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This study newly applies measurements from two geostationary satellites, the Advanced Himawari Imager (AHI) onboard the geostationary satellite Himawari-8 and the Geostationary Ocean Color imager (GOCI) onboard the geostationary satellite COMS, to evaluate a unique regional aerosol-transport model coupled to a non-hydrostatic icosahedral atmospheric model (NICAM) at a high resolution without any nesting technique and boundary conditions of the aerosols. Taking advantage of the unique capability of these geostationary satellites to measure aerosols with unprecedentedly high temporal resolution, we focus on a target area (115°E-155°E, 20°N-50°N) in East Asia in May 2016, which featured the periodic transport of industrial aerosols and a very heavy aerosol plume from Siberian wildfires. The aerosol optical thickness (AOT) fields are compared among the AHI, GOCI, MODIS, AERONET and NICAM data. The results show that both AHI- and GOCI-retrieved AOTs were generally comparable to the AERONET-retrieved ones, with high correlation coefficients of approximately 0.7 in May 2016. They also show that NICAM successfully captured the detailed horizontal distribution of AOT transported from Siberia to Japan on the most polluted day (18 May 2016). The monthly statistical metrics, including correlation between the model and either AHI or GOCI, are estimated to be >0.4 in 42–49% of the target area. With the aid of sensitivity model experiments with and without Siberian wildfires, it was found that a long-range transport of aerosols from Siberian wildfires (from as far as 3000 km) to Japan influenced the monthly mean aerosol levels, accounting for 7–35% of the AOT, 26–49% of the surface PM2.5 concentrations, and 25–66% of the aerosol extinction above 3 km in height over Japan. Therefore, the air pollutants from Siberian wildfire cannot be ignored for the spring over Japan.

A rapid decrease in PM2.5 concentrations in China has been observed in response to the enactment of strong emission control policies. From 2012 to 2017, total emissions of SO2 and NOx from China decreased by approximately 63% and 24%, respectively. Simultaneously, decreases in the PM2.5 concentration in Japan have been observed since 2014, and the proportion of stations that satisfy the PM2.5 environmental standard (daily, 35 µg/m3; annual average, 15 µg/m3) increased from 37.8% in fiscal year (FY) 2014 (April 2014 to March 2015) to 89.9% in FY 2017. However, the quantitative relationship between the PM2.5 improvement in China and the PM2.5 concentration in downwind regions is not well understood. Here, we (1) quantitatively evaluate the impacts of Chinese environmental improvements on downwind areas using source/receptor analysis with a chemical transport model, and (2) show that these rapid emissions reductions improved PM2.5 concentrations both in China and its downwind regions, but the difference between SO2 and NOx reduction rates led to greater production of nitrates (e.g., NH4NO3) due to a chemical imbalance in the ammonia–nitric acid–sulfuric acid–water system. Observations from a clean remote island in western Japan and numerical modeling confirmed this paradigm shift.

Monitoring particulate matter is essential to alert the public about health risks. The Terra/MODIS true color image clearly captured a yellow band over Hokkaido prefecture in Japan on 7 March 2016. We investigated whether this event was an Asian dust (Kosa) transport or not with the ground-based observations in Sapporo and Takikawa in Hokkaido and NASA's MERRA-2 re-analysis data. The timing of increased particle number concentrations (PNCs; greater than 0.5 mu m) was clearly measured by a low-cost aerosol sensor at Sapporo and Takikawa in the early afternoon. For this particle size range, the PNC by this aerosol sensor had greater agreement with another commercial instrument for the 1-hourly mean data. The lidar data at Takikawa and NASA's AERONET at Sapporo also implied the increased dust particles (i.e., dominance of non-spherical and coarse particles, respectively), which supported that the PNC increase was due to the dust transport. The hourly PM2.5 data in Sapporo significantly increased in the evening rather than around the noon to early afternoon. We concluded that this event was judged as an Asian dust (Kosa) event in Hokkaido starting from the early afternoon, which was, however, not reported by Japan Meteorological Agency (JMA) based on their visible observations.

To identify the unusual climate conditions and their connections to air pollutions in a remote area due to wildfires, we examine three anomalous large-scale wildfires in May 2003, April 2008, and July 2014 over East Eurasia, as well as how products of those wildfires reached an urban city, Sapporo, in the northern part of Japan (Hokkaido), significantly affecting the air quality. NASA’s MERRA-2 (the Modern-Era Retrospective analysis for Research and Applications, Version 2) aerosol re-analysis data closely reproduced the PM2.5 variations in Sapporo for the case of smoke arrival in July 2014. Results show that all three cases featured unusually early snowmelt in East Eurasia, accompanied by warmer and drier surface conditions in the months leading to the fires, inducing long-lasting soil dryness and producing climate and environmental conditions conducive to active wildfires. Due to prevailing anomalous synoptic-scale atmospheric motions, smoke from those fires eventually reached a remote area, Hokkaido, and worsened the air quality in Sapporo. In future studies, continuous monitoring of the timing of Eurasian snowmelt and the air quality from the source regions to remote regions, coupled with the analysis of atmospheric and surface conditions, may be essential in more accurately predicting the effects of wildfires on air quality.

[1] We examined possible responses of cyclone activities to the bimodal path states of the Kuroshio Current [i.e., large meander (LM) and non-LM (NLM)] by using the long-term reanalysis data and the 20th century hindcast experiment of a high-resolution atmosphere-ocean coupled model. Compared with a seasonal mean cyclone track frequency for the LM and NLM periods, a primary cyclone track shifts southward in association with the meander of Kuroshio Current. Composite analyses of the hindcast experiment showed remarkable atmospheric responses accompanying the Kuroshio LM. The Kuroshio LM causes a decrease in latent heat flux in the south of Japan and a southward shift of the near-surface baroclinic zone. Distinctive decreases in thermodynamic fluxes inhibit the rapid development of cyclones in the meander region, eventually inducing positive sea level pressure anomalies downstream from that region.

Spatial patterns of future atmospheric warming presented by climate model projections indicate that the temperatures of the lower troposphere will increase markedly at high latitudes in the Northern Hemisphere and also the upper troposphere at low-middle latitudes. In this study, potential impacts of the spatial patterns of future atmospheric warming on Asian dust emission were investigated by performing numerical experiments using a dynamical downscaling method. After three significant Asian dust outbreak events were reproduced as control runs, initial and boundary conditions of the control runs were updated by adding differences in atmospheric variables between the future climate (2091–2100) and the recent climate (1991–2000) to execute pseudo global warming runs. The results showed that the dust emission fluxes in the main Asian dust sources (MADSs), i.e., the Taklimakan and Gobi deserts, decrease markedly in the future climate condition. The future decreases in the dust emission fluxes are likely to be caused by a combination of the relatively large increases in sea level pressures (SLPs) in MADSs and the small SLP changes to the north of MADSs, which reduces the meridional SLP gradients between the two areas and consequently weakens cold air outbreaks. The large SLP increases in MADSs may be due to a northward shift of storm tracks and increased atmospheric stabilities caused by the large upper-tropospheric warming at low-middle latitudes. The small SLP changes to the north of MADSs corresponded to the large increases in surface air temperatures, which would be influenced by the large lower-tropospheric warming at high latitudes.

The main contributor of aerosol particulate soluble iron to Japan and the Pacific Ocean has been investigated using data obtained during the research campaign entitled “A Study on Dust and Sand Storms” conducted by the Ministry of the Environment, Japan. The concentration of particulate soluble iron was not correlated to total iron concentration. Particulate iron solubility ranged from less than 1% to 6%. It was low when the air mass was dominated by kosa aerosols, and high when the air mass was dominated by pollutants. Durations for the kosa and pollution events over Jeju Island, Matsue, and the Pacific Ocean in April and May 2007 were estimated using a Chemical Weather Forecasting System (CFORS). The estimated durations of the pollution events at Jeju and Matsue were slightly shorter than those of the kosa events. The calculated duration of the pollution event over the Pacific Ocean was only three hours, much shorter than that of the kosa event. Kosa aerosols are the main contributor of soluble iron to the Pacific Ocean; however anthropogenic aerosols should not be discounted as contributors of soluble iron to an area off the coast of the Asian continent.

During the Typical Asian Dust episode from the end of March-early April 2007 (TAD-2007), the mass concentration of suspended particulate matter (SPM) began to increase rapidly on the morning of 1 April in Hokuriku and Tohoku but remained low in Kanto. Ground-level lidar and rawinsonde sounding in Hokuriku observed a dust layer at ∼2 km corresponding to the base of the temperature inversion. In Kanto, which is leeward of Japan's central mountain ranges, SPM increase began from the east coast and then advanced westward after 18 JST with easterly winds. Merged CloudSat and CALIPSO datasets indicated that clouds over mainland Japan and the Sea of Japan were located in the upper-level atmosphere (> 6 km). Continuous meteorological observations showed that cloud condensation and rainfall were not observed over the mountains during the daytime of 1 April. These results suggest that the delay in the SPM increase in Kanto was caused by dust being indirectly transported to that region by flowing around the central mountains.

We investigated the influence of meteorological variability on the interannual variation of springtime boundary layer ozone over Japan during 1981–2005 by multiyear simulations with the Models-3 Community Multiscale Air Quality (CMAQ) modeling system and the Regional Emission Inventory in Asia (REAS). CMAQ/REAS generally reproduced the observed interannual variability of springtime ozone over Japan, showing year-to-year variations larger than the annual rate of increase of the long-term trend. We then analyzed the influence of the interannual variation of meteorology in simulated results by using the fixed emissions for 2000 and meteorological fields for each year. As a reference parameter, we calculated the area-weighted surface pressure anomaly over the Pacific Ocean east of Japan. When the anomaly has a large negative value, polluted air masses from continental Asia tend to be transported directly to Japan by westerly winds. In contrast, when the anomaly has a large positive value, influence of the outflow from continental Asia tends to be small because the westerly components of wind fields around Japan are comparatively weak. Instead, southerly winds are relatively strong and transport clean air masses from the Pacific Ocean to Japan. Consequently, springtime ozone over Japan is higher (lower) than in ordinary years when the anomaly has a large negative (positive) value. In general, the interannual variation of springtime ozone over Japan is sensitive to the outflow from continental Asia. We also found some correlation between springtime ozone over Japan and the El Niño-Southern Oscillation, indicating that higher and lower springtime ozone over Japan are related to La Niña and El Niño, respectively. Differences in the meridional displacement and diversity of cyclone tracks near Japan between El Niño and La Niña years may be responsible for interannual variations in the springtime boundary layer ozone over Japan.

Intensive aircraft- and ground-based measurements of ultrafine to supermicron particles in the Osaka metropolitan area, Japan, were carried out on 17–19 March 2003, in order to investigate vertical profiles of size-resolved particles in the urban atmosphere. Differently sized particles were observed at different altitudes on 19 March. Relatively higher concentrations of ultrafine particles (31 nm) and submicron particles (0.3–0.5 μm) were measured (100–200 cm−3) at altitudes of 300 and 600 m, whereas supermicron particles (2–5 μm) were present (300–600 cm−3) at higher altitudes (1300 m in the morning and 2200 m in the afternoon). The chemical composition analysis showed that supermicron particles evidently comprised mainly soil particles mixed internally with anthropogenic species such as carbonaceous components and sulfate. Numerical simulation using the Chemical weather FORecasting System (CFORS) suggested the long-range transport of soil dust and black carbon from the Asian continent. Total number concentrations of particles sized 10–875 nm ranged from 4.8×103 to 3.0×104 cm−3 at an altitude of 300 m and from 7.3×102 to 4.8×103 cm−3 at an altitude of 1300 m. Total number concentrations of particles sized 10–875 nm correlated very well with NOX concentrations, and, therefore, ultrafine and submicron particles were likely emitted from urban activities such as car traffic and vertically transported. Number size distributions at lower altitudes obtained by aircraft measurements were similar to those obtained by ground measurements, with modal diameters of 20–30 nm on 18 March and about 50 nm on 19 March.

Recent research indicates that the cooling trend in the tropical Pacific Ocean over the past 15 years underlies the contemporaneous hiatus in global mean temperature increase. During the hiatus, the tropical Pacific Ocean displays a La Niña-like cooling pattern while sea surface temperature (SST) in the Indian Ocean has continued to increase. This SST pattern differs from the well-known La Niña-induced basin-wide cooling across the Indian Ocean on the interannual timescale. Here, based on model experiments, we show that the SST pattern during the hiatus explains pronounced regional anomalies of rainfall in the Asian monsoon region and thermodynamic effects due to specific humidity change are secondary. Specifically, Indo-Pacific SST anomalies cause convection to intensify over the tropical western Pacific, which in turn suppresses rainfall in mid-latitude East Asia through atmospheric teleconnection. Overall, the tropical Pacific SST effect opposes and is greater than the Indian Ocean SST effect., Unique cooling in the tropical Pacific and warming in the Indian Ocean over the past 15 years is postulated to have an effect on Asian rainfall. Here, based on a numerical modelling experiment, the authors investigate this relationship and provide insight into the atmospheric dynamics at play.

Vertical profiles of springtime ozone (O3) over the Osaka metropolitan area in Japan were investigated by means of aircraft-based observations in March 2001 (Period I) and March 2003 (Period II). The observations in Period I were carried out during an episode of high O3 concentration under stagnant meteorological conditions, whereas in Period II, pollution levels were low and the weather was cloudy with strong winds. The elevated O3 concentration in the surface mixing layer in Period I was a result of local photochemical production as well as trans-boundary air pollution transported from East Asia. In contrast, the O3 concentration in Period II was very low, corresponding to the O3 level of ‘clean continental’ air masses from the northern Asian continent. On the basis of the vertical O3 profiles observed in Period I, we estimated the contributions of local photochemical production and trans-boundary air pollution to the O3 within the mixing layer, local photochemical production contributed −3% to 33% and trans-boundary air pollution contributed 8%–27%. We found that the trans-boundary air pollution made a notable contribution to the springtime O3 in the Osaka metropolitan area, its contribution being comparable to that of local photochemical production.

In Mongolia and northern China, most of the dust storms in spring occur in association with a passage of cold front formed at the leading edge of cold air outbreak. In this study, we propose an index to evaluate a cold frontal activity by identifying “cooling days”. These are days when there is a strong decrease in daily mean surface air temperature of greater than 5 K, using the European Centre for Medium-Range Weather Forecasts 45-year reanalysis data for 1967 through 2002.The geographical distribution of seasonal mean cooling day frequencies showed that Mongolia is the most frequent area over Eurasia with the highest frequency of cooling days in spring.Interannual variations in the area-averaged frequency of cooling days in Mongolia showed that most of the years with less-than-average frequencies in spring occurred in the last 20 years. To determine the main cause(s) for the differences in cooling day frequencies, we conducted a composite study for the 8 years with the highest area-averaged spring cooling day frequencies (active years), and the 8 years with the lowest (inactive years). This study revealed that there were no apparent differences between the active and inactive years with regard to geographical route of cold air, or total number of cooling events. In contrast, the cooling intensity in the inactive years was approximately 20% below that in the active years. These results suggest that recent weakening of cooling intensity over eastern Mongolia has resulted from warming over Siberia.

数日スケールの急激な気温変動イベントを対象とし,54年間(1948～2001年)のNCEP-NCAR再解析データを用いて,北半球全域における時空間分布と温暖化傾向に対応した発生回数の変化について調査した.本研究で使用した気温急変イベントは,個々の地点における日平均地上気温が3日以内で±20K変化した場合と定義した.発生回数の多い4地域(西シベリア,アラスカ,カナダ北西部,カナダ南東部)における広域気温急変イベントの発生期間は,大部分が寒候期(11月から2月)に集中していた.発生回数の極大時期は,西シベリア地域では11月・12月,北米大陸上では12月・1月と地域差がみられた.また,寒候期平均気温偏差に対する気温急変現象の平均発生回数を調査したところ,4地域ともに温暖偏差時の発生回数は寒冷偏差時に比べて15～25%程度減少しており,この傾向はとりわけ西シベリア地域とカナダ北西部地域で顕著であった.上記2地域における総観場の時間発展を明らかにするため,典型事例の合成図解析を行った.その結果,カナダ北西部地域での気温低下時には,アラスカ上でブロッキング高気圧が形成され,引き続いてカナダ北西部への寒気の南下がみられた.また,西シベリア地域における気温上昇時には,西シベリア低地から東アジア方面への寒冷域の南東への進行とそれに付随したシベリア高気圧の発達がみられた.これは,典型的な東ブジアへの寒気の吹出しパターンと対応していると考えられる.これらの結果は,将来的な温暖化傾向に伴い,地域的には日々のスケールの気温変動性が減少する可能性のあることを示唆している.