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 the 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 to 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 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 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 and over 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.
The 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 five-category non-equilibrium method, the three-category non-equilibrium method, and the bulk equilibrium method. 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 using various observation data in East Asia. The simulated mass, size, and deposition of SO42− and NH4+ agreed well with the observations, whereas those of NO3−, sea salt, and dust needed improvement. The simulated surface mass concentration (PM10 and PM2.5) and spherical extinction coefficient agreed well with the observations. The simulated aerosol optical thickness (AOT) and dust extinction coefficient were significantly underestimated.
In this study, we describe 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 (MLIT). 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 formed around Tokyo by 0900 Japan Standard Time (JST; UTC + 9 hours), whereas surface rainfall continued in the southeast of the Kanto Plain (most of Chiba and the 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 (ASL) 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.
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 of 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 the 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 produced a high atmospheric instability, and water vapor transported 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 enabled 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.
Aircraft landing and taking off at Narita International Airport in Japan frequently report low-level wind shear (LLWS), which is a local variation in the wind vector, and turbulence when the prevailing wind is southwesterly, which is crosswind to the runway. On 20 June 2012, just before touchdown, an arriving aircraft at this airport encountered LLWS that consisted of a sudden change in the wind vector from a headwind component of 5 knots (2.6 m s−1) to a tailwind component of 10 knots (5.1 m s−1). This caused a rather hard landing. As the aircraft approached, none of cumulonimbus clouds, fronts, or wind shear lines were observed around the airport. Further analysis of the data measured by the landing aircraft and observations made by the Doppler lidar revealed that the LLWS was caused by horizontal roll vortices that developed in the atmospheric boundary layer (ABL) over the Shimofusa tableland surrounding the airport. The axes of these horizontal roll vortices were nearly parallel to the mean wind direction, while their horizontal and vertical scales were approximately 800 m and 500 m, respectively.
In our present study, we demonstrate that the existence of such horizontal roll vortices that cause LLWS can be effectively detected by a single Doppler lidar that utilizes backscattering from aerosols.
Although the LLWS associated with horizontal roll vortices has a smaller magnitude than those caused by a microburst, gust front, or front, landing aircraft often encounter these horizontal roll vortices just before touchdown with a much higher probability than other phenomena since horizontal roll vortices occur at a horizontal spacing of approximately 800 m over a wide area during the daytime hours of a clear day.
Previous modeling studies have indicated that the Oyashio front in the subarctic Pacific Ocean significantly affects the atmosphere on meso- to basin scales; however, 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, the 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, the near-surface atmospheric properties were largely different over the western half and the eastern half.
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 during the 38 years from 1979 to 2016. Among the 965 TCs, 90 TCs (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 TCs, such that UL-TCs tended to remain at weak intensity.
The Okazaki heavy rainfall event, which occurred at midnight on 28 August 2008 around Okazaki city, Japan, was produced by a quasi-stationary, band-shaped precipitation system. It remained quasi-stationary for approximately five hours over Okazaki city and the surrounding area, producing prolonged, heavy precipitation. This study presents sensitivity numerical experiments to examine the surrounding mountainous topography's effect on 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 the eastern mountains produced a low-level convergence south of Okazaki city, resulting in a quasi-stationary precipitation system with prolonged precipitation, as observed near Okazaki city. The convergence was formed by sustained easterly and northerly winds blowing in western Okazaki city. The easterlies were maintained by a westward shift of southeasterly inflow from the Pacific Ocean due to the enhanced pressure gradient on the upstream side of the eastern 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 prolonged heavy precipitation.
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 the urban environment. The objectives of this study are to examine (i) the temporal variability of the RET in the PRD and (ii) the underlying causes responsible for the temporal variation in the RET across space inside the PRD. The results indicate the following. (1) The RET in the 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 the RET in Guangzhou and Zengcheng. (3) The dominant factors triggering RET variation varied across space in the PRD. In detail, the decline of sunshine duration (SD) decreasing Rn and the decline of wind speed (WS) weakening energy exchange were the dominant factors in decreasing RET in Guangzhou and Zengcheng. In contrast, the daily maximum temperature, daily minimum temperature, and relative humidity (RH), which were the factors causing the increase of vapor pressure deficit (VPD), were responsible for the RET increase in Taishan, Zhongshan, and Shenzhen. Overall, our results indicated that the RET in PRD exhibited a strong spatial heterogeneity due to differences in land use change and climatic conditions. Therefore, the improvement of water resources management and urban environment in the PRD should consider the spatial variation and underlying forces of RET changes.
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 by 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.
In this study, the mechanism for precipitation hotspots (PHs) of locally developed afternoon thunderstorms in the Taipei Basin is investigated using a three-dimensional Vector Vorticity equation cloud-resolving Model (VVM) with an idealized topography and surface properties. A 500 m horizontal grid resolution is used in all experiments. The results show that the local circulation is a key for PHs at the south of the Taipei Basin. The two valleys guide background southwesterly (SW) flow along with the sea breezes to penetrate into the basin. The urban heat island (UHI) effect enhances the sea breeze convergence at the south of the basin and produces strong convection there. The interactions between cold pools generated by the convection and the sea breezes produce northward propagating new convective cells. Besides, the background wind direction is important in determining the location of sea breeze convergence. If the background wind direction changes from westerly (W) to west-northwesterly (WNW), there might be no precipitation at all in the basin. This study suggests that the idealized experiments also provide a useful framework for studying the impacts of future climate changes on the PHs in the Taipei Basin by applying the pseudo–global warming approach.
A novel method for detecting tropical cyclones in high-resolution climate model simulations is proposed herein and subjected to examination. The proposed method utilizes a two-dimensional scatterplot based on two quantities that represent the radial gradient and the tangential asymmetry of mid-to upper-level thickness around a simulated vortex. A comparison between the modeled and observed tropical cyclones using the nonhydrostatic regional climate model (NHRCM) with 20-km grid spacing under reanalysis-driven boundary conditions for one year revealed that no cyclones were missed and there was only one false alarm over a part of the western North Pacific near Japan. The simulated vortices were classified into two categories: tropical cyclones and extratropical cyclones. These two groups, having specific features, were also found in the results using present-day climate datasets, indicating that the tropical cyclones were reasonably distinguished from extratropical cyclones, although a one-by-one comparison could not, in principle, be conducted. Comparison of the results obtained from datasets with 5 km and 20 km grid spacing demonstrated that the detection scheme was only weakly dependent on the horizontal resolution. This dependence was further reduced by using the radial gradient over the outer radii instead of near the center of the vortex. The resolution-independent feature in this method is due to a procedure in which the tangential asymmetry of mid-to upper-level thickness is utilized instead of the relative vorticity at 850 hPa, often used in conventional schemes. This procedure allows the method to identify tropical cyclones without the need to determine a grid-dependent threshold. The method proposed here provides a useful tool for detecting tropical cyclones in high-resolution climate simulations.
In August 2016, a monsoon gyre persisted over the western North Pacific and was associated with the genesis of multiple devastating tropical cyclones (TCs). A series of hindcast simulations were performed using the nonhydrostatic icosahedral atmospheric model (NICAM) to reproduce the temporal evolution of this monsoon gyre. The simulations that were initiated at dates during the mature stage of the monsoon gyre successfully reproduced its termination and the subsequent intensification of the Bonin high, whereas 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. The high predictability of the termination is likely due to the predictable midlatitudinal signals that intensify the Bonin high.