Model depiction of the atmospheric flows of radioactive cesium emitted from the Fukushima Daiichi Nuclear Power Station accident
© The Author(s). 2017
Received: 10 June 2016
Accepted: 5 January 2017
Published: 23 January 2017
In this study, a new method is proposed for the depiction of the atmospheric transportation of the 137Cs emitted from the Fukushima Daiichi Nuclear Power Station accident. This method employs a combination of the results of two aerosol model ensembles and the hourly observed atmospheric 137Cs concentration at surface level during 14–23 March 2011 at 90 sites in the suspended particulate matter monitoring network. The new method elucidates accurate transport routes and the distribution of the surface-level atmospheric 137Cs relevant to eight plume events that were previously identified. The model ensemble simulates the main features of the observed distribution of surface-level atmospheric 137Cs. However, significant differences were found in some cases, and this suggests the need to improve the modeling of the emission scenario, plume height, wet deposition process, and plume propagation in the Abukuma Mountain region. The contributions of these error sources differ in the early and dissipating phases of each event, depending on the meteorological conditions.
KeywordsFukushima Nuclear Power Station accident Aerosols Radioactive materials 137Cs Chemical transport modeling Ensemble models
A wide area of northeastern Japan, the Tohoku and Kantou regions, was contaminated by the radioactive material emitted from the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS) of the Tokyo Electric Power Company (TEPCO), as manifested by various environmental investigations (Nakajima et al. 2014). The accident was caused by the Great East Japan Earthquake, which struck at 14:46 Japan Standard Time (JST; Coordinated Universal Time, UTC+ 9 h) on 11 March 2011.
Model simulation of the atmospheric 137Cs concentration at surface level
In this study, two aerosol transport models were employed to simulate the atmospheric transportation of 137Cs. The first model uses an online dynamic core of the nonhydrostatic icosahedron atmospheric model (NICAM; Tomita and Satoh 2004; Satoh et al. 2008) coupled with the spectral radiation-transport model for aerosol species (SPRINTARS; Takemura et al. 2000; Dai et al. 2014); we will refer to this as the N-model. The NICAM model is implemented with an air-mass flux-type dynamic scheme of high mass conservation, and it can cover a global-to-regional simulation with its three grid systems, which include a quasihomogeneous global grid, a stretched grid, and a diamond regional grid (Uchida et al. 2015). We adopted the diamond grid system for a regional simulation with the following settings: 10 s time step, 5-km grid resolution, and 40 layers, with the lowest layer being 20 m thick. The cloud microphysics scheme was the NICAM single-moment scheme with six water categories (NSW6), which is a simplified version of Lin’s scheme (Lin et al. 1983). The turbulent diffusion calculation adopted a partial condensation scheme (Mellor and Yamada 1982) and an eddy diffusion scheme with a level 2.0 turbulent scale adjustment scheme (Nakanishi and Niino 2004). The numerical domain was a rhombus area, with apexes at 26.5° N, 144.4° E; 35.8° N, 132.0° E; 50.6° N, 133.4° E; and 39.9° N, 147.7° E; it covered the Tohoku region (the northeastern part of the Japanese islands, including the Fukushima and Miyagi prefectures, as shown in Fig. 2) and the Kantou Plain region (the area including the Tokyo, Saitama, Chiba, Kanagawa, and Ibaraki prefectures). After release from the FDNPS, the atmospheric 137Cs was treated as a sulfate aerosol particle and assumed to undergo transportation, hygroscopic growth, dry and wet deposition, and gravitational settling, according to the scheme of Takemura et al. (2000). The present numerical experiment assumed a monodisperse sulfate-equivalent particle with a radius of 0.24 μm, taking into account the observed size distribution by Kaneyasu et al. (2012) and an in-cloud scavenging coefficient defined by Takemura et al. (2000) as 0.8 (Goto et al. 2015a).
The other model was a meteorological model, the weather research and forecast model (WRF) version 3.1 (Skamarock et al. 2008), coupled with a three-dimensional chemical transport model, the Models-3 Community Multiscale Air Quality (CMAQ) version 4.6 (Byun and Schere 2006); we will refer to this as the W-model. The CMAQ had been modified for radioactive material transport by the National Institute for Environmental Studies for a FDNPS simulation (Morino et al. 2011; SCJ 2014). We employed the WRF modules of scalar positive-definite advection, full diffusion, without the sixth-order horizontal-advection noise filter, and the planetary boundary parameterization of the Mellor-Yamada-Janjic scheme (Wang et al. 2012). In the present simulation, all the 137Cs was assumed to be in particulate phase (Sportisse 2007), and the chemical and aerosol processes were not calculated because no detailed process has been well reported. The deposition schemes were those of Byun and Ching (1999) and Byun and Schere (2006). The mean particle radius was assumed to be 0.5 μm, and the geometrical dispersion σ g was assumed to be 1.1 (Sparmacher et al. 1993; Sportisse 2007; Kaneyasu et al. 2012). The model domain was a 700 × 700 km2 area, covering most of the Tohoku and Kantou regions, with a grid resolution of 3 km and 34 layers, with the lowest layer approximately 60 m thick. The simulation period was 14–24 March, and the analysis was performed by comparing the model ensemble results with the observed atmospheric 137Cs concentration at surface level. The total deposition over Japanese land was simulated as 2.12 PBq by the N-model and 2.21 PBq by the W-model (Morino et al. 2013). These values belong to the small deposition group in the SCJ model comparison (SCJ 2014), which gave a mean value of total deposition of 2.92 PBq.
Daily correlation coefficients between observed and simulated 137Cs concentrations based on N-model, W-model, and two-model ensemble (N + W) results for 137Cs
N + W
Results and discussion
Structure of atmospheric 137Cs plumes
In this section, we examine the plume formation and dissipation during each of three periods: 15–16 March, 18–19 March, and 20–21 March 2011.
The observed high-concentration area of plume P2 gradually spread from 9:00 to 12:00 in the Central Tokyo area, which is similar to simulations that show that the plume slightly shifted to the east, as shown in Fig. 6a, panels 9:00 to 12:00. Figure 9c shows that the ensemble model cannot account for this shift of the high-concentration area, but the emission scenario of Katata et al. (2015) gives a better result (Fig. 10). The SPM data suggest that dissipation of the P3 plume occurred during the evening of 15 March through the morning of 16 March, as shown in Fig. 6, panels 3/15/15:00 to 3/16/7:00, and Fig. 9d, e. The N-model adequately simulated the timing and distribution of the plume dissipation following a reduction in the transport of radioactive material, due to a patch of precipitation between FDNSP and Kantou; however, the W-model simulation showed a slower dissipation.
The wind vector fields shown in Fig. 9c, d indicate that a weak cyclonic motion around the Central Fukushima area was generated until noon of 15 March; this was probably due to solar heating. It cause a gradual spread of plume P2 toward the western part of the Tokyo area, as indicated by a thick arrow in Fig. 9d. The wind field also transported plume P3 toward the Nakadori region in the afternoon of 15 March, when high concentrations of more than 100 Bq m−3 were observed consecutively from southern to northern sites (Fig. 6, panels 12:00 to 15:00, and Fig. 9c, d). In the late afternoon, a southeasterly wind became dominant at the FDNSP, and consequently, a part of the P3 plume took a northern detour to the edge of the Abukuma mountains and into the Nakadori region from north to south, as shown by Fig. 6b, panels 15:00 to 18:00, and Fig. 9e. Tsuruta et al. (2014) also identified this northern branch. However, Figs. 6b and 9e show that without the plumes crossing the Abukuma mountains, the simulated plumes were too narrow to explain the elevated observed concentrations at the SPM sites in the Nakadori region. This problem is probably due to the spatial resolution (3–5 km), which is too coarse to simulate the fine-scale orographic horizontal advection of the plume traversing the Abukuma Mountains.
Another notable difference between observations and the model is that in plume P2, a medium-level concentration of between 1 and 10 Bq m−3 was persistently observed in a wide area of Kantou from the evening of 15 March until the morning of 16 March (Fig. 6b, panels 3/16/0:00 to 7:00; Fig. 9f), whereas the simulation results, in particular, those of the W-model, show a much smaller concentration in the southern part of the Hamadori and Kantou regions. Thus far, it is difficult for us to identify the cause of the model failures for plumes 2 and 4, but it is natural to assume that this is due to the difficulty of accurately simulating transport in a strongly sheared, thin atmosphere.
A subsequent low-pressure system passed the Japanese islands during 20–21 March (Fig. 4e, f). Time sequences of observed and two-model atmospheric 137Cs concentrations with precipitation maps are shown in Fig. 7, and model ensemble results are shown in Fig. 13 for several characteristic times. The figures indicate the dominant westerly wind on 19 March through the early morning of 20 March at the FDNPS blew the plume toward the Pacific, as shown in Fig. 13a (labeled A) and Fig. 7a, panel 6:00; however, during the morning of 20 March, there was an increasing easterly component due to a moving low-pressure system; this blew the plume back westward toward the Hamadori and established plume P7 (labeled B in Fig. 13a), which had a wide distribution covering the Hamadori and its offshore areas, as shown in Fig. 7a, panels 9:00 to 12:00, and Fig. 13a, b. This plume movement is indicated by a thick arrow in Fig. 13a. The simulated distribution of plume P7, however, was very different in the N- and W-models, resulting in a two-branch pattern of the ensemble mean (A and B in Fig. 13a).
The arrival of P7 at the Choshi Peninsula between 12:00 and 13:00, and its gradual westward spread until 15:00, as shown by the rotation of the plume indicated by a thick arrow in Fig. 13a, was successfully simulated by the N-model, as shown in Fig. 7a, panels 12:00 to 15:00. On the other hand, the W-model failed to simulate this phenomenon. Since precipitation was not involved in the transport process on the morning of 20 March, this failure was considered to be a consequence of more transportation to the upper atmosphere near the emission source, as suggested by the shift to the north (relative to the results of the N-model) of the high concentration around the FDNPS (Fig. 7a, panels 12:00 to 13:00).
Northwestern transport started around 13:00 and lasted until that night, forming plume P8 in the Tohoku region (Fig. 7a, panels 15:00 to 21:00, and Fig. 13b, c). Each model satisfactorily simulates the transportation of plume P8 to the northern part of the Nakadori region, but they are both too coarse to simulate the successive increase in the concentration of 137Cs observed from north to south along the Nakadori channel.
Plume routes and time series
During a 6-hour period on 15 March, plume P2 spread over a large area of the Kantou region. The coverage was not uniform, and the plume was narrow, as indicated by the peak concentration at sites 9, K1, K2, and K3 in Fig. 16b. However, the simulated arrival time was too early, as indicated by the time series and as was already suggested by Fig. 9a.
Plume P3 transported the radioactive material toward the Nakadori region, crossing the Abukuma Mountains, as indicated by the successive peaks over time from the south to the north at sites E and C in the afternoon of 15 March (Fig. 16a); the models were too coarse to simulate the detailed progress of the plume along the Nakadori channel. The progress of plumes P2 and P3 is indicated by thick white arrows in Fig. 15.
As shown in Fig. 9f, plume P4 produced a high concentration of 137Cs in the Choshi Peninsula, as shown by the peaks at sites 12 and 15, but the plume did not reach Kantou.
Short-duration plumes P5 and P6 were observed at site J on 18 and 19 March, and they were simulated by the models, but the simulated arrival time was earlier than the observed time. Moreover, the concentration was largely underestimated for the duration of the period.
In the early afternoon of 20 March, plume P7 was transported over a long distance, first to the ocean and then toward land by a clockwise rotation of the plume route, as indicated by the thick white arrow in Fig. 15; it covered a large part of the northern Kantou region, as shown in Fig. 16b.
Plume P8 was transported northward and was redirected southward along the Nakadori region at the northern edge of the Abukuma Mountains. Consequently, the high-concentration area moved from north to south, as illustrated by the time series of observations at sites B, C, and E. However, the models failed to indicate this phenomenon, underestimating the concentration at sites in the Nakadori region, as also shown in Fig. 7a, panels 15:00 to 21:00. At the same time, the wind direction turned from northwest to north at the FDNPS, with the result that the northern part of P8 covered sites H and J, as illustrated by the thick northeastward arrow.
Plume P9 began on the morning of 21 March, taking a southern route until it collided with the weather front located in the southern part of the Kantou region. The simulated pattern was too short to correspond with the peaks at sites 9, 12, and 15 at 9:00; however, this is due to the complex transport and precipitation processes discussed in the preceding sections. It is difficult to determine if the peaks at sites K1 and K2 on 21 March were caused by a persistent tail from plume P7 or by plume P9, because the observed concentration remained high without a clear separation between plumes, as indicated by the distribution maps shown in Figs. 7b and 13d–f.
As has been discussed in the preceding sections, we found that a combined analysis of observed and model ensemble data is a useful method for analyzing the development of plumes and the distribution of radioactive materials, but neither approach alone is adequate. Although the SPM observational data are unique and of high density, they are not sufficient to show the detailed distribution structure of the atmospheric 137Cs, because the transport mechanism was complex, varied over time, and depended on the local meteorological and geographical conditions. Although in some cases, the models failed to simulate the exact location and time of arrival of the plumes at the SPM sites, the spatial and temporal development of the plume structure was adequately simulated, making it possible to understand how the atmospheric 137Cs was distributed.
The following statement is characteristic of the target area and period, and it is relevant to the atmospheric transportation of radioactive materials: during the analysis period in the spring season in East Asia, migratory pressure systems periodically brought radioactive materials to the Japanese land area, producing somewhat similar plume development patterns. For instance, two peaks can be seen in the time series: one due to plumes P2 and P3 followed by P4, during 15–16 March, and the other due to plumes P7 and P8 followed by P9, during 20–21 March. The first peak (P3 on 15–16 March) was caused by a change in the wind field to northeasterly and later to southeasterly, as a migratory low-pressure system progressed toward the Japanese islands. The second peaks (P4 and P9) were caused by a northerly wind after the low-pressure system had passed from the Japanese islands to the Pacific. When the height of the aerosol layer was overestimated, plumes P4 and P9 deviated eastward due to the westerly wind in the upper layers, such as at 900 hPa, as shown in Figs. 9f and 13f.
Future tasks include improving the present method, such as by conducting sensitivity tests for (1) different emission scenarios, for example, those by Katata et al. (2015); (2) plume height based on different model-layering setups; (3) wet deposition processes with different parameterizations; and (4) material transport across and along the northern edge of the Abukuma Mountains. Relevant to task 4, a report by Sekiyama et al. (2015) has claimed that there was no significant difference between 3-km and 500-m grid simulations of the JMA nonhydrostatic model for horizontal transport of radioactive material. However, in view of the differences in model parameterization for orographic waves, further analysis of this is necessary. It is also necessary to increase the spread of the model ensemble in order to obtain a more accurate reconstruction of the plume transportation. Another interesting and unresolved problem is the area of mid-level concentration, from 1 to 10 Bq m−3, in the Kantou region that persisted from the evening of 20 March to the morning of 21 March. It is also important to carefully evaluate the performance of the model when simulating the vertical stratification of the atmosphere, which controls the dry deposition process. A future study should address aerosol survival under strong but stochastic precipitation conditions. On the other hand, SCJ (2014) suggested that all of the models evaluated tended to underestimate wet deposition in weak precipitation conditions. Consequently, it may be necessary to develop a nonlinear parameterization of the precipitation rate.
We have analyzed only 25% of the total SPM sampling tapes to date, so additional analysis is necessary. Further efforts are also needed to collect missing observational data, in particular, in the Hamadori region, in order to investigate the detailed atmospheric transportation processes that could not be addressed by the present study. Some of the SPM tapes were discarded by the network before we could retrieve them. Therefore, a special tape conservation effort would be required for large-scale disaster events. We expect the present study to be useful for future research.
Community multiscale air quality
Fukushima Daiichi Nuclear Power Station
Japan Atomic Energy Agency
Japan Aerospace eXploration Agency
Japan Meteorological Agency
Japan standard time
Mesoscale objective-analysis data
Ministry of Education, Culture, Sports, Science and Technology, Japan
Nonhydrostatic icosahedral atmospheric model
NICAM single-moment scheme with six water categories
Science Council of Japan
Suspended particulate matter
Spectral radiation-transport model for aerosol species
Tokyo Electric Power Company
United Nations Scientific Committee on the Effects of Atomic Radiation
Coordinated universal time
Weather Research and Forecast Model
We greatly appreciate the kind support of H. Tomita and H. Yashiro of RIKEN/AICS. We also thank all the local governments who offered the used SPM filter tapes at the request of the Ministry of the Environment, Japan. We have cited the weather maps of Japan Meteorological Agency. We used the meteorological data sets of the Japan Meteorological Agency.
Parts of this research were supported by funds from MOE/GOSAT2, JST/CREST/EMS/TEEDDA, JAXA/EarthCARE&GCOM-C, MEXT/KAKENHI/Innovative Areas 2409, MOEJ/ERTDF/S-12 and 5-1501, and the National Regulation Agency, Japan.
TN was the principal investigator of the modeling and SPM data analysis projects, the chair of the SCJ Model Comparison Committee, designed the combined analysis, and drafted the manuscript. SM, DG, JU, TT, and MS conducted the numerical simulations with NICAM-SPRINTARS. YM and TO conducted the numerical simulations with WRF-CMAQ. HT, TO, and ME conducted the SPM tape analysis. All co-authors participated in discussions about the results and commented on the original manuscript. All authors read and approved the final manuscript.
Correspondence and requests for materials should be addressed to TN (email@example.com).
The authors declare that they have no competing interests.
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