Radioactive plumes can spread far and wide depending on wind conditions. The plumes often frequently reached the Tokyo metropolitan area, which is approximately 200 km away from the Fukushima Daiichi nuclear power plant, under spatially heterogeneous wind fields in March 2011. To reduce exposure to radioactive plumes, the behaviour of the plumes must be known. However, the transport mechanism of radioactive plumes is not fully understood. Using a regional climate model, we show that multiple diurnal cycle processes play a key role in the frequent transport of radioactive plumes to the Tokyo metropolitan area. The observed data and hindcast results indicate that the radioactive plume moves along the local winds, which comprise the northeasterly local wind (NELW) associated with the meso-scale low-pressure system (meso-low) and the northerly sea wind (NSW) during the night. The long-term analysis and sensitivity simulations also show the nocturnal processes that the NELW caused by the meso-low and the NSW are formed east of the Tokyo metropolitan area and from Fukushima offshore east of the Tokyo metropolitan area, respectively, when neither winter monsoons nor extra-tropical cyclones are predominant. These findings indicate that the radioactive plumes could reach faraway places frequently via nocturnal local processes.
This research focuses on dam reservoir operation effective in flood mitigation and water resource reservation on a seasonal scale. Based on the relationship between discharge characteristics in the upper watershed of Chao Phraya River and flood occurrences in the lower watershed, it was clarified that the dam reservoir operation most effective in the rainy season was determining the lowest reservoir volume in August for the Sirikit Dam reservoir and in July for the Bhumibol Dam reservoir, and storing water until November. Furthermore, by the probability evaluation on the free reservoir capacities of both dams estimated from the predetermined lowest reservoir volume and inflow volume in both dams, the dam reservoir operation considering the importance of flood mitigation and water resource reservation on a seasonal scale can be achieved.
Due to the massive earthquakes and tsunami on March 11th, 2011 in Eastern Japan, Fukushima Daiichi nuclear power plant was severely damaged and some reactors were exploded. Then radioactive particles were widely spread out. In this study, we modified the stable isotope module of RSM (IsoRSM, Yoshimura et al.) to enable to simulate the transport of the radioactive tracers, namely iodine 131 and cesium 137, by including the dry and wet deposition processes. The control experiment with 10km resolution and the emission estimated by Chino et al. (2011) showed reasonable temporal results for Toukatsu area (eastern part of Tokyo metropolis and western part of Chiba prefecture), i.e., on March 22th, the tracers from Fukushima was reached and precipitated in a significant amount as wet deposition. Thus, we conducted 4 experimental simulations to analyze the uncertainty due to different meteorological patterns and different parameters for wet and dry deposition and diffusion. Though the temporal patterns of deposition of radioactive particles were moderately similar each other in all experiments, we revealed that the deposition parameters and boundary conditions can cause the uncertainties largely in the distribution of deposition.
To reproduce more accurate deposition maps of radioactive materials released in the Fukushima Daiichi Nuclear Power Plant accident in March 2011, our study focuses on the uncertainty of atmospheric transport simulations caused by precipitation. In our new method, simulated wet deposition distribution of 137Cs is modified by high‐resolution radar rain gauge data of observed precipitation. Sensitivity experiments are conducted to examine the impact of using both different reanalyzed meteorological data sets as boundary condition and the observation data of precipitation as the redistribution of simulated precipitation. Among the results, the experiment modified by high‐resolution radar rain gauge data realized the most accurate cumulative 137Cs deposition from 18 to 27 March. While the meteorological field is reasonably simulated in the atmospheric transport model in the experiments, the results showed that applying observed precipitation also contributes to improve the accuracy of simulated wet deposition amount.
The contaminated air mass traveled over Japan and the radionuclides were deposited into and contaminated terrestrial ecosystems; this phenomenon was discovered by field measurements (
Abstract. Resuspension of 137Cs from the contaminated ground
surface to the atmosphere is essential for understanding the environmental
behaviors of 137Cs and estimating external and inhalation exposure of
residents. Kajino et al. (2016) assessed the 137Cs resuspension flux
from bare soil and forest ecosystems in eastern Japan in 2013 using a numerical
simulation constrained by surface air concentration measurements. However,
the simulation was found to underestimate the observed deposition amounts by
2 orders of magnitude. The reason for this underestimation is that the
simulation assumed that resuspended 137Cs is carried by submicron
aerosols, which have low deposition rates. Based on the observational
indications that soil dust and bioaerosols are the major carriers of
resuspended 137Cs, a new simulation is performed with higher deposition
rates constrained by both surface concentrations and deposition amounts. In
the new estimation, the total areal annual resuspension of 137Cs in
2013 is 25.7 TBq, which is equivalent to 0.96 % of the initial deposition
(2.68 PBq). Due to the rapid deposition rates, the annual redeposition
amount is also large at 10.6 TBq, approximately 40 % of the resuspended
137Cs. The resuspension rate through the atmosphere (0.96 % yr−1)
seems slow, but it (2.6 × 10−5 d−1) may not be negligibly
small compared to the actual decreasing trend of the ambient gamma dose rate
obtained in Fukushima Prefecture after the radioactive decay of 137Cs
plus 134Cs in 2013 is subtracted (1.0–7.9 × 10−4 d−1): resuspension can account for 1 %–10 % of the decreasing rate due
to decontamination and natural decay through land surface processes. The
current simulation underestimated the 137Cs deposition in Fukushima
city in winter by more than an order of magnitude, indicating the presence
of additional resuspension sources. The site of Fukushima city is surrounded
by major roads. Heavy traffic on wet and muddy roads after snow removal
operations could generate superlarge (approximately 100 µm in
diameter) road dust or road salt particles, which are not included in the
model but might contribute to the observed 137Cs at the site.
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