Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2
DOI 10.1186/s40645-017-0117-x

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

Model depiction of the atmospheric flowsof radioactive cesium emitted from the
Fukushima Daiichi Nuclear Power Station
accident
Teruyuki Nakajima1,2*, Shota Misawa1, Yu Morino3, Haruo Tsuruta1,4, Daisuke Goto3, Junya Uchida1,
Toshihiko Takemura5, Toshimasa Ohara3, Yasuji Oura6, Mitsuru Ebihara6 and Masaki Satoh1

Abstract
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 theneed 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 anddissipating phases of each event, depending on the meteorological conditions.
Keywords: Fukushima Nuclear Power Station accident, Aerosols, Radioactive materials, 137Cs, Chemical transportmodeling, Ensemble models

Introduction
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 wascaused 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.
Takemura et al. (2011) show that the negative anomaly
of a 500-hPa height over the Okhotsk Sea area along 145°
E made the westerly jet stronger than the climatological* Correspondence: terry-nkj@nifty.com
1
Atmosphere and Ocean Research Institute (AORI), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan
2
Earth Observation Research Center (EORC), Japan Aerospace Exploration
Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan
Full list of author information is available at the end of the article

mean during mid-March; consequently, 70 to 80% of theradioactive material from the FDNPS was driven to the
Pacific Ocean and the rest of the globe (Takemura et al.
2011; Stohl et al. 2012; Mészáros et al. 2016). The
remaining material spread over and deposited onto theland area of Japan, producing characteristic hot spot
patterns (Yasunari et al. 2011; JAEA 2012; SCJ 2014). The
total emission of 137Cs into the atmosphere until the endof April was estimated to be 14.6 ± 3.5 PBq (SCJ 2014).
The ratio of the total deposition over the Japanese landarea to the total atmospheric emission was estimated as
20 ± 6%, according to the airborne monitoring conductedby the Ministry of Education, Culture, Sports, Science,
and Technology, Japan (MEXT 2011), whereas the ratio
was calculated as 27 ± 10% based on the multi-modelintercomparison by the Science Council of Japan (SCJ
2014). To date, this inconsistency has not been fully
understood, owing to the lack of observation data, which
is attributable to instrumental damage and electric outages© The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

as well as modeling uncertainties. In addition, there is
still great uncertainty in the emission time series of theradioactive material, as shown in Fig. 1. Yumimoto etal. (2016) conducted an inverse analysis to optimallyestimate the emission rate using the time series of thedeposition map, but the result is very different fromthat of Katata et al. (2015).
Recently, Tsuruta et al. (2014) developed a method to
directly measure the hourly time series of the atmospheric137
Cs concentration at surface level, from the aerosol sampling tapes of the national suspended particulate matter(SPM) network. The SPM network monitors air pollution
by employing beta-ray attenuation counters. Four laboratories, namely, those of Tokyo Metropolitan University, theNuclear Professional School of the University of Tokyo,
the Japan Atomic Energy Agency, and the Japan Chemical
Analysis Center, retrieved the atmospheric loading fromthe hourly aerosol spots on the SPM tape. This method offers the potential for studying the atmospheric transport of137
Cs, although the data is from surface level, during theentire post-accident period; the SPM dataset has high temporal and spatial sampling, with observations every hour at
90 out of 400 sites (Fig. 2). In Fig. 2, it can be seen that the
Nakadori region is a channel basin area between the Ou
and Abukuma mountains, while the Hamadori region is acoastal region to the east of the Abukuma mountains. The
FDNPS is located in the northern part of the Hamadori region. In this report, we compare the ensemble results of
two aerosol transport models with SPM data. An important purpose of the comparison is to investigate the validityof the combined use of SPM data and multi-model simulations to depict the transportation of atmospheric 137Cs
over the Japan land area. Once validated, further analysis
can be performed on a larger volume of SPM data, such as
the most recent data from 99 SPM sites, which has recently been made available to the public (Oura et al. 2015).
In addition, the results could be a useful input for our
second model intercomparison, which is intended as afollow-up to the first comparison, which was made by the
SCJ (SCJ 2014), and this can contribute to future discussions of the use of models in emergency protocols.
Tsuruta et al. (2014) identified nine plumes, as listed
in Fig. 3, that transported particulates to the land area ofPage 2 of 18

Japan and in which the maximum atmospheric 137Csconcentration exceeded 10 Bq m−3, based on a synoptic
analysis using a time series of the SPM data and thewind vector field. For purposes of comparison, we selected plumes P2 to P9 in the period 14–24 March 2011.
In this period, there were two migrations of low pressure
systems over Japan; these occurred on 15 and 20 March,according to the weather maps shown in Fig. 4.

Methods/Experimental
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 thisas the N-model. The NICAM model is implemented
with an air-mass flux-type dynamic scheme of highmass conservation, and it can cover a global-to-regional
simulation with its three grid systems, which include aquasihomogeneous global grid, a stretched grid, and adiamond regional grid (Uchida et al. 2015). We adopted
the diamond grid system for a regional simulation withthe 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 singlemoment 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 theJapanese 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, theatmospheric 137Cs was treated as a sulfate aerosol particle

Fig. 1 Time series of the 137Cs emission rate from the FDNPS, as estimated by Terada et al. (2012), Katata et al. (2015), and Yumimoto et al. (2016)

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 2 Names of key regions and locations of SPM sites at the time of accident for the present study. The Tohoku region is the northeastern part of theJapanese islands and includes the Fukushima and Miyagi prefectures; the Kantou region is the area that includes the Tokyo, Saitama, Chiba, Kanagawa, andIbaraki prefectures. The FDNPS is located in the northern part of the Hamadori region, a coastal area to the east of the Abukuma Mountains. The Nakadori
region is a channel basin area between the Ou and the Abukuma mountains. Open circles are SPM monitoring sites managed and maintained by local
governments in eastern Japan before the accident. The base map was modified by using the original map in Fig. 1 of Tsuruta et al. (2014)

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

Fig. 3 Plumes identified by Tsuruta et al. (2014). Horizontal bars
show the period with high 137Cs concentrations (>10 Bq m−3).
Closed and open circles indicate areas in the Hamadori, Nakadori,
and Kantou regions where the concentrations were larger orsmaller, respectively, than 100 Bq m−2

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 ofPage 4 of 18

0.24 μm, taking into account the observed size distribution by Kaneyasu et al. (2012) and an in-cloudscavenging coefficient defined by Takemura et al.
(2000) as 0.8 (Goto et al. 2015a).
The other model was a meteorological model, theweather 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 Schere2006); 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 thepresent simulation, all the 137Cs was assumed to be inparticulate 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 toFig. 4 a–f Weather maps based on JMA analysis at 9:00 JST in the analysis periodNakajima et al. Progress in Earth and Planetary Science (2017) 4:2

be 0.5 μm, and the geometrical dispersion σg was assumed tobe 1.1 (Sparmacher et al. 1993; Sportisse 2007; Kaneyasu etal. 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 themodel ensemble results with the observed atmospheric 137Csconcentration at surface level. The total deposition overJapanese land was simulated as 2.12 PBq by the Nmodel and 2.21 PBq by the W-model (Morino et al.
2013). These values belong to the small depositiongroup in the SCJ model comparison (SCJ 2014), whichgave a mean value of total deposition of 2.92 PBq.
Both models were nudged with three-hourly 5 × 5 km
mesoscale objective analysis data (MANAL) from the Japan
Meteorological Agency (JMA). We selected the emission
scenario of Terada et al. (2012), which has been used inmany previous studies and is thus suitable for comparison
with them. Figure 5 shows two examples of the simulated
and observed atmospheric 137Cs used to study the performance of both the N- and the W-model for specific cases,
taken from the detailed time sequences as shown in Figs. 6
and 7 with precipitation maps of the JMA analysis and the
N-model. These two examples were 15 March, 12:00 JST(3/15/12 h), and 21 March, 9:00 JST (3/21/9 h), when highconcentration plumes of more than 100 Bq m−3 were observed in the Kantou region. In Fig. 5, observed concentrations are indicated by color-coded circles, and the data
below the detection limit of 0.1 Bq m−3 are indicated by triangles. On 15 March, a dry process governed the transportation, and there was no significant precipitation until latethat evening, as indicated by the JMA analysis shown in
Fig. 6. The distribution of the atmospheric 137Cs was wellsimulated by the two models for a wide range of concentrations, from 0.1 to 100 Bq m−3. It should be noted that theobserved high-concentration area was confined to a narrow
region, which was consistent with the simulated plume-wise
distribution starting from the FDNPS for the Kantou region.
The plumes simulated by the N-model were less diffused(narrower) than those simulated by the W-model. Judging
by the correlation coefficients of the observed and simulated
distributions, reality is assumed to fall between these two results; this is further discussed in the following paragraph.
For the March 21 event, the results of the N- and Wmodels differed significantly, as can be seen in Fig. 5c, d,respectively. Morning precipitation occurred in the
Kantou region on 21 March, as will be shown in Fig. 7b.
However, the simulated atmospheric concentration of137
Cs was generally smaller than the observed values,
suggesting that the simulated wet deposition was toostrong for the W-model. Wet deposition during the total
period over land in Japan was simulated as 1.68 and2.12 PBq by the N- and W-model, respectively.

Page 5 of 18

As indicated in Fig. 5, the accuracies of the simulated
plume patterns of the two models differed from event toevent. These differences were also reported by SCJ (2014).
One way to reconcile the differences and construct statistically optimum distributions is to use a multi-model ensemble method to calculate the weighted mean concentration
C k for the kth grid point of the N-model at the jth time
step, as given byC k;j ¼ 
C N;k;j C W;k;j
1
1= 2 þ 2 ;
þ 2
σN σW
σ 2N
σW

ð1Þ

where CN,k,j and CW,k,j are the values of the N- and Wmodel, respectively. The grid values of the W-model arelinearly interpolated onto the N-model grid. The quantities σN and σW are the daily root mean square deviations of the respective model values from the observed
values. Figure 8 shows a comparison of the observed
values with the values obtained by the N-model alone
and with the two-model ensemble means constructed by
the proposed method. The daily correlation coefficientsfor specific days (15, 20, and 21 March) are listed inTable 1. The correlation coefficients were calculatedusing the daily means of the observed and simulated
values at the surface level, as linearly interpolated at thesite locations. The ensemble method constructs an
atmospheric 137Cs distribution that had a correlation coefficient better than that of either model alone. However,
20 March was an exception; the correlation coefficientof the two-model ensemble method was not as good asthat of the N-model, although it was superior to that of
the W-model. The 3-day average of the correlation coefficients was improved to 0.51 by the two-model ensemble method, from 0.48 obtained with the N-model and0.46 with the W-model.

Results and discussion
Structure of atmospheric 137Cs plumes

In this section, we examine the plume formation anddissipation during each of three periods: 15–16 March,
18–19 March, and 20–21 March 2011.
15–16 March

Land plume events in the Kantou and Tohoku regions
lasted from the morning of 15 March until noon of thenext day. Time sequences of the 137Cs concentration of
the two models and the observations are shown in
Fig. 6a, b, along with precipitation maps of the JMA
analysis and those of the N-model. Two-model ensemble means and the wind vectors of the JMA analysis areshown in Fig. 9 for several characteristic times. During
the morning of 15 March, a low-pressure system with aweak trough structure was located in the Pacific region
off the Kantou region (Fig. 4a). A northeasterly windNakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 5 Comparison of the atmospheric 137Cs (Bq m−3) for events on 15 March, 12:00 and 21 March, 9:00, as estimated by the NICAM-SPRINTARS
model (N; a, c) and the WRF-CMAQ model (W; b, d). Observed values are indicated by color-coded circles (Bq m−3); triangles indicate data below
the detection limit (0.1 Bq m−3) of SPM measurements

prevailed at the northern edge of the low-pressure
system until 9:00, replacing the southwesterly wind associated with the high-pressure system of the previous
day (not shown). This northeasterly wind transported
radioactive material from the FDNPS southward toform plume P2, departing from the FDNPS at midnight
and arriving at the Kantou region around 4:00 (Fig. 6a,panel 4:00), at which point the concentration exceeded

several Bq m−3, according to observations by the SPM
network. Figure 6a, panels 0:00 to 4:00, indicates thatthe arrival time and location of the tongue of simulated
plume P2 appear to be consistent with the observed
concentrations shown by color-coded circles. The
model simulation of plume transportation was relatively
easy in this early phase during which there was no
precipitation.

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 6 Maps of the atmospheric 137Cs concentration at surface level as simulated by the N- and W-models for the periods of a 3/15/0 h to 3/15/
12 h and b 3/15/15 h to 3/16/9 h. The observed concentrations are indicated by colored circles. Precipitation maps are also shown for the JMA
analysis and the N-model

Simulation results of the maximum stage of plume P2
in the Kantou region is, however, different from the
SPM observations; a thick simulated plume as large astens of Bq m−3 arrived around 6:00 at the Saitama,
Tokyo, and Kanagawa prefectures (Fig. 6a, panels 6:00 to
9:00, and Fig. 9a, b), whereas the observed data did notNakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 7 Maps of the atmospheric 137Cs concentration at surface level as simulated by the N- and W-models for the periods of a 3/20/6 h to 3/20/
21 h and b 3/21/1 h to 3/21/11 h. The observed concentrations are indicated by colored circles. Precipitation maps are also shown for the JMA
analysis and the N-modelshow an increase to more than 10 Bq m−3 until 9:00, indicating a later arrival. Such a large difference in the arrival
time of the plume cannot be explained by model errors,

given that there were relatively simple meteorological conditions with a dry process, and the N- and W-models had
similar results, as shown in Fig. 6a, panels 0:00 to 9:00.

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 8 Scatter plots of the observed atmospheric 137Cs concentration (Bq m−3) and the values obtained by the N-model and the two-model ensemble
mean (a, b). The area in which the data was below the detection limit (0.1 Bq m−3) is shaded

One possible cause of this is the large emission fromFDNSP around 22:00 on 14 March, in the emission
scenario of Terada et al. (2012). Figure 1 shows that the
updated scenario of Katata et al. (2015), on the other
hand, suggests a large emission occurred later, around1:00 on 15 March. To investigate this possibility, Fig. 10shows the simulation results of the N-model for the
scenario of Terada et al. (2012) and that of Katata et al.
Table 1 Daily correlation coefficients between observed andsimulated 137Cs concentrations based on N-model, W-model,
and two-model ensemble (N + W) results for 137Cs
Day (JST)

N

W

N+W

2011/3/15

0.51

0.52

0.55

2011/3/20

0.57

0.50

0.55

2011/3/21

0.36

0.37

0.44

Average

0.48

0.46

0.51

(2015). It can be seen in the figure that with the scenario
of Katata et al. (2015), the high-concentration area inthe Kantou region, becomes smaller and is more consistent with the observations.
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 theplume 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 highconcentration area, but the emission scenario of Katata
et al. (2015) gives a better result (Fig. 10). The SPM datasuggest 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

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 9 a–f Maps of observations (colored circles) and model ensemble means of atmospheric 137Cs concentration from 3/15/9 h to 3/16/9 h. Areas
identified as plumes with concentration larger than 10 Bq m−3 are encircled and labeled with the name of the plume. JMA-MANAL wind vectors
at 1000 hPa are superimposed. Movement of the P3 plume route is indicated by a thick arrow in panel dmaterial, due to a patch of precipitation between FDNSP
and Kantou; however, the W-model simulation showed aslower dissipation.
The wind vector fields shown in Fig. 9c, d indicate thata weak cyclonic motion around the Central Fukushima
area was generated until noon of 15 March; this wasprobably due to solar heating. It cause a gradual spreadof plume P2 toward the western part of the Tokyo area,
as indicated by a thick arrow in Fig. 9d. The wind fieldalso transported plume P3 toward the Nakadori region
in the afternoon of 15 March, when high concentrationsof 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 detourto the edge of the Abukuma mountains and into the
Nakadori region from north to south, as shown byNakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 10 Atmospheric 137Cs distributions simulated by the N-model at 3/15/6 h and 3/15/9 h with emission scenarios by Terada et al. (2012) and
Katata et al. (2015)

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.
Subsequently, precipitation occurred over the Nakadoriand Hamadori regions during the night of 15 March, lasting until the morning of 16 March and dissipating plumeP3, as shown in Fig. 6b, panels 7:00 to 9:00, and Fig. 9f.
These figures indicate that the next plume, P4, was transported by a northerly wind that became dominant duringNakajima et al. Progress in Earth and Planetary Science (2017) 4:2

the morning of March 16, due to increased activity of the
low in the Pacific (Fig. 4b). The SPM sites in the Choshi
Peninsula observed a high concentration for several hours
during the morning of 16 March, although the simulated
plume failed to reach the target area (Fig. 9f). A possible
explanation for the model failure is that the simulated
aerosol height was higher than the actual height. According to the JMA analysis, there was considerable wind
shear during the morning of 16 March, i.e., a northeasterly
wind prevailed below 600 m, and a northwesterly wind
prevailed above 600 m, as indicated by the vertical profilesof the atmosphere at the JMA Tsukuba station (36.06° N,
140.13° E) shown in Fig. 11. In a trajectory analysis using
MANAL data, Miyasaka (Takafumi Miyasaka, Universityof Tokyo, personal communication, 2015) estimated thatthe aerosol height for the air mass that reached the Kantou region should be lower than 500 m. However, at thelatitude 36.7° N, the simulated aerosols were distributed
up to an altitude of 1500 m. Consequently, the simulated
aerosols were transported to the Pacific area once they
were uplifted to that level; this is indicated by the factthat the plume is not parallel to the 1000-hPa wind vector. There is a report that NICAM-SPRINTARS tends
to overestimate the vertical transport of aerosols (Gotoet al. 2015b). Another possible reason is overestimationof the wet deposition by the early morning precipitation
along the plume on 16 March; the simulation showed
this to be shifted slightly southward and was more thanthe MANAL objective analysis, as indicated by Fig. 6b,panel 3:00.
Another notable difference between observations andthe 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/

Fig. 11 Vertical profiles of temperature (T), relative humidity (RH), winddirection (WD), and wind speed (WS) versus geopotential height at 3/16/
9 h for the JMA Tsukuba station. Wind directions and speeds are at theheights corresponding to the standard isobaric surfaces and characteristic
points in the wind profile as determined by the JMA routine
measurement protocol

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16/0:00 to 7:00; Fig. 9f), whereas the simulation results, inparticular, those of the W-model, show a much smaller
concentration in the southern part of the Hamadori andKantou 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 ofaccurately simulating transport in a strongly sheared, thin
atmosphere.
18–19 March

Figure 12a, b shows the observed and simulated plumes P5
and P6 in the short-period events of 18 and 19 March. A
migrating high-pressure system passed the Japanese islandson those days (Fig. 4c, d), and a weak wind condition wasestablished around the FDNPS. The northwesterly component of the wind field turned southwesterly along the outskirts of the high-pressure zone at approximately 15:00 on
18 March (not shown). The P5 plume driven by the southwesterly wind was subsequently observed at sites north of
the FDNPS. It should be noted that the wind vector at 18:00
was relatively perpendicular to the plume with this transportmechanism. A similar oblique relation between the windvector and the plume axis was also simulated for plume P3(Fig. 9d) and plumes P7 and P9 (shown below in Fig. 13b, f)
under quick changes in the dominant wind direction. Thesephenomena indicate that the temporal change in winddirection must be accurately reproduced in order to successfully simulate the plume. Moreover, it is also necessary
to have higher resolution of the land-sea breeze simulation.
20–21 March

A subsequent low-pressure system passed the Japaneseislands during 20–21 March (Fig. 4e, f ). Time sequencesof observed and two-model atmospheric 137Cs concentrations with precipitation maps are shown in Fig. 7, andmodel ensemble results are shown in Fig. 13 for several
characteristic times. The figures indicate the dominantwesterly wind on 19 March through the early morningof 20 March at the FDNPS blew the plume toward thePacific, 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 amoving low-pressure system; this blew the plume backwestward toward the Hamadori and established plumeP7 (labeled B in Fig. 13a), which had a wide distribution
covering the Hamadori and its offshore areas, as shownin 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-branchpattern 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Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

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Fig. 12 Maps of observations (colored circles) and model ensemble means of atmospheric 137Cs concentration in a 3/18/18 h and b 3/19/12 h.
Areas identified as plumes with concentration larger than 10 Bq m−3 are encircled and labeled with the name of the plume. JMA-MANAL wind vectors
at 1000 hPa are superimposedby a thick arrow in Fig. 13a, was successfully simulatedby 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 consequenceof more transportation to the upper atmosphere near
the emission source, as suggested by the shift to thenorth (relative to the results of the N-model) of the highconcentration around the FDNPS (Fig. 7a, panels 12:00
to 13:00).
Northwestern transport started around 13:00 andlasted 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 ofplume 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.
Plumes P7 and P8 started to dissipate at around 3:00
on March 21 (Fig. 7b, panels 1:00 to 7:00, and Fig. 13e).
At this time, plume P9 started traveling to Kantou, due
to a northeasterly wind ascribed to the combined effectof a migratory system and the Okhotsk low-pressure system (Fig. 4f ). Plume P9 reached the Kantou region ataround 9:00 on 21 March and collided with the weather

front located at the southern part of the Kantou region,
as shown by the wind vectors in Fig. 13f. The plume
simulated by the N-model had a tongue shape and covered the land area of the northern Chiba prefecture; thiswas consistent with the distribution of the observed sites
characterized by concentrations of more than 100 Bq m−3
around the Chiba prefecture and Tokyo Bay, as depicted
in Fig. 7b, panels 9:00 to 10:00. However, the N-model
plume was too short, and it shifted to the northern area
without reaching the observed sites. In this event, the Wmodel also failed to transport 137Cs to the Kantou region.
Although not shown by a figure, we found that changing
the emission scenario to that of Katata et al. (2015) did
not improve the results. Another possibility is that theheight of the aerosol layer was overestimated. Figure 14
shows the vertical profile of meteorological parameters at
the JMA Tsukuba station at 9:00 on 21 March. The figure
indicates that there was a strong temperature inversion ataround 300 m geopotential height. Wind was northeasterly below the inversion and northwesterly above. Therefore, it can be assumed that the modeled aerosol layer was
too high, and the aerosols were transported eastward by
the northwesterly. This also might be due to the stronger
precipitation that was simulated by the models in the
Kantou region during the time period 3:00 to 7:00
(Fig. 13b, panels 3:00 to 7:00). These two reasons may

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

Page 14 of 18

Fig. 13 a–f Maps of observations (colored circles) and model ensemble means of atmospheric 137Cs concentration from 3/20/9 h to 3/21/9 h.
Areas identified as plumes with concentration larger than 10 Bq m−3 are encircled and labeled with the name of the plume. JMA-MANAL wind vectors at
1000 hPa are superimposed. Movement of the P7 plume route is indicated by a thick arrow in panel a. See the main text for explanation of labels (a, b)

Fig. 14 Vertical profiles of temperature (T), relative humidity (RH), wind direction (WD), and wind speed (WS) versus geopotential height at 3/21/9 h
for the JMA Tsukuba station. Wind directions and speeds are at the heights corresponding to the standard isobaric surfaces and characteristic points inthe wind profile as determined by the JMA routine measurement protocol

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

explain the quick dissipation of simulated plume P7, in
contrast to the observed persistent survival of the plumein the morning of 21 March (Fig. 7b, panels 3:00 to 9:00,
and Fig. 13e, f).
Plume routes and time series

Figure 15 presents a summary of the plume routes
analyzed in the preceding sections, and Fig. 16 shows
the time series of the observations and the model ensemble mean 137Cs concentrations at several sites.
The relevant sites are B, C, E, and J in the Tohoku
region and sites 9, 12, and 15 in the Kantou region,
as defined by Tsuruta et al. (2014), as well as at the
newly added sites K1, K2, and K3.
During a 6-hour period on 15 March, plume P2 spreadover 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, andK3 in Fig. 16b. However, the simulated arrival time wastoo 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 thesouth to the north at sites E and C in the afternoon of
15 March (Fig. 16a); the models were too coarse to

Page 15 of 18

simulate the detailed progress of the plume along the
Nakadori channel. The progress of plumes P2 and P3 isindicated 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 atsite J on 18 and 19 March, and they were simulatedby 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 wastransported over a long distance, first to the ocean andthen toward land by a clockwise rotation of the plumeroute, as indicated by the thick white arrow in Fig. 15; itcovered a large part of the northern Kantou region, as
shown in Fig. 16b.
Plume P8 was transported northward and was redirectedsouthward along the Nakadori region at the northern edgeof the Abukuma Mountains. Consequently, the highconcentration 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 thisphenomenon, underestimating the concentration at sites
in the Nakadori region, as also shown in Fig. 7a, panels

Fig. 15 a, b Schematic diagrams of transport routes analyzed in the present study for plumes P2, P3, P4, P7, P8, and P9. Thick arrows indicate thegeneral trend of the movement of each plume

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

Page 16 of 18

Fig. 16 Time series of observed (dots) and two-model ensemble mean (lines) atmospheric 137Cs concentrations (Bq m−3) from 14 to 24 March ain the Tohoku region at sites J, B, C, and E (see Fig. 15) and b in the Kantou region at sites 9, 12, 15, K1, K2, and K3 (see Fig. 15)

15:00 to 21:00. At the same time, the wind direction
turned from northwest to north at the FDNPS, with theresult 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 thepeaks at sites 9, 12, and 15 at 9:00; however, this is dueto 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 werecaused 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.

Conclusions
As has been discussed in the preceding sections, wefound 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 ofhigh 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 geographicalconditions. Although in some cases, the models failed tosimulate the exact location and time of arrival of theplumes at the SPM sites, the spatial and temporaldevelopment 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 atmospherictransportation 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, twopeaks can be seen in the time series: one due to plumes
P2 and P3 followed by P4, during 15–16 March, and theother due to plumes P7 and P8 followed by P9, during
20–21 March. The first peak (P3 on 15–16 March) wascaused 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 theJapanese islands to the Pacific. When the height of theaerosol 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

Nakajima et al. Progress in Earth and Planetary Science (2017) 4:2

that there was no significant difference between 3-km and
500-m grid simulations of the JMA nonhydrostatic modelfor horizontal transport of radioactive material. However,in view of the differences in model parameterization for
orographic waves, further analysis of this is necessary. It isalso necessary to increase the spread of the model ensemblein order to obtain a more accurate reconstruction of theplume 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 theevening of 20 March to the morning of 21 March. It is also
important to carefully evaluate the performance of themodel when simulating the vertical stratification of theatmosphere, 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 evaluatedtended 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 samplingtapes to date, so additional analysis is necessary. Further
efforts are also needed to collect missing observationaldata, in particular, in the Hamadori region, in order toinvestigate 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 tapeconservation effort would be required for large-scaledisaster events. We expect the present study to be usefulfor future research.
Abbreviations
CMAQ: Community multiscale air quality; FDNPS: Fukushima Daiichi Nuclear
Power Station; JAEA: Japan Atomic Energy Agency; JAXA: Japan Aerospace
eXploration Agency; JMA: Japan Meteorological Agency; JST: Japan standard
time; MANAL: Mesoscale objective-analysis data; MEXT: Ministry of Education,
Culture, Sports, Science and Technology, Japan; NICAM: Nonhydrostatic
icosahedral atmospheric model; NSW6: NICAM single-moment scheme with
six water categories; SCJ: Science Council of Japan; SPM: Suspended particulatematter; SPRINTARS: Spectral radiation-transport model for aerosol species;
TEPCO: Tokyo Electric Power Company; UNCEAR: United Nations ScientificCommittee on the Effects of Atomic Radiation; UTC: Coordinated universal time;
WRF: Weather Research and Forecast Model

Acknowledgements
We greatly appreciate the kind support of H. Tomita and H. Yashiro of
RIKEN/AICS. We also thank all the local governments who offered theused 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.

Funding
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.

Page 17 of 18

Authors’ contributions
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 readand approved the final manuscript.
Authors’ information
Correspondence and requests for materials should be addressed to TN(terry-nkj@nifty.com).
Competing interests
The authors declare that they have no competing interests.
Author details
1
Atmosphere and Ocean Research Institute (AORI), The University of Tokyo,
5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8568, Japan. 2Earth Observation
Research Center (EORC), Japan Aerospace Exploration Agency (JAXA), 2-1-1
Sengen, Tsukuba, Ibaraki 305-8505, Japan. 3National Institute forEnvironmental Studies (NIES), 16-2 Onogawa, Tsukuba, Ibaraki 305-8506,
Japan. 4Remote Sensing Technology Center of Japan, 3-17-1 Toranomon,
Minato-ku, Tokyo 105-0001, Japan. 5Research Institute for Applied Mechanics,
Kyushu University, Kasuga Park 6-1, Kasuga, Fukuoka 816-8580, Japan.
6
Graduate School of Science and Engineering, Tokyo Metropolitan University,
1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan.
Received: 10 June 2016 Accepted: 5 January 2017

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