near-surface vortex developed along a gust front beneath the mini-supercell. In the 50-m-resolution simulation, the near-surface vortex was even more intense enough to be called a tornado-like vortex (TLV). Strong updraft beneath the low-level mesocyclone promoted the rapid amplification of the TLV through vortex stretching.
Cold-air damming (CAD) in the Kanto region is an important forecast target because of its association with heavy snowfall and heavy rainfall that can cause substantial societal impacts. In this study, we conducted sensitivity experiments using the Japan Meteorological Agency (JMA) non-hydrostatic model (JMA-NHM) to investigate factors affecting CAD forecast in 30 cases for which the JMA operational Mesoscale Model (MSM) exhibited large forecast errors, especially a warm bias in the low-level cold-air field. We combined two representations of orography with different effective resolutions and two planetary boundary layer (PBL) schemes to evaluate their relative importance in CAD cold-air forecast. The 1-km-equivalent, more detailed orography tended to strengthen the damming of low-level cold air compared with the 5-km-equivalent orography. Differences between PBL schemes had impacts comparable to or larger than those of orographic refinement, through differences in heat and momentum diffusion over the warm ocean. Moreover, the PBL impacts emerged earlier than the orographic impacts. These results indicate that, toward future high-resolution CAD forecasting, not only orographic representation but also the PBL scheme requires attention. The impacts of these settings on actual forecast accuracy, beyond the relative sensitivities examined here, remain to be evaluated.
The Transformed Eulerian Mean (TEM) and Mass-Weighted Isentropic Mean (MIM) frameworks are widely used to diagnose wave-mean flow interactions. While their continuity and thermodynamic equations are nearly identical, their zonal momentum equations differ. The MIM system utilizes mass-weighted means for all wind velocities, whereas the TEM system mixes the Eulerian zonal mean flow and the residual mean flow. To resolve this discrepancy, this study derives a modified TEM zonal momentum equation by incorporating the time evolution equation of the zonal bolus velocity. In this new formulation, all mean flows are transformed into residual mean flows, making the structure consistent with the MIM system. The vertical component of the wave activity flux is reorganized into contributions from diabatic heating perturbations, the meridional flux of zonal momentum induced by meridional distortion of isentropic surfaces, and form drag. Analysis using ERA5 reanalysis data demonstrates that the vertical wave activity flux can be newly interpreted in terms of form drag and isentropic surface distortion, revealing that the tropospheric vertical flux of zonal momentum corresponds predominantly to the meridional flux of zonal momentum. The zonal and meridional bolus velocities are driven primarily by stationary and transient disturbances, respectively.
This study presented a statistical analysis of line-shaped mesoscale convective systems (LS-MCSs, called “Senjo-Kousuitai”, Kato 2020) in northern Japan using a 64-year high-resolution reanalysis dataset. A significant increase in LS-MCSs frequency and rainfall intensity is identified, particularly along the Pacific side, suggesting a growing risk of extreme precipitation in northern Japan. The synoptic-scale circulation patterns exhibited a strong correspondence with the spatial orientation of LS-MCSs. Pacific-side events were frequently associated with tropical cyclones, whereas the Sea of Japan side events were linked to quasi-stationary frontal systems. An environmental diagnostic analysis, based on the recently-proposed criteria for LS-MCS occurrence (so-called 6-conditions) revealed that approximately 60% of the events satisfy all conditions at the time of initiation. Upper-level moistening and ascent meet thresholds before initiation, while low-level moisture transport and convective inhibition become increasingly satisfied their criteria toward the onset. The results also demonstrate sensitivities to threshold values. Composite analysis further indicates that moisture flux convergence and upstream disturbances, such as mesoscale low-pressure systems, contribute to LS-MCSs organization. Overall, this study provides new insights into the climatology and environmental mechanisms of LS-MCSs in northern Japan, contributing to improved understanding and prediction of extreme rainfall events.
Extreme high temperature and humidity threaten high-latitude regions that are less adapted to such conditions. While marine heatwaves (MHWs) can contribute to hot and humid conditions over land, the modulation of these impacts by the day-to-day atmospheric circulation remains poorly understood. This study focused on Hokkaido, the northernmost prefecture in Japan, to investigate how the day-to-day atmospheric circulation regulated the distribution and intensity of MHW-related heatstroke risk during August 2023. Regional atmospheric model simulations indicated that the MHWs that developed south of Hokkaido increased the Wet Bulb Globe Temperature (WBGT) by approximately 2 °C in the Pacific coastal areas around Kushiro and Tomakomai. Backward trajectory analysis showed that the intensity of MHW-induced heatstroke risk was governed by the day-to-day atmospheric circulation. On days when MHWs substantially impacted the WBGT in the southeastern coastal region, air parcels passed over regions with high sea surface temperatures (SSTs). Conversely, when trajectories bypassed these MHW areas, the impact remained low. These results demonstrate that the day-to-day atmospheric circulation may influence the impact of MHWs on WBGT over land. Our findings suggest that the relative positioning of airflow pathways and the SST distribution should be considered to improve heatstroke risk prediction in coastal regions.
