Although the influence of the El Niño–Southern Oscillation (ENSO) on central Pacific rainfall is well recognized, quantitative assessments of contemporaneous and lagged relationships using official multi-decadal station observations remain scarce. We analyze monthly precipitation at Betio on Tarawa atoll for 1949–2025 provided by the Kiribati Meteorological Service, in relation to Niño 3 sea surface temperature (SST) anomalies. When the year is defined from May to April to include the boreal-winter peak of ENSO, annual precipitation at Betio is strongly related to the Niño 3 SST anomaly (r = 0.83). Monthly precipitation is moderately related to the concurrent Niño 3 SST anomaly (r = 0.55). Lag correlations remain statistically significant when Niño 3 SST leads rainfall by up to about 7 months, after the effective sample size is reduced to account for serial correlation. These results quantify the ENSO–rainfall linkage at Betio and indicate a practical lead time for drought early warning on a low-lying atoll.
This study investigates the synoptic and mesoscale atmospheric conditions of the precipitation system causing the August 2026 Chiba Heavy Rainfall Event in the Kanto Plain, Japan (13–14 August 2026). Synoptically, peripheral flow from a high-pressure system northeast of Japan advected a deep moist layer, which backward trajectories traced to tropical regions far east. Additionally, an approaching cutoff low from the southeast and an associated shortwave trough maintained remarkably low temperatures (> 2σ below climatology) at 500–300 hPa, inducing strong atmospheric instability. Mesoscalely, cold-air damming (CAD) existed prior to the heavy rainfall due to the high-pressure ridge. Convergence between CAD-induced low-level cold northeasterlies and southeasterly warm-moist air formed a coastal front (CF) along the Chiba coast. Diabatic cooling from CF-initiated precipitation markedly intensified the CAD and northeasterlies, accentuating the CF. Consequently, convective cells repeatedly generated on the CF, organizing and sustaining the precipitation system just west of the CF. A mesoscale low formed on the CF enhanced upwind water vapor flux and vertical shear. As the mesoscale low developed and moved, the environment degraded, leading to system decay. These findings demonstrate that the extreme heavy rainfall resulted from combined upper-level cooling, deep moisture advection, and intense CAD-induced coastal frontogenesis.
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.
