An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.

This study investigates the influence of strong southerly moisture flux on an extreme rainfall event over western Japan in early July 2018, by using a global atmospheric reanalysis dataset. During its peak period from 5 to 7 July, extensive and unprecedented rainfall observed along the well-defined quasi-stationary Baiu front was attributed to two branches of extremely moist inflow from the southern confluence into western Japan. One was a shallow southerly airstream enhanced by the surface North Pacific Subtropical High, and the other was a deeper southwesterly airstream accompanying enhanced convection over the East China Sea. Both the vertically integrated moisture flux from the south and its convergence into western Japan reached the highest levels for 60 years due to an overwhelming contribution from the intensified southerlies. Anomalous diabatic heating associated with the active convection over the East China Sea acted to maintain the southwesterly moisture flux by inducing low-level cyclonic potential vorticity anomalies. During the rainfall event, a strong meander of the upper-level subtropical jet associated with the intensified surface North Pacific Subtropical High accompanied an amplified upper-level trough over the Korean Peninsula, which acted to induce ascent dynamically along the Baiu front.

A sky-radiometer network was developed in order to investigate the characteristics of aeolian dust on the way of transportation from the source region to the Japan area. From the analysis of the skyradiometer data, optical thickness in the visible to near infrared regions and size spectrum from radius 0.01 to 5 mm can be obtained. Five observation sites are set at Aksu, Qira, Shapotou, Qingdao, and Beijing in China, and four observation sites at Naha, Fukuoka, Nagoya and Tsukuba in Japan. The data of six observation sites (Qira, Shapotou, Naha, Fukuoka, Nagoya and Tsukuba) are automatically transferred to Meteorological Research Institute (MRI) through telephone lines.The following results were obtained from the data analysis of intensive observation period (IOP) in April, 2002. (1) The data with Å ngström exponent between 0.0 and 0.5 corresponds to the dust event day data. (2) The contribution of the particles with radius greater than 0.5 mm, which correspond to coarse dust particles, to the total optical thickness frequently exceeds more than @70% in the source region, and exceeds in Qingdao and sites in Japan on the dust event day. (3) The contribution of particles with radius greater than 0.5 mm to the total volume is more than @80% on the dust event day. (4) The retrieved volume spectrum in the source region, Aksu and Qira, is not dependent on the optical thickness. This means that the floating aerosols mainly consist of dust particles in the source region. (5) The total volume observed in Japan sites is one third of that in the source region. (6) When the size distribution for the coarse mode (r > 0:5 mm) is approximated by log-normal size distribution, effective radius r eff is 2.1 to 2.3 mm in China sites and 1.6 to 1.8 mm in Japan sites. r g is about 0.7 mm at Aksu, Qira, and Shapotou and about 0.5 mm at other sites (ln r g is the center of log-normal size distribution). The width of size distribution ðs g Þ is scattered between 0.67 and 0.87. These difference of size distribution among the observation sites are caused by the modification of air mass including aeolian dust; coarser dust particles are partially removed during the transportation, and the air mass including aeolian dust as a main component are partially contaminated by the aerosol into the atmosphere from anthropogenic activities. The observation network of ADEC caught this change clearly.

To investigate aerosol optical properties, the Meteorological Research Institute has been continuously measuring scattering and absorption coefficients since January 2002 by using an integrating nephelometer and one-and three-wavelength absorption photometers in dry air conditions at Tsukuba, Japan. We used these optical data to investigate trends of aerosol properties and climatology from 2002 to 2013. The results showed that most aerosol characteristics had seasonal variation and decreasing or increasing trends significant at the 95 % confidence level. From 2002 to 2013, the extinction coefficient at 550 nm and absorption coefficient at 530 nm had statistically significant decreases of-1.5 × 10-6 and-5.4 × 10-7 m-1 year-1 , respectively. In the same period, the scattering coefficient showed a non-significant decrease of-8.8 × 10-7 m-1 year-1. The single scattering albedo (SSA) at 550 nm had a significant increasing trend of 7.4 × 10-3 year-1. Asymmetry factors did not show a significant trend. The increasing trend in the extinction Ångström exponent was significant, whereas the trend in the effective radius was not significant. The increasing trend of 2.1 × 10-2 year-1 in the absorption Ångström exponent from 2006 to 2013 was significant. This tendency suggests a compositional change of light-absorbing aerosol. Frequency distributions of aerosol properties were investigated during 2006-2012. In this period, absorption coefficients were measured by the three-wavelength absorption photometer. The most frequent values of the extinction coefficient at 550 nm, the absorption coefficient at 530 nm, and the SSA at 550nm were 25 × 10-6 , 3.0 × 10-6 m-1 , and 0.905, respectively. The analysis using the extinction Ångström exponent showed that aerosol characteristics were dependent on the extinction Ångström exponent. The aerosol characteristics estimated from optical data were consistent with those derived from radiometer data. Therefore, ground-based monitoring of aerosol optical properties is useful for monitoring aerosol characteristics and interpreting variations in the surface radiation budget.

To investigate the absorption of aeolian dust transported from the Chinese Desert to Japan, in situ measurements of single scattering albedo (SSA) using Particle and Soot Absorption Photometer (PSAP; Radiance Research) and Nephelometer M903 (Radiance Research) were made at Qira, China; Beijing, China; and Tsukuba, Japan. SSA measured by PSAP and M903 at Qira was between 0.91 and 0.93, and that at Beijing and Tsukuba was between 0.80 and 0.90. We also analyzed data obtained by the sky-radiometer network on dust transported from the Taklimakan Desert to Japan. The SSA retrieved from sky-radiometer was between 0.91 and 0.97. The SSA measured at Qira is consistent with the SSA inferred from the sky-radiometer. This means that unpolluted aeolian dust does not have large absorption. The SSA at Beijing and Tsukuba is lower than the SSA inferred from the sky-radiometer. The large absorption of aerosol is mainly due to black carbon (BC). The above facts indicate that the BC is distributed near the ground surface and the larger part of the aeolian dust layer does not include the BC.