Heavy snowfall, caused by snow clouds over the Sea of Japan, can severely affect social and economic activities in Japan. Therefore, snow clouds, which form and develop mainly over the ocean and bring heavy snowfall to populated coastal plains, have been extensively studied from the perspective of disaster prediction and prevention. Most studies analyzed data acquired by aerological, meteorological satellite, and radar observations, or conducted numerical simulations. Because of the difficulties involved in accessing cloud systems over the ocean, however, few in situ observation data have been available, and up until the middle 1990s, many problems remained unsolved or their analysis and simulation results had not been validated. Here we review knowledge gained from instrumented aircraft observations, made from the middle 1990s through the early 2000s, particularly the development of a convectively mixed boundary layer and the inner structures of longitudinal-mode cloud bands, Japan-Sea polar-air mass convergence zone cloud bands, and a polar low. Unsolved problems relating to the inner structures and precipitation mechanisms of snow clouds and the expected contributions of aircraft observations to further progress in these areas of atmospheric science are also briefly discussed.

 In this study, the characteristics of wind gusts in Japan in the period from 2002 to 2017 were examined using surface meteorological data recorded at 151 weather observatories throughout Japan. This study does not focus on particular phenomena, such as tornadoes and downbursts, which cause wind gusts. A wind gust is defined on the basis of the gust factor and the amount of increase and decrease of the 3-s mean wind speed from the 10-min mean wind speed. A total of 3,531 events were detected as wind gusts. The frequency of wind gusts with more than 25 m s⁻¹ averaged across all observatories is 0.97 per year, which is four or five orders of magnitude higher than the tornado encounter probability in Japan. The frequency of wind gusts in the coastal region is approximately three times higher than that in the inland area. Wind gusts occur most frequently in September and least frequently in June. Wind gusts have high activities during daytime, especially in the afternoon. Approximately half of the events are the typhoon-associated wind gusts (WGTYs), which occurred within a radius of 800 km from the typhoon center. Most of the WGTYs occur from August to October. Approximately half of the WGTYs occur in the right-front quadrant of a typhoon with respect to the typhoon motion. The frequency of WGTYs is high in western Japan, whereas the northern and eastern parts of Japan are characterized by a high frequency of wind gusts without a typhoon. In addition, persistent strong winds, which meet the same conditions as wind gusts but without a rapid decrease in the wind speed, were investigated. The frequency of such strong winds is high on the Japan Sea coast, especially in December. The effects of the observational environment on the frequency of wind gusts were also discussed.

 The southeasterly gale in Xinjiang, China is a severe local weather phenomenon that is occasionally observed near the northwest opening of the Tianshan Grand Canyon. On 8 June 2013, a strong southeasterly gale attacked Urumchi with an average ground wind velocity of 15 m s⁻¹ and wind gusts that reached 30 m s⁻¹. The gale lasted for over 24 h, making it the strongest wind in the last 20 years. Through observations and numerical simulation, this study describes the southeasterly gale's formation. The large-scale, topographic forcing from the Tianshan Mountains led to an intensification of the pressure gradient across the Tianshan Grand Canyon, creating a favorable gale condition. When air currents traveled through the canyon, establishing a critical layer with zero-wind velocity, the orographic forcing activated a non-linear process. The air current sank on the canyon's northwest opening due to unstable stratification which in turn strengthened the gale.

 Typhoons are considered as one of the most powerful disaster-spawning weather phenomena. Recent studies have revealed that typhoons will be stronger and more powerful in a future warmer climate and be a threat to lives and properties. In this study, we conduct downscaling experiments of an extreme rain-producing typhoon, Typhoon Lionrock (2016), to assess the impacts of climate change on resulting hazards by assuming pseudo global warming (PGW) conditions. The downscaled precipitations over the landfall region in the present climate condition agree well with the Radar-Automated Meteorological Data Acquisition System (Radar-AMeDAS) observations. A typhoon track in the future climate similar to that in the present climate is successfully reproduced, with a stronger wind speed (by ∼ 20 knots) and lower central pressure (by ∼ 20 hPa) under the PGW condition. The changes in precipitation amounts associated with the typhoon under PGW condition are analyzed over seven individual prefectures in the northern part of Japan. The typhoon in the warming climate produces more precipitation over all prefectures. Iwate, Aomori, Akita, Miyagi, and Hokkaido are projected to have relatively more precipitation associated with the typhoon in the warming climate. The overall analysis suggests that Typhoon Lionrock under PGW may increase the risk of flooding, damages to infrastructures, and lives staying along the typhoon track.

 This study aims to improve the understanding of the differences in surface air temperature data between observations and reanalysis since the beginning of the 20th century and addresses the reanalysis data error. The anomaly correlation, standard deviation, and linear trend of temperature during 1909-2010 in eastern China were analyzed based on the homogenized observation data obtained from 16 stations and two sets of 20th century monthly mean surface air temperature reanalysis data (20CR and ERA20C). The results show that the inter-annual and decadal variabilities were consistent between reanalysis and observations in eastern China after 1979. The reanalysis data exhibited a large fluctuation during the 1960s. The average 20CR temperature was lower than the observations during 1920-1950. The inter-annual and decadal variability for winter and spring were consistent with the observations. The correlation and standard deviation ratio between the reanalysis and observations demonstrated a high consistency of their inter-annual variability and dispersion. The ERA20C data were generally closer to the observations than the 20CR data for the period 1979-2010. The linear trends of surface air temperature showed clear warming in both reanalysis datasets and the observations but the reanalysis trends were significantly smaller than the observational trends for annual mean temperature and most of the seasonal mean temperatures after the 1950s. Overall, ERA20C was generally closer to the observational temperatures than 20CR during 1909-2010, but this consistency does not necessarily indicate ERA20C's suitability for climate change research because of the systematic bias referenced to the observational data.

 There is an increasing need for accurate winter agrometeorological forecasts, which is facilitated by a better understanding of the evolution process of nighttime air-temperature distribution. However, studies on how air-temperature distributions evolve in mesoscale plains have been limited. To clarify how the low temperatures in winter nights form, we analyzed the effects of topography and boundary-layer wind on the temperature distribution of the Tokachi region for a winter night using numerical simulations by the Japan Meteorological Agency Nonhydrostatic Model (JMA-NHM) with horizontal grid spacing of 2 and 5 km. We also analyzed vertical profiles of boundary-layer atmospheric conditions.

 The results show that although boundary-layer wind is expected to affect the temperature distribution over the entire Tokachi region, the effects were generally confined to the northwestern part. Widespread effects over the Tokachi region were found only under strong wind conditions. We found that the mountain pass in the northwestern part of the Tokachi region is an important wind path, and the downslope winds as well as the sensible heat transfer by turbulent mixing in the boundary layer also was important in the evolution of the air-temperature distribution. On the night we considered, a moderate boundary-layer wind was maintained throughout the night, but the surface wind speed decreased from the northern and southern parts of the Tokachi region; this can be attributed to the development of an inversion layer. A drainage flow was observed to originate from the southern part of the Tokachi Plain, reaching the central part of the Tokachi region in the night. We find that radiative cooling and sensible heat transfer by turbulent mixing in the surface layer do not adequately explain the temporal change in observed surface air temperatures. The development of an inversion layer and katabatic drainage flow drastically change the temperature distribution, despite moderately strong wind conditions in the boundary layer.

 A simple conceptual model of the resonant interaction in a typhoon-like vortex between vortex Rossby waves (VRWs) and gravity waves (GWs) that are caused by the VRWs is presented. It is well-known that the VRWs in the central region of the vortex can grow by the interaction with the GWs in the outer region, but a simple conceptual model for their interaction has not yet been proposed. The proposed conceptual model is based on the buoyancy-vorticity formulation (BV-thinking), and is different from that for the barotropic and baroclinic instabilities based on PV interactions (PV-thinking).

 We consider disturbances of the first baroclinic mode on a basic barotropic vortex. The disturbance vertical vorticity ζ of the VRW in the central region has a large amplitude on the upper and lower levels, whereas the disturbance buoyancy b and radial vorticity η of the GW have a large amplitude on the middle level. The central VRW propagates (relative to the fluid) anticyclonically, but moves cyclonically because of the strong cyclonic advection by the vortex. The outer cyclonically-propagating GW is also weakly advected cyclonically by the vortex. As a result, the counter-propagating VRW and GW (satisfying Rayleigh's condition) may be phase-locked with each other (satisfying Fjørtoft's condition).

 By the counter-propagation and phase-lock, the circulation around ζ of the VRW enhances b of the GW, which in turn enhances η. Simultaneously, the circulation around η of the GW enhances ζ of the VRW. As a result, the VRW and GW grow simultaneously.

 We analytically show the possibility of the resonant interaction, and numerically obtain the growing solution in the system linearized about the basic vortex.

 Future changes in precipitation extremes and role of tropical cyclones are investigated through a large ensemble experiment, considering 6,000 years for the present and 5,400 years under +4 K warming, using a 60-km mesh Meteorological Research Institute atmospheric general circulation model version 3.2. As in the previous findings of the authors, the annual maximum 1-day precipitation total (Rx1d) is projected to increase in the warmer world in the future almost globally, except in the western North Pacific where a projected decrease of tropical cyclone frequency results in only small change or even reduction of Rx1d. Furthermore, a large ensemble size enables us to investigate the changes in the tails of the Rx1d distribution. It is found that 90- and 99-percentile values of the Rx1d associated with tropical cyclones will increase in a region extending from Hawaii to the south of Japan. In this region, the interannual variability of the Rx1d associated with tropical cyclones is also projected to increase, implying an increasing risk of rare heavier rainfall events because of global warming.

 Using brightness temperature data from passive microwave satellite imagery, this study examines tropical cyclones (TCs) with concentric eyewall (CE) in the western North Pacific between 1997 and 2011. The identified CEs are divided into two types according to the characteristics of the eyewall replacement cycle (ERC) in the microwave imagery: a CE with a typical ERC (T-ERC) and a CE without an ERC (N-ERC). It is indicated that 88% T-ERCs reach peak intensity near (0.2 h after on average) CE formation, whereas 90% N-ERCs reach peak intensity prior to (22.0 h on average) CE formation.

 In general, N-ERCs tend to occur when there are strong interactions between the environment and the CE, whereas T-ERCs occur in a relatively quiet environment. The three-dimensional conceptual models of the environmental configurations for both CE types are proposed. Specifically, N-ERCs are accompanied by stronger southwesterly and southeasterly inflows, active low-level trough, and stronger subtropical high (SH) and South Asia high (SAH), compared with T-ERCs. For N-ERCs, the stronger inflows may bring in a large amount of moisture, and the active low-level trough may result in a large vertical wind shear (VWS). The stronger SH and SAH may contribute to changes in the intensity and direction of the VWS for N-ERCs, and hence trigger the development of local convection in the outer eyewall. The asymmetries in the convection of the outer eyewall may weaken the ability to cut off the radial inflow to the inner eyewall. Consequently, N-ERCs fail to finish the ERC and weaken rapidly in intensity, even though the moisture remains sufficient after CE formation.

 A set of atmospheric dispersion-deposition model (ADM) integrations were conducted with a hypothetical emission of radioactive materials consisting of ¹³⁷Cs, ¹³¹I, and ¹³⁴Cs from the Tomari Nuclear Power Plant (NPP) in Hokkaido, Japan, which is a snow climate site. Each integration was driven by Japan Meteorological Agency's (JMA's) mesoscale model (MSM) analysis data with 5-km horizontal resolution. The initial conditions were those on each day from January 2010 to December 2016, and the integration period was at most four days. The target was the area within 30 km of the plant. Extending a unit-mass emission concept, the measure of the relative risk is the probability of exceeding the threshold of the maximum effective dose rate based only on exposure from groundshine. Considering that the measure increased monotonically with the ratio of the total emission amount to the threshold, we evaluated the probabilistic risk with its median. The results suggested that the risk was higher in the eastern part of the target area owing to the prevailing westerly flow. The frequent snowfall in winter drags radioactive materials down in the target region, even under an active turbulent condition with strong vertical shear. The composite analysis of wind direction averaged over the target area revealed that the risk was high in the leeside but that mountains effectively blocked the inflow of the radioactive materials. The results were insensitive to wet deposition parameterization. The risk was reduced when we replaced the emission altitude with a higher one than the standard setting. The snow shielding effect was negligible on the short-term radioactivity just after the emission but was substantial on the seasonal change in radioactivity.

 Typhoon Nepartak was a category 5 tropical cyclone in 2016 that resulted in significant societal impacts. The tropical cyclone went through a rapid intensification (RI), with an increase of maximum wind speed of 51 m s⁻¹ and a decrease of minimum sea level pressure of 74 hPa in 42 h. The real-time forecast from the U.S. Navy Coupled Ocean/Atmosphere Mesoscale Prediction System – Tropical Cyclone (COAMPS-TC), initialized at 1200 UTC 3 July, predicted the track and intensity reasonably well for Super Typhoon Nepartak and captured the storm's RI process. Positive interactions among primary and secondary circulations, surface enthalpy fluxes, and mid-level convective heating are demonstrated to be critical for the RI. The storm structure variations seen from the simulated satellite infrared brightness temperature during RI bear considerable resemblance to the Himawari-8 satellite images, although the forecast inner core is too broad, presumably due to the relatively coarse resolution (5 km) used for the real-time forecasts at the time.

 This study found that regional snowfall distributions in a Japan-Sea side area of Japan are controlled by intraseasonal jet variability, particularly the 10-day-timescale quasi-stationary Rossby waves across the Eurasian continent and the atmospheric blocking over the East Asian region. This study was mainly focused on the Niigata area, which is representative of heavy snowfall areas in Japan. Based on previous studies, three types of dominant snowfall distributions were defined: (1) the plain (P) type, which is characterized by heavy snowfall events predominant in coastal regions of the Niigata area; (2) the mountain (M) type, which occurs in the mountainous regions; and (3) the PM type, which occurs across the whole Niigata area.

 Our results revealed that all distribution types were related to the southward shift of the westerly jet over Japan associated with an intensified trough, i.e., cyclonic anomalies, originating from quasi-stationary Rossby waves along westerly jets over Eurasia (Eurasian jets). The cyclonic anomalies were found to be also related to blocking cyclones because the frequency of blocking events considerably increased in the East Siberian region. The mechanisms leading to trough intensification were different among the events of the three snowfall types. The formation of Siberian blocking with relatively different positions and different paths of quasi-stationary Rossby wave packet propagation along Eurasian jets was evident in the distribution types. Therefore, local-scale snowfall distributions in the Japan-Sea side area are determined by anomalous large-scale circulations, which can be evidently distinguished in the global reanalysis data.

 In this study, we investigated the impact of urbanization on surface air temperatures and the urban heat island (UHI) for Sendai City. We estimated the impact of urbanization during the 150-year period by comparing the 1850s to the 2000s case. We used the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution and three land-use datasets: one for potential natural vegetation (PNV) data, and the other two for realistic land-use data (the 1850s and 2000s). First, the results from the control simulation (2000s land-use case) were verified against observations. The results show that the WRF model reasonably reproduced the diurnal variation of the observed surface air temperatures in the 2000s land-use case at six stations of the Miyagi prefecture. The model mean biases ranged from −0.29 to −1.18°C in August (10-year average) and from −0.44 to −1.50°C in February (10-year average). Second, the impact of urbanization on the surface air temperature distribution in and around Sendai City was evaluated. For the 1850s land-use case, the very small urban area of Sendai City led to a negligible UHI. Note that this case yields nearly the same surface air temperatures as experiments using PNV. If we compare the simulated monthly mean surface air temperatures in the central part of Sendai City between the 1850s and 2000s land-use cases, we find that the monthly mean temperature for February in the 2000s is 1.40°C higher than that in the 1850s, whereas that for August is 1.30°C. Similarly, we find a considerable nocturnal (1800-0500 JST) average surface air temperature increase of 2.20°C in February and 2.00°C in August.

 This study aims to investigate the potential impact of surface observations with a high spatial and temporal density on a local heavy rainstorm prediction. A series of Observing System Simulation Experiments (OSSEs) are performed using the Local Ensemble Transform Kalman Filter with the Japan Meteorological Agency non-hydrostatic model at 1-km resolution and with 1-minute update cycles. For the nature run of the OSSEs, a 100-m resolution simulation is performed for the heavy rainstorm case that caused five fatalities in Kobe, Japan on July 28, 2008. Synthetic radar observation data, both reflectivity and Doppler velocity, are generated at 1-km resolution every minute from the 100-m resolution nature run within a 60-km range, simulating the phased array weather radar (PAWR) at Osaka University. The control experiment assimilates only the radar data, and two sensitivity experiments are performed to investigate the impact of additional surface observations obtained every minute at 8 and 167 stations in Kobe. The results show that the dense and frequent surface observations have a significant positive impact on the analyses and forecasts of the local heavy rainstorm, although the number of assimilated observations is three orders of magnitude less than the PAWR data. Equivalent potential temperature and convergence at the low levels are improved, contributing to intensified convective cells and local heavy rainfalls.

 An extreme cold surge event caused record-breaking low temperatures in East Asia during 20-25 January 2016. The planetary- and synoptic-scale feature of the event is investigated quantitatively using the isentropic cold air mass analysis with a threshold potential temperature of 280 K. Because cold air mass is an adiabatically conservative quantity, it is suitable for tracing and examining the extreme cold surges. We further introduced a metric named mean wind of cold air mass, which divides the factor of cold air mass evolution into convergence and advection parts. The new metric allowed us to trace the evolution of the cold air mass with dynamic consistency for a period of more than a week.

 A thick cold air mass built up over southern Sakha by a convergent cold air mass flow during 16-18 January. It migrated westward and reached Lake Baikal. On 20 January, an intense Siberian High developed, with an eastward-moving mid-upper-level ridge, producing a strong surface pressure gradient over the coastal regions of the Asian continent. This ridge and a cutoff low to the adjacent east formed a northerly flow in the mid-upper troposphere. The resultant southward flow through the troposphere blew the cold air mass over 480 hPa in thickness to the subtropical region of East Asia, causing strong cold surges there on 24 and 25 January.

 The abnormality of the event is further quantified using extreme value theory. The cold air mass gradually became rare along the path of the cold air mass from Lake Baikal to eastern China, which experienced as thick a cold air mass as once in 200 years. The cold air mass itself shows little change in thickness. Therefore, the migration of a cold air mass over 540 hPa in thickness from northern Siberia is the major cause of this cold surge extreme.

 The boreal summer intra-seasonal oscillation (BSISO) is the predominant sub-seasonal variability over the East Asia (EA) and western North Pacific (WNP) region and critical for seasonal forecast of the EA summer monsoon. This study examines the theoretically estimated predictability and practical prediction skill of the EAWNP BSISO in the Beijing Climate Center Climate System Model version 2 (BCC_CSM2.0), which is one of the participants in the Sub-seasonal to Seasonal Prediction Project. The results from the uninitialized free run of BCC_CSM2.0 show that the model reasonably simulates EAWNP BSISO in terms of its variance, propagation, and structure. Measured by the bivariate correlation (> 0.5) and root mean square error (< √2) between the predicted and observed real-time BSISO index, the prediction skill and predictability of EAWNP BSISO are about 14 and 24-28 days, respectively. The initial/target strong BSISO cases have a relatively higher prediction skill than the initial/target weak BSISO cases. For the theoretically estimated BSISO predictability, a similar dependence on target amplitude occurs in the model, while no significant dependency on initial amplitude is found. Moreover, diagnosis of the phase dependence reveals that BSISO is less skillful for the prediction starting from active or active-to-break transition phases of WNP rainfall, whereas it is more predictable when prediction is targeting extreme dry/wet phases of WNP rainfall. Finally, systematic errors are found in BCC_CSM2.0 such as the underestimation of BSISO amplitude and the faster phase speed.