The characteristics of the raindrop size distribution (DSD) in summer in the western (Nilek) and central (Urumqi) regions in the Tianshan Mountains of China were studied based on three years of second-generation Particle Size Velocity (Parsivel²) disdrometer data. The FengYun-2G satellite remote sensing data and the ERA5 reanalysis product were used to reveal the possible thermo-dynamical and microphysical processes that caused the dissimilarities in DSD between Urumqi and Nilek. The DSD in Nilek is significantly different from that in Urumqi. The concentration of mid- and large-size drops is higher in Nilek than in Urumqi. The DSD characteristics for six rain rate classes and two rain types (convective and stratiform) are studied. It is found that the raindrops in Nilek have overall higher mass-weighted mean diameters (D_m) and lower logarithm of normalized intercept parameters (log₁₀ N_w) than those in Urumqi, which is true for different rain rates and rain types. Convective clusters in Urumqi are similar to maritime clusters, whereas those in Nilek are more similar to continental clusters, according to a classification standard of convective clusters. The radar reflectivity, rain rate relations, and the shape and slope relations for rainfall in Urumqi and Nilek are also different. The DSD variability in the two regions may be attributed to differences in convective intensity that are closely related to the specific terrain of the Tianshan Mountains.

Iwamizawa, situated on the Sea of Japan side of Hokkaido, is a city in Japan that experience frequent heavy snowfall events. Warm surface-layer ocean anomalies over the Sea of Japan can induce heavy snowfall over the Sea of Japan side of Japan; however, the relationship between ocean temperature over the northern Sea of Japan and snowfall events at Iwamizawa remains uncertain. This study used reanalysis data to investigate atmospheric and oceanic circulation anomalies associated with each anomalous heavy snowfall winter month at Iwamizawa. During all anomalous snowfall winter months at Iwamizawa, a cold air anomaly with northwesterly winds existed over the Far East that was associated with a dipole pattern with anticyclone anomalies over the northern coast of the Eurasian Continent and cyclonic anomalies extending zonally over the Far East and northern Pacific Ocean. The surface cold air temperature and strong wind speed anomalies are major factors for anomalous upward turbulent heat flux over the northern Sea of Japan during all anomalous snowfall winter months at Iwamizawa. Additionally, during anomalous snowfall in January, warm surface-layer ocean anomaly over the northern Sea of Japan, which preceded the heavy snowfall events at Iwamizawa by 2 months, plays an important role in the upward turbulent heat flux anomaly. This preceding warm ocean temperature anomaly was associated with a strong Tsushima Warm Current anomaly. Results showed that the warm surface-layer ocean anomaly over the northern Sea of Japan that precedes anomalous cold advection from the Eurasian Continent also has a large impact on producing heavy snowfall events over the western Hokkaido coastal regions near Iwamizawa in January.

During the autumn rainy season, typhoons located far from Japan sometimes cause significant precipitation in Japan. In the current study, we characterized remote precipitation events in September for 40 years from 1980 to 2019. We also analyzed cases in which remote precipitation did not occur despite approaching typhoons, as well as cases in which heavy precipitation was not affected by typhoons. We characterized the environmental fields of the remote precipitation cases by comparing them with these other two types of cases.

Statistical analysis showed that remote precipitation tended to occur when the typhoons were located over the southern or southwestern oceans of mainland Japan and when the tracks of the typhoons were northward or changing to the northeast. The composite analysis of the remote precipitation cases showed that the subtropical high was retreating to the east for 2 days before the remote precipitation. By contrast, the cases in which remote precipitation did not occur showed the opposite pattern; the subtropical high was strengthening to the west when typhoons were approaching over the southern or southwestern oceans of the Japanese archipelago. Furthermore, the remote precipitation occurred to the equatorward jet streak entrance of the 200 hPa jet, whereas the 200 hPa jet streak was shifted to the west in the cases where remote precipitation did not occur. The vertical cross-section of the northward water vapor flux showed that the northward water vapor inflow from the middle troposphere was larger in cases of remote precipitation than in cases in which heavy precipitation was not caused by typhoons. In addition, dynamical analysis showed that the area of remote precipitation corresponded to the region of 800–600 hPa mean quasi-geostrophic forcing for ascent and 925 hPa frontogenesis.

Nocturnal temperature inversion (NTI) is an important factor characterizing the local climate in mountainous areas. In central Japan, most of the mountain slopes are covered by forests, but the effects of their leaf expansion/fall on the NTI variations in basins have not been clarified. According to a 3-year leaf area index observation in the mixed forest of the Sugadaira Highland (1320 m a.s.l), Nagano Prefecture, Japan, we identified weakening of the NTI associated with leaf expansion and strengthening after leaf fall in a small basin. Using digital elevation and land-cover data, we defined the distribution of the deciduous and mixed forests in the catchment area of nocturnal cold-air drainage. The estimated timings of leaf expansion/fall at the catchment scale based on the effective cumulative temperature almost coincided with the NTI changes. Micrometeorology observations showed that NTI at the forest floor and downslope winds at the adjacent grassland strengthened during the dormant (leafless) season in the nighttime when the radiative cooling is strong. Calm and clear nights were selected during the spring dormant season and the summer growing season for 22 nights and 30 nights, respectively. The heat loss during the cold-air pool development was estimated and converted to storage heat flux in the forest areas. The storage heat flux was 3.8 W m⁻² more on average in the growing season than in the dormant season, and it was less than that of forests estimated in previous studies (several 10 W m⁻²), indicating that an increase in storage heat flux of the forests with leaf expansion could cancel nocturnal radiative cooling and weaken gravity currents at the forest floor.

Knowledge about tropical cyclone (TC) size is essential for disaster prevention and mitigation strategies, but due to the limitations of observations, TC size data from the open ocean are scarce. In this paper, several models are developed to identify TC size parameters, including the radius of maximum wind (RMW) and the radii of 34 (R34), 50 (R50), and 64 (R64) knot winds, using various machine learning algorithms based on infrared channel imagery of geostationary meteorological satellites over the western North Pacific (WNP). Through evaluation and verification, the trained and optimized support vector machine models are proposed for RMW and R34, whereas the general regression neural network models are set up for R50 and R64.

According to the independent-sample evaluations against aircraft observations (1981–1987)/Joint Typhoon Warning Center best track data (2017–2019), the mean absolute errors of R34, R50, R64, and RMW are 54/58, 34/38, N/A/21, and 25/25 km, respectively. The corresponding median errors are 39/46, 34/31, N/A/17, and 17/19 km, respectively. There is an overall slight underestimation of the parameters, which needs to be analyzed and improved in a future study. Despite aircraft observations of TCs in the WNP having ceased in the late 1980s, this new dataset of TC sizes enables a thorough estimation of wind structures covering a period of 40 years.

Typhoon Hagibis (2019) was a large and intense tropical cyclone that had significant societal impacts in Japan. It went through a period of explosive rapid intensification (RI), with an increase of maximum wind speed from 60 kt to 160 kt in 24 h, immediately followed by a secondary eyewall formation (SEF) and an eyewall replacement cycle (ERC). Operational forecasts from Coupled Ocean/Atmosphere Mesoscale Prediction System—Tropical Cyclone (COAMPS-TC) failed to capture Hagibis' explosive RI, peak intensity, and associated inner core structural evolution. Four COAMPS-TC sensitivity experiments, initialized at 1200 UTC 5 October 2019, were conducted to study the impact of horizontal resolution on prediction of Typhoon Hagibis' RI and structure. Results indicate that RI of the storm to Category 4 intensity can be simulated with the finest grid spacing at 4 km, but use of 1.33 km for the finest grid spacing facilitates more realistic prediction of the explosive intensification rate, Category 5 peak intensity, and small inner core accompanying the RI. Our sensitivity experiments indicate that realistic simulation of Hagibis' SEF/ERC requires a very intense storm with a small inner core as a prerequisite for its occurrence. Therefore, the finest grid spacing at 1.33 km is necessary but not sufficient to capture the SEF/ERC. The simulation of the RI and SEF/ERC is also sensitive to the resolution of the outermost grid, which has impacts on the storm's moisture distribution by modulating the flow of moist air from the deep tropics into the TC. While these results have implications for the grid configuration of operational models similar to COAMPS-TC, additional work is needed to gain systematic understanding of the physical processes associated with simulation of explosive RI and SEF/ERC.

To compare the predictability of two stratospheric sudden warming (SSW) events occurring in 2009 and 2010, ensemble forecast experiments are conducted using an Atmospheric General Circulation Model. It is found that the predictable period of the vortex-splitting SSW in 2009 is approximately 7 days that is much shorter than that of the vortex-displacement SSW in 2010. The latter event is predictable more than 13 days in advance. The ensemble spread in the upper stratosphere for medium-range forecasts is found to be enlarged just prior to the onset of the 2009 SSW event, whereas no such enlargement is seen for the 2010 SSW event.

Stability analysis of the zonally asymmetric basic states specified by the ensemble mean forecast using a nondivergent barotropic vorticity equation reveals that the extremely distorted polar vortex in the upper stratosphere just before the onset of the 2009 SSW event is highly unstable to infinitesimal perturbations, whereas there is no such unstable mode with an extremely large growth rate during the 2010 SSW event. In addition, the most unstable mode during the onset of the 2009 SSW event has a similar horizontal structure to the 1st EOF of the ensemble spread. Thus, it is suggested that a predictability barrier inherent in the upper-stratospheric circulation, characterized by the presence of dynamically unstable modes with large growth rates, limits the predictable period of the 2009 SSW event.

Although a number of studies have been conducted of the lightning activity in hurricanes and typhoons, little information has been obtained on the three-dimensional (3-D) structure of the lightning, or how it is related to the precipitation structures within the storms. Here, we utilize observational data from the 3-D Tokyo Lightning Mapping Array (Tokyo LMA), a Japan Meteorological Agency C-band Doppler radar, and the Japanese Lightning Detection Network (JLDN) to conduct a study of the lightning activity during Typhoon Faxai (2019) in comparison with the storm's precipitation structure. This is done for the dissipating stage of the typhoon, when the eyewall was well within the range of the instruments and undergoing a surge in lightning activity. Of particular interest in the surge was the occurrence of numerous positive cloud-to-ground (+CG) lightning flashes. Detailed study of the Tokyo LMA and JLDN data shows that, out of 52 flashes during the surge, 29 flashes or 56 % produced positive strokes to ground, an unheard of number considering that, from the lightning and 3-D radar structures, the storm appeared to be normally electrified, and under such circumstances would produce negative rather than positive strokes to ground. It also focuses attention on the question of how +CGs are produced in tropical cyclones in the first place. Based on a lack of −CG strokes and the LMA observations showing that the +CG strokes are produced mid-way or toward the end of normal polarity intracloud (IC) flashes, it appears that the dissipating storm cells have a depleted or horizontally sheared mid-level negative charge, such that an IC flash propagating into and through upper positive storm charge effectively funnels a steadily increasing amount of positive charge into the mid-level initiation region, eventually causing the positive breakdown of the IC flash to turn downward toward ground, producing a +CG stroke.

In recent years, “senjo-kousuitai”, characterized as a band-shaped area of heavy rainfall, has frequently caused river floods and landslides in Japan. Preventing and mitigating such disasters requires skillful forecasts of accumulated rainfall for several hours with an adequate lead time. The immediate very-short-range forecast of precipitation (VSRF) provided by the Japan Meteorological Agency is well suited to this purpose, representing a blended forecast of hourly accumulated precipitation for up to 6 h ahead based on extrapolation and numerical weather prediction. This study examined the predictability of the VSRF for 3-h accumulated precipitation associated with 21 senjo-kousuitai events that occurred in Kyushu in 2019 and 2020. Predictability was evaluated based on forecast accuracy at each forecast time (FT; 1–6 h) using categorical and neighborhood verification techniques. Overall, the VSRF product was useful for heavy rainfall areas of ≥ 80 mm (3h)⁻¹ up to an FT of 2 h at the original grid spacing of 1 km, but with large uncertainty in the accuracy of the forecasts. After that FT, it was not possible to obtain a useful precipitation forecast for the threshold of ≥ 80 mm (3h)⁻¹, even if displacement errors at municipal or larger scale (15–31 km) were tolerated. Further analysis showed that the VSRF is less skillful in the stage of senjo-kousuitai formation at shorter FTs (1–2 h) owing to limitations of the extrapolation forecasts. The poor skill during this period affects the timing of both issuance of warnings and decision-making regarding evacuation, representing major challenges for future development of forecasting methods and systems for senjo-kousuitai.