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In summer 2023, record-high temperatures were observed in many parts of the Northern Hemisphere, including Japan, where summer-mean temperature was the highest over the last 126 years. Under an unprecedented heatwave in late July through September, record-high temperatures were successively observed particularly over northern and eastern Japan. The late-July heatwave is attributable primarily to the markedly-intensified North Pacific Subtropical High over Japan, accompanied by the poleward-deflected subtropical jet (STJ). This situation occurred under the influence of the Pacific–Japan pattern driven by northwestward-moving enhanced tropical convection over the western North Pacific and the Silk-Road pattern. The enhanced convection was influenced by upper-level cyclonic vortices detached from the intensified mid-Pacific trough. Seemingly, it was also under the remote influence from positive sea-surface temperature (SST) anomalies in the western equatorial Pacific as well as negative ones in the central–eastern equatorial Indian Ocean, considered as remnant and delayed impacts of long-lasted La Niña until the preceding winter. The August heatwave occurred under the persistent poleward-shift of STJ as well as warm, moist low-level southerlies and their downslope-wind effects. Both extremely high SST around northern Japan and a long-term warming trend in air temperature could also contribute to the record-setting air temperature.

An eyewall replacement cycle is often seen in tropical cyclones, when a secondary eyewall forms outside the inner eyewall, and the inner eyewall disappears. Although this cycle significantly affects the intensity of tropical cyclones, the mechanisms of secondary eyewall formation (SEF) are diverse, and most are complementary. Some studies have suggested that dry air inflow and diabatic cooling may have an important role in SEF via the mesoscale descending inflow (MDI). Here, we use numerical experiments to investigate the role of the middle tropospheric dry inflow in SEF. Idealized experiments were conducted using the plane version of the Nonhydrostatic Icosahedral Atmospheric Model. The control experiment produced SEF with a dry air inflow in the middle troposphere and associated MDI. In sensitivity experiments, in which the water vapor in the middle troposphere was increased in the outer areas of the tropical cyclone, the onset of SEF was delayed. These results reveal the two distinct processes of SEF: the angular momentum transport by MDI and the unbalanced dynamics in the boundary layer.

Shallow coastal seawater response during the passage of near-landfall intensification (NLI) tropical cyclones (TCs) and non-NLI TCs was examined using oceanic and atmospheric reanalysis data and observations. The sea surface temperature ahead of the NLI-TC track is maintained or even increases when NLI-TC is approaching the land. The magnitude of the wind stress, which play an important role in the NLI process, is related to the zonal surface wind on the right side of the tracks. Coastal mixed layer warming can be explained by Ekman transport under sustained wind stress due to surface wind forcing. The successive deepening of the coastal ocean boundary layer and the increase in warming in the subsurface seawater temperature by an average of 0.3°C, could maintain thermal capacity in a certain degree. This shallow coastal water response could partly explain the NLI progress in the northern South China Sea, indicating the importance of coastal ocean dynamics and air-sea interactions.

This study investigated three-dimensional structure of an equilibrium drop size distribution within a convective system that spawned heavy rainfall over northern Kyushu in western Japan on 10 July 2023. Ground-based optical disdrometer observations showed that the drop size distribution shape became bimodal (the peaks are at 0.7 and 1.0 mm in diameter) and then reached an equilibrium state during the rapid increase in precipitation intensity. Analyses of vertical profiles of polarimetric measurements showed that within the convective system collisional coalescence was dominant mainly at 1.5-4 km height, whereas collisional breakup was dominant below 1.5 km height. These processes were inferred to enhance the precipitation intensity. The equilibrium drop size distribution continued at least one minute during the event, and its spatial scale, diagnosed by a radar-derived parameter to be several kilometers, suggested that the equilibrium drop size distribution was a meso-γ-scale phenomenon.

At a site on the Sea of Japan side of central Japan, the concentrations of ionic components in PM_2.5 were measured semi-continuously for about 15 months using filter collection and ion chromatography (filter method). Continuous measurements of sulfate particles were simultaneously performed in summer and autumn using a commercial sulfate monitor (Sulfate Particle Analyzer, SPA). High concentrations of sulfate ion (SO₄²⁻) were sometimes observed from spring to summer, and the high SO₄²⁻ was thought to be due not only to trans-boundary pollution from the Asian continent but also the influence of volcanic plumes. A comparison between the SPA and the filter method showed that the sulfate concentrations measured by the SPA method tended to be about 20% lower than those by the filter method. High concentrations of sulfate particles were observed not only from volcanoes from the Kyushu District such as Sakurajima, but also from volcanic smoke derived from Nishinoshima in the Ogasawara Islands. In recent times, sulfate particles from volcanos may be important contributors to PM_2.5 in Japan.

Ensemble dynamical downscaling experiments were performed to investigate the influence of East Siberian blocking on a heavy snowfall event that occurred over Fukui City, Japan, in early February 2018 and was associated with the development of the Japan Sea Polar airmass Convergence Zone (JPCZ). The downscaling experiments simulated the enhancement of the East Asian cold air stream and its flow along two routes: the western route, which runs from the Eurasian Continent via the Yellow Sea and the Korean Peninsula; and the northern route, which originates in the Sea of Okhotsk and runs via the northern Japan Sea. As a result, the location and strength of the simulated JPCZ in the downscaling experiments are consistent with those in the Japanese regional reanalysis dataset. For the sensitivity experiments, the blocking that develops over East Siberia just prior to the formation of the JPCZ was removed, and the results indicate that the East Siberian blocking contributes significantly to JPCZ development by enhancing the East Asian cold air stream along the western route. Additional data analyses based on the 20-year reanalysis revealed that East Siberian blocking can enhance both the western and northern routes of the cold air streams.

The relationship between maximum sustained wind speed (V_max) and minimum sea level pressure (P_min) of tropical cyclones (TC), which is called the wind–pressure relationship (WPR), is investigated by using best track data in which aircraft observations are used. On average, for given V_max (P_min), P_min (V_max) varies by 8.5 hPa (11.0 kt) between the 25th and 75th percentiles, and it varies by 17.1 hPa (22.6 kt) between the 10th and 90th percentiles; corresponding variations in the Dvorak Current Intensity (CI) numbers are also quantified. Also investigated is an adjusted WPR in which environmental conditions are incorporated through multiple linear regression. Its utilization reduces the variations to 6.9 hPa (9.5 kt) between the 25th and 75th percentiles and 13.0 hPa (18.9 kt) between the 10th and 90th percentiles. These remaining variations indicate intrinsic variability of WPR, suggesting a need for further utilization of observations to improve the intensity estimation of TCs.

This study investigates the precipitation characteristics and disturbances of widespread heavy rainfall during the Akisame using meteorological data from the Japanese 55-year Reanalysis, precipitation data from the Automated Meteorological Data Acquisition System (AMeDAS), the Global Satellite Mapping of Precipitation (GSMaP) and the Global Precipitation Measurement (GPM) data. First, in the climatological field during the Akisame, northward water vapor transports (WVTs) prevail over eastern Japan (EJPN) due to the eastward shift of the North Pacific subtropical high and the existence of an anticyclone over the continent. This situation differs from the Baiu. Under humid conditions during the Akisame, well-organized precipitation systems with large stratiform precipitation area accompanying strong convective precipitation bring heavy rainfall, as in the Baiu. Second, the analysis of widespread extreme precipitation events (WEPEs) conducted for the Baiu in WJPN (Baiu/WJPN) by Shibuya et al. (2021) is performed for 4 cases: Baiu/EJPN, Baiu/WJPN, Akisame/EJPN and Akisame/WJPN. In the composite of the Akisame/EJPN case, WVTs toward EJPN is enhanced by a northeast-southwest dipole structure of geopotential height anomaly. We newly revealed that WEPEs occur during the Akisame associated with the overlap of the climatological field with anomalous disturbances that are different from those in the Baiu.

It is generally considered that warm rain is less likely to occur in urban areas where the air is polluted. However, heavy precipitation from shallow convective clouds is occasionally reported in Tokyo. In this study, we observed microphysical characteristics of warm convective precipitation in Tokyo on 19-20 August 2019 using an X-band polarimetric radar, a Ka-band radar, a cloud droplet spectrometer and an optical disdrometer. The radar reflectivity and the specific differential phase from the X-band radar tended to increase in the lower layers, suggesting accretion growth of raindrops. On the other hand, the differential reflectivity decreased in the lower layers, suggesting the presence of low concentrations of large raindrops near the echo top. According to range height indicators, precipitation clouds were composed of streak-like echoes. The Z-R relationship on the ground was close to that of the Marshall-Palmer raindrop size distribution. Mean cloud number concentration (N_c) was 370 cm⁻³, which was larger than the average of low-level clouds in Tokyo (213 cm⁻³). Parcel model simulations suggested that warm rain could be initiated when N_c < 1200 cm⁻³, although the threshold of N_c depends on the cloud base temperature.