Scientifically-based methods of rainmaking (one of planned weather modification technologies) appeared brilliantly just after World War II, and are currently used in about 40 nations and regions worldwide. The rainmaking methods introducing dry-ice pellets or artificially generated aerosols which act as cloud condensation nuclei and/or ice nuclei into clouds, bypass the rate-limiting processes in rain and snow formation mechanisms in natural clouds by producing precipitation embryos in the form of large droplets in warm clouds or ice crystals in cold clouds, and lead to precipitation enhancement. In this paper, while describing the necessity, history, principle and problems of rainmaking, the leading-edge trend of the planned weather modification research is briefly introduced.

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A cloud simulation chamber facility run by the Meteorological Research Institute (MRI) has been used to investigate the details of the fundamental processes of cloud formation. To overcome the current lack of understanding on aerosol-cloud-precipitation interactions and to reduce the uncertainty in the estimation of aerosol indirect effects, we are attempting to evaluate CCN/IN abilities of various types of specific aerosol particles so as to link aerosol particles and cloud particles from the experimental approaches. In this report, the laboratory studies on the assessments of hygroscopic seeding and glaciogenic seeding materials at MRI are introduced. We tested two prospective techniques for delivering the salt particles that can be applied to warm-based clouds using airborne platform; salt micro-powder and pyrotechnic (flare) methods. We also investigated a widely-used delivery technique of silver iodide particles from the ground-based smoke generator into supercooled clouds, changing dissolving agent and hygroscopic substances in AgI acetone solutions to improve CCN and IN abilities.

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As one of the physical evaluation techniques of seeding effects, cloud seeding experiments were conducted utilizing an instrumented aircraft, with the use of dry-ice pellets in cold clouds and hygroscopic flares and salt micro-powders in warm clouds as a seeding agent. We demonstrated from the in-situ measurements that dry-ice pellet seeding of mixed-phase orographic clouds were effective, based on the comparison of estimated radar reflectivity factor, ice water content, or precipitation intensity from the 2D images of ice particles between seeded portions and their surroundings. The observational results we obtained so far showed that hygroscopic seeding of warm clouds might be effective under limited conditions.

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Numerical simulation is one of the powerful tools for weather modification research. In the last decade, remarkable progress has been made in the modeling study of precipitation enhancement. The subjects of numerical simulations widely range from investigation of elementary processes to seasonal evaluation of cloud seeding effect. Potential applications of the models will expand with the performance of the super-computer in the future. The present article reviews the current status of numerical models and simulations in precipitation enhancement studies, including the latest achievements in terms of optimal seeding method, snowfall enhancement in a dam catchment by glaciogenic seeding, idealized simulations of hygroscopic seeding, and rainfall enhancement in a dam catchment by hygroscopic seeding.

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Atmospheric fine aerosol measurement campaign was performed in the summer of 2013 in Kanto area, Japan. In this paper, approaches to source apportionment of the PM_2.5 by Positive Matrix Factorization (PMF) are described. Data used for PMF analysis is a chemical analysis data (organic and elemental carbon, ionic species, and elemental composition) of the PM_2.5. Sampling of the PM_2.5 was undertaken at three sites of Kudan (Tokyo Metropolitan), Kazo (Saitama Prefecture) and Maebashi (Gunma Prefecture) in Kanto area. Based on the PMF analysis at the three sites, the contributions of the following six sources were estimated; local soil (percentage source contribution of three sites average: 9.7%), mobile sources (21.2%), smoke sources (5.7%), biomass burning (19.5%), secondary nitrate (11.9%), and secondary sulfate (31.9%). The concentrations of secondary sulfate seemed to vary with the two-day cycle rather than the one-day cycle at the three sites. The concentrations of secondary nitrate showed diurnal variation with the minimum in the daytime. On the other hand, the particles concentrations with biomass burning and mobile sources unlike the secondary nitrate was higher during the daytime.

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Manganese perovskite La_0.75Sr_0.25MnO₃ (LSM) fine particles for magnetic hyperthermia therapy were synthesized by an ultrasonic spray pyrolysis method modified with a mist classifier installed between a sprayer and a reactor to produce particles with the size range of 50–200 nm. The mist classification suppressed the generation of particles larger than 500 nm. From an X-ray diffraction measurement, it was found that the phase of synthesized particles was La_0.75Sr_0.25MnO₃ with and without the mist classification. The crystallite size of synthesized particles decreased with decreasing the nozzle diameter of classifier. Curie temperature estimated from the magnetization measurement at various temperatures was reduced from T_C＝323 K (50°C) to T_C＝297–312 K (24–39°C) when the mist was classified. The production rates also decreased from 118 mg h^-1 to 39 - 4 mg h^-1 by using the mist classifier.

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Number size distribution of ambient aerosols was measured using a wide-range particle spectrometer (WPS) at the Cape Hedo Atmosphere and Aerosol Monitoring Station in Okinawa and at the Fukue Atmosphere Monitoring Station in Nagasaki, Japan. Both sites are located in westernmost Japan, but in different proximity to the north-eastern region of China, which air masses arriving at these sites often pass through. At each site field measurements were conducted in February 2012 and 2013, respectively. The measurement results at Cape Hedo showed a bimodal distribution with maximum at 50 nm and 200 nm throughout the observation period. In contrast, the size distribution at Fukue Island varied: a bimodal distribution with maximum at 50 nm and 150 nm was observed between 1:00 a.m. and 4:00 a.m. local time on February 23rd, while a monomodal distribution with maximum at ~15 nm, which was very likely new particle formation, was observed at 12:00 p.m. local time on February 24th. During the latter episode the number concentration exceeded 40000 cm^-3, and growth of these nucleation mode particles was also observed. Such particle formation and growth were not observed at Cape Hedo. The study here demonstrates difference in size distribution of ambient aerosols contained in transboundary air masses and provides valuable information on aerosol size distribution in East Asian region.

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