By using the Weather Research and Forecasting (denoted as WRF) model driven by two super high resolution global models, High Resolution Atmospheric Model (denoted as HiRAM) and Meteorological Research Institute Atmospheric General Circulation Model (denoted as MRI), this study investigates the dynamical downscaling simulation and projection of extreme precipitation activities (including intensity and frequency) in Taiwan during the Mei-Yu seasons (May and June). The analyses focus on two time period simulations: the present-day (1979-2003, historical run) and the future (2075-2099, RCP8.5 scenario). For the present-day simulation, our results show that the bias of HiRAM and MRI in simulating the extreme precipitation activities over Taiwan can be reduced after dynamical downscaling using the WRF model. For the future projections, both the dynamical downscaling models (i.e., HiRAM-WRF and MRI-WRF) project that extreme precipitation will become more frequent and more intense over western Taiwan, but less frequent and less intense over eastern Taiwan. The east-west contrast in the projected changes in extreme precipitation in Taiwan are found to be a local response to the enhancement of southwesterly monsoonal flow over the coastal regions of South China, which leads to an increase in water vapor convergence over the windward side (i.e., western Taiwan) and a decrease in water vapor convergence over the leeward side (i.e., eastern Taiwan). Further examinations of the significance of the projected changes in extreme precipitation that affect the agriculture regions of Taiwan show that the southwestern agriculture regions will be affected by extreme precipitation events more frequently and more intensely than the other subregions. This finding highlights the importance of examining regional differences in the projected changes in extreme precipitation over the complex terrain of East Asia.

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 Recent climate warming and rapid urban development in the Pearl River Delta (PRD) of China exerted great impacts on the reference evapotranspiration (RET), which in turn affects the management of water resources and the quality of urban environment. The objectives of this study are to examine (i) the temporal variability of RET in PRD, and (ii) the underlying causes responsible for the temporal variation in RET across space inside the PRD. The results indicate that: (1) The RET in PRD had an overall increasing trend caused by the increase of construction land during 1960-2016. (2) The increase of surface albedo caused by land cover conversion from woodland to grassland played an important role in the noticeable decline of RET in Guangzhou and Zengcheng. (3) The dominant factors triggering RET variation varied across space in PRD. In detail, the decline of sunshine duration decreasing Rn, and the decline of wind speed weakening energy exchange, were the dominant factors in decreasing RET in Guangzhou and Zengcheng. In contrast, daily maximum temperature, daily minimum temperature and relative humidity, which were the factors causing the increase of vapor pressure deficit, were responsible for RET increase in Taishan, Zhongshan and Shenzhen. Overall, our results indicated that RET in PRD exhibited strong spatial heterogeneity due to differences in land use change and climatic conditions. Therefore, the improvement of water resources management and urban environment in PRD should consider the spatial variation and underlying forces of RET changes.

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 The Okazaki heavy rainfall event, which occurred at midnight on 28 August 2008 around Okazaki city in Japan, was produced by a quasi-stationary band-shaped precipitation system. This precipitation system remained quasi-stationary for approximately 5 hours over Okazaki city and the surrounding area, and produced prolonged intense precipitation. This study presents sensitivity numerical experiments to examine the impact of surrounding mountainous topography on the quasi-stationarity of the precipitation system using the Weather Research and Forecasting (WRF) model with 500 m horizontal resolution. In an experiment without the mountains to the east of Okazaki city, the quasi-stationary precipitation system was not reproduced. On the other hand, experiments including eastside mountains produced a low-level convergence in south of Okazaki city, resulting in the quasi-stationary precipitation system and prolonged precipitation as observed near Okazaki city. The convergence was formed by sustained easterlies together with northerly winds blowing in west of Okazaki city. The easterlies were maintained by westward shift of southeasterly inflow from the Pacific Ocean due to the enhanced pressure gradient on the upstream side of the eastside mountains in the low-level atmosphere with low Froude numbers (Fr < 0.5). The easterlies also steadily supplied warm and moist air to the quasi-stationary system, leading to the prolonged intense precipitation observed in the Okazaki heavy rainfall event.

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 We investigated extremely heavy precipitation that occurred around the Kinugawa River, Japan, in September 2015, and the probability of extreme precipitation occurrence, using data from a large ensemble forecast more than 1,000 members that were dynamically downscaled to 1.6 km horizontal grid spacing. The observed event was statistically rare among simulated cases and 3-day accumulated precipitation around the target area was equivalent to the 95th percentile among all simulated ensemble members. Our results show that this extreme precipitation event occurred under specific conditions: two coexisting typhoons at close proximity that produces a high atmospheric instability, and water vapor transport from the Pacific Ocean. We also assessed the probability of extreme precipitation in mountainous areas other than the Kinugawa River case. Heavy precipitation also occurred southwest of the Kinugawa River region due to two typhoons, similar to the Kinugawa River case. The tracks of these typhoons shifted marginally; however, there was a difference in the water vapor supplied to the area, causing heavy precipitation. The large-ensemble downscaled data used in this study hence enable us to evaluate the occurrence probability of a torrential rainfall event that was rarely observed, which may contribute to updating a disaster mitigating plan for possible similar disasters in future.

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 This study describes the spatial distribution of the melting layer (ML) in a winter stratiform precipitation system associated with a south-coast cyclone (SCC) on 30 January 2015 over the Kanto Plain, Japan, using an X-band polarimetric radar at Funabashi operated by the Ministry of Land, Infrastructure, Transport and Tourism. The detailed horizontal distribution of surface precipitation types based on Weather Reports from citizens provided by Weathernews Inc. (WNI reports) was also investigated in relation to the ML structure.

 Surface precipitation in the Kanto Region started with rain and then changed to snow around Tokyo. According to WNI reports, a large dry snow area had formed around Tokyo by 0900 Japan Standard Time (UTC + 9 hours), while surface rainfall continued in the southeast of the Kanto Plain (most part of Chiba and southern part of Kanagawa). A boundary line between the surface dry snow and rain areas became clear in the eastern part of Kanagawa and the northwestern part of Chiba. This boundary then gradually moved inland.

 Polarimetric ML signatures suggesting the presence of melting snow were continuously observed above the rainfall area in the southeast of the Kanto Plain. The polarimetric ML signatures, on the other hand, approached the ground near the surface dry snow-rain boundary while the surface snowfall was predominant around Tokyo. During the mature snowfall period around Tokyo, the ML vertically extended below 1 km above sea level near the surface dry snow-rain boundary, which indicates the presence of a local horizontal temperature gradient and a surrounding ~0°C near-isothermal layer. It is suggested that this vertically extending ML coincided with the edge of a cold air mass in the lower atmosphere, which often forms during snowfall associated with SCCs in the Kanto Region.

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 Aircrafts making landing and takeoff at Narita International Airport (Narita Airport) in Japan report frequently low-level wind shear (LLWS), a local variation of wind vector, with turbulence when the prevailing wind is southwesterly, which is crosswind to the runway direction. On 20 June 2012, an arrival aircraft at Narita Airport encountered a LLWS, which consisted of a sudden change of the wind vector from head wind component of 5 knots (2.6 m s^-1) to tail wind component of 10 knots (5.1 m s^-1), just before the touchdown and made a hard landing. None of cumulonimbus clouds, a front or a wind shear line was observed around the airport during her approaching and landing. Analyses of the data measured by the landing aircraft and the observations by the Doppler lidar at the airport revealed that the LLWS was caused by horizontal roll vortices, which developed in the atmospheric boundary layer (ABL) over the Shimofusa Tableland around the airport. The horizontal roll vortices had their axes nearly parallel to the mean wind direction, and their horizontal and vertical scales were approximately 800 m and 500 m, respectively. The present study demonstrated that existence of the horizontal roll vortices causing LLWS can be effectively detected by a single-Doppler lidar which utilizes backscattering from aerosols.

 Although the LLWS associated with the horizontal roll vortices has smaller magnitude than those caused by a microburst, a gust front and a front, a landing aircraft just before touchdown encounters the horizontal roll vortices with much higher probability than the other phenomena mentioned here since the horizontal roll vortices occurs at a horizontal spacing of approximately 800 m over a wide area during daytime of a clear day.

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 Previous modeling studies have indicated that the Oyashio front in the subarctic Pacific Ocean significantly affects the atmosphere on meso- to basin-scales, but there were no in situ observations that captured oceanic imprints on the atmosphere in this region as far as the authors know. We present in situ evidence of atmospheric responses to the Oyashio front by using a total of 103 radiosondes launched around the Oyashio front in April 2013 with continuous surface meteorology and ceilometer observations. Composite profiles showed that the low-level atmosphere below 1000 m was statically stable on the cold side of the Oyashio front, but unstable and mixed on the warm side. In the atmosphere on the warm side, relative humidity dropped sharply at an altitude of around 1000 m, an indication that the mean cloud top was at this altitude. While the frequency of cloud base height peaked at 50–100 m in the cold areas, cloud bases were distributed at higher altitudes in the warm areas. These differences in the atmospheric boundary layer and cloud base heights across the front were clearer under conditions of southerly winds compared with those of northerly winds. Above a local sea surface temperature minimum with a width of approximately 400 km, where the ocean mixed layer depth is known to reach a local maximum, a large horizontal air temperature gradient was observed below an altitude of 1000 m. This horizontal gradient corresponded to a sea level pressure (SLP) anomaly of 1.2 hPa, comparable to observations of SLP anomalies in the Kuroshio Extension region. Furthermore, we found that narrow warm ocean streamers moistened the overlying atmosphere, affecting downward longwave radiation. Over the wide streamer located between 146.4°E and 147.0°E on 5 April, near-surface atmospheric properties were largely different between over the western half and the eastern half.

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 This study examined the statistical characteristics of tropical cyclones (TCs) for which the cyclogenesis (TCG) process was modulated by upper tropospheric cold lows (UCLs) over the western North Pacific (WNP) during the 38 years from 1979 to 2016. Among the 965 TCs, 90 (9 %, 2.4 per year) were defined as having TCG influenced by UCLs in the northwest quadrant of the TC region (UL-TCs). Most UL-TCs occurred in the summer, with large variability in the annual occurrence rate of UL-TCs during June to October, ranging from 0 to approximately 30 %. The annual variation was related to the activity of the Tibetan high and the summer temperature anomaly over Japan. The extremely hot summer of 2016 was partly enhanced by the intense Tibetan high, when 4 UL-TCs also occurred. The average location of UL-TCs at the time of TCG and tropical storm formation (TSF) was significantly farther to the north than the average location of TCs not formed under the influence of UCL (N-UL-TCs). Many UL-TCs occurred in lower tropospheric environments associated with the shear line or confluence regions. The UL-TCs tended to move northward, and the occurrence rate of UL-TCs that made landfall in Japan was approximately double that of other countries. The atmospheric environmental parameters around UL-TCs at the time of TCG were more favorable for the development of TCs than those around N-UL-TCs. In contrast, the atmospheric and oceanic environmental parameters around UL-TCs at the time of TSF were less favorable for the development of TSs, such that UL-TCs tended to remain at weak in intensity.

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 Model performance of a regional-scale meteorology – chemistry model (NHM-Chem) has been evaluated for the consistent predictions of the chemical, physical, and optical properties of aerosols. These properties are essentially important for the accurate assessment of air quality and health hazards, contamination of land and ocean ecosystems, and regional climate changes due to aerosol-cloud-radiation interaction processes. Currently, three optional methods are available: the 5-category non-equilibrium, 3-category non-equilibrium, and bulk equilibrium methods. These three methods are suitable for the predictions of regional climate, air quality, and operational forecasts, respectively. In this paper, the simulated aerosol chemical, physical, and optical properties and their consistency were evaluated by using various observation data in East Asia. The simulated mass, size, and deposition of SO₄^2- and NH₄⁺ agreed well with the observations, whereas those of NO₃^-, sea-salt, and dust needed improvement. The simulated surface mass concentration (PM₁₀ and PM_2.5) and spherical extinction coefficient agreed well with the observations. The simulated aerosol optical thickness and dust extinction coefficient were significantly underestimated.

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 In August 2016, a monsoon gyre persisted over the western North Pacific and was associated with the genesis of multiple devastating tropical cyclones. A series of hindcast simulations was performed using the nonhydrostatic icosahedral atmospheric model (NICAM) to reproduce the temporal evolution of this monsoon gyre. The simulations initiated at dates during the mature stage of the monsoon gyre successfully reproduced its termination and the subsequent intensification of the Bonin high, while the simulations initiated before the formation and during the developing stage of the gyre failed to reproduce subsequent gyre evolution even at a short lead time. These experiments further suggest a possibility that the development of the Bonin high is related to the termination of the monsoon gyre. High predictability of the termination is likely due to the predictable mid-latitudinal signals that intensify the Bonin high.

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 Global warming experiments using three different 60 km-mesh atmospheric global circulation models are studied to characterize ensemble mean future changes in monthly East Asian precipitation for June to August. During the summer, wetting and drying effects due to changes in mean vertical motion play a key role in future precipitation changes, as does the “wet-get-wetter” effect due to increased moisture. The former processes are related adiabatically to the projected modification of 500 hPa horizontal atmospheric circulation, which is characterized by two cyclonic circulation anomalies extending over the eastern Eurasian Continent (C1) and the western North Pacific Ocean (C2) for each month.

 Over Japan, the western edge of C2 shifts from a region south of the Japanese Islands to northern Japan during June–August, representing a delayed northward movement or southward shift of the westerly jet over the western North Pacific in the future compared with the present-day climatology. Most regions of Japan lie within the northeasterly wind and associated downward motion zones of C2, leading to significant uncertainties in the future precipitation over Japan by the offset against the “wet-get-wetter” effect and possibly even a future decrease in precipitation. A wetter future climate is anticipated under weak subsidence or the upward vertical motion zone of C2, such as western Japan in August away from C2, and the Southwest Islands of Japan in June in the C2 southwesterly wind zone.

 Over the eastern Eurasian Continent, C1 is distributed mainly over northeastern China in June, central and southern China in July and August respectively. During these months, most of the eastern regions are located within the southwesterly-to-southeasterly wind zone of C1, indicating wet future conditions due to enhanced upward motion. This tendency drives a further increase in precipitation in future wetter East Asian climate via the “wet-get-wetter” effect and the increased evaporation.

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 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 participants in the Sub-seasonal to Seasonal Prediction Project. Results from the uninitialized free run of BCC_CSM2.0 show that the model reasonably simulates the 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 compared to the initial/target weak BSISO cases. For the theoretically estimated BSISO predictability, 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.

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“Kamae, Y., W. Mei, and S.-P. Xie, 2017: Climatological relationship between warm season atmospheric rivers and heavy rainfall over East Asia”. J. Meteor. Soc. Japan, 95, 411-431, https://doi.org/10.2151/jmsj.2017-027

Due to authors’ mistakes, corrections are needed for Kamae et al. (2017).

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 A set of atmospheric dispersion-deposition model integrations was conducted with a hypothetical emission of radioactive materials consisting of ¹³⁷Cs, ¹³¹I, and ¹³⁴Cs from the Tomari Nuclear Power Plant in Hokkaido, Japan, which is a snow climate site. Each integration was driven by Japan Meteorological Agency’s meso-scale model 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 4 days. The target was the area within 30 km of the plant. Extending a unit-mass emission concept, the measure of 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 due to the prevailing westerly. 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 for 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 a wet deposition parameterisation. 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.

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 An extreme cold surge event caused record-breaking low temperatures in the 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 the threshold potential temperature of 280 K. Because cold air mass is adiabatically conservative quantity, it is suitable for tracing and examining the extreme cold surges. We further introduce 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 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.

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 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 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 south-ward 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 the 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 were 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.

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 This study investigates the impact of urbanization on surface air temperature and the urban heat island (UHI) for Sendai City. We estimate the impacts of the urbaniza-tion during the 150-year period by comparing the 1850s to the 2000s case. We use the Weather Research and Forecasting (WRF) model with 1-km horizontal resolution and three land-use datasets, one for potential natural vegetation (PNV) data, the other two for realistic land-use data (the 1850s and 2000s). Results from the control simulation (2000s land-use case) are firstly verified against observations. The results show that the WRF model reasonably well reproduces the diurnal variation of the observed surface air temperatures in the 2000s land-use case at six stations in Miyagi prefecture. The model mean biases range 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). Secondly, the impacts of urbanization on the surface air temperature distribution in and around Sendai City are evaluated. In the 1850s land-use case, the very small urban area of Sendai City results in a negligible UHI. This case gives nearly the same surface air temperatures as experiments using the PNV. Comparing 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 considerable nocturnal (1800–0500 JST) average surface air temperature increases of 2.20°C in February and 2.00°C in August.

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 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 5 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.

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 Typhoon Nepartak was a category 5 tropical cyclone of 2016 and had significant societal impacts. It 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 Coupled Ocean/Atmosphere Mesoscale Prediction System – Tropical Cyclone (COAMPS-TC), starting from 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.

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 A large amount of snowfall caused by snow clouds over the Sea of Japan sometimes severely affects 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 long been intensively studied from the perspective of disaster prediction and prevention. Most studies have analyzed data acquired by aerological, meteorological satellite, and radar observations, or have 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 remained unvalidated. Here, knowledge gained from instrumented aircraft observations made from the middle 1990s through the early 2000s is reviewed, in particular with regard to 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.

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 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) in order 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 7 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.

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 This study aimed to improve understanding of the differences in surface air temperature data between observations and reanalysis since the beginning of the 20th century and to address the reanalysis data error. The anomaly correlation, standard deviation, and linear trend of temperature during 1909–2010 in eastern China was analyzed based on homogenized observation data 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 variability 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.

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 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 a moderately strong wind condition in the boundary layer.

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 A simple conceptual model of the resonant interaction in a typhoon-like vortex between vortex Rossby waves (VRWs) and gravity waves (GWs), which 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. While, 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) anticy-clonically, but moves cyclonically because of the strong cyclonic advection by the vortex. The outer cyclonically propagating GW is weakly advected also 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. At the same time, 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.

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 Future changes in precipitation extremes and role of tropical cyclones are investigated by a large ensemble experiment, 6,000 years for the present and 5,400 years under +4 K warming, with a 60-km mesh atmospheric general circulation model (MRI-AGCM3.2). As in the previous findings by the authors, the annual maximum 1-day precipitation total (Rx1d) is projected to increase in the future warmer world almost all over the world, 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, the large ensemble size enables us to investigate changes in the tails of the Rx1d distribution. It is found that 90- and 99-percentile values of Rx1d associated with tropical cyclones will increase in a region extending from Hawaii to the south of Japan. In this region, interannual variability of Rx1d associated with tropical cyclones is also projected to increase, implying an increasing risk of rare heavier rainfall events by global warming.

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 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). Furthermore, N-ERCs can be classified into four categories. 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, while 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 large amount of moisture, and the active low-level trough may result in large vertical wind shear (VWS). The stronger SH and SAH may contribute to changes in 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.

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 In this study, the characteristics of wind gusts in Japan in the period of 2002–2017 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 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 over 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.

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 The Southeasterly Gale in Xinjiang, China is a severe local gale weather phenomenon which occasionally happens near the northwest opening of 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 gust speed that reached 30 m s^-1. The gale lasted for over 24 hours making it the strongest wind in latest 20 years. Through observations and numerical simulation, this study represents the formation of this southeasterly gale incident. The large-scale topographic forcing of the Tianshan Mountains led to the intensification of pressure gradient across the Tianshan Grand Canyon, and therefore an advantageous condition for the generation of the southeasterly gale had been provided. When air currents travelled through the canyon, a critical layer with zero wind velocity was established, and nonlinear process was activated by orographic forcing. It is suggested that air current sank on the northwest opening of the canyon due to unstable stratification and thus strengthened the gale.

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