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Earth Syst. Sci. Data, 14, 463–477, 2022
https://doi.org/10.5194/essd-14-463-2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
Homogenized century-long surface incident solar
radiation over Japan
Qian Ma, Kaicun Wang, Yanyi He, Liangyuan Su, Qizhong Wu, Han Liu, and Youren Zhang
State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth
System Science, Beijing Normal University, Beijing 100875, China
Correspondence: Kaicun Wang (kcwang@bnu.edu.cn)
Received: 6 July 2021 – Discussion started: 23 August 2021
Revised: 17 December 2021 – Accepted: 19 December 2021 – Published: 4 February 2022
Abstract. Surface incident solar radiation (Rs) plays a key role in climate change on Earth. Rscan be directly
measured, and it shows substantial variability on decadal scales, i.e. global dimming and brightening. Rscan
also be derived from the observed sunshine duration (SunDu) with reliable accuracy. The SunDu-derived Rshas
been used as a reference to detect and adjust inhomogeneity in the observed Rs. However, both the observed Rs
and SunDu-derived Rsmay have inhomogeneity. In Japan, SunDu has been measured since 1890, and Rshas
been measured since 1961 at ∼100 stations. In this study, the observed Rsand SunDu-derived Rswere first
checked for inhomogeneity independently using the statistical software RHtests. If confirmed by the metadata of
these observations, the detected inhomogeneity was adjusted based on the RHtests quantile-matching method.
Second, the two homogenized time series were compared to detect further possible inhomogeneity. If confirmed
by the independent ground-based manual observations of cloud cover fraction, the detected inhomogeneity was
adjusted based on the reference dataset. As a result, a sharp decrease of more than 20 W m−2in the observed
Rsfrom 1961 to 1975 caused by instrument displacement was detected and adjusted. Similarly, a decline of
about 20 W m−2in SunDu-derived Rsdue to steady instrument replacement from 1985 to 1990 was detected
and adjusted too. After homogenization, the two estimates of Rsagree well. The homogenized SunDu-derived
Rsshow an increased at a rate of 0.9 W m−2per decade (p < 0.01) from 1961 to 2014, which was caused by
a positive aerosol-related radiative effect (2.2 W m−2per decade) and a negative cloud cover radiative effect
(−1.4 W m−2per decade). The brightening over Japan was the strongest in spring, likely due to a significant
decline in aerosol transported from Asian dust storms. The observed raw Rsdata and their homogenized time
series used in this study are available at https://doi.org/10.11888/Meteoro.tpdc.271524 (Ma et al., 2021).
1 Introduction
Surface incident solar radiation (Rs) plays a vital role in
atmospheric circulation, hydrologic cycling, and ecologi-
cal equilibrium; therefore, its decrease and increase, termed
global dimming and brightening (Wild et al., 2005; Shi et
al., 2008), have received widespread interest from the pub-
lic and scientific community (Allen et al., 2013; Xia, 2010;
Wang et al., 2013; Tanaka et al., 2016; Ohmura, 2009; He
et al., 2018). In addition, the impact factors such as clouds
and aerosols on the variation in Rshave been widely studied
(Wild et al., 2021; Qian et al., 2006; Feng and Wang, 2021a).
Ground-based observations of Rsare the first recommen-
dation for detecting global dimming and brightening. How-
ever, observational data may be inevitably ruined by artifi-
cial shifts, which my lead to the variability in Rswith large
uncertainties. Wang et al. (2015) point out that instrument
replacements and the reconstruction of the observational net-
work introduced substantial inhomogeneity into the time se-
ries of observed Rsover China for 1990–1993. Manara et
al. (2016) also show the instrument changes from the Rob-
itzsch pyranometer to the Kipp & Zonen CM11 pyranometer
before 1980 caused no clear dimming in Italy. Until recently,
Wild et al. (2021) use a well-maintained data series at a site
Published by Copernicus Publications.
464 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
in Germany with long time duration to investigate the dim-
ming and brightening in central Europe under clear-sky con-
ditions and point out that the aerosol pollutants are likely ma-
jor drivers in the Rsvariations. Augustine and Hodges (2021)
use Surface Radiation Budget (SURFRAD) Network obser-
vations to explore the variability in Rsover the US from 1996
to 2019 and find that cloud fraction can explain 62 % of the
variation of Rs, while aerosol optical depth (AOD) only ac-
counts for 3 %. Both studies also indicate the measurement
instruments have been changed over the observational time
periods, which may introduce non-climatic shifts and inho-
mogeneity in the raw data series.
Homogenizing the observed Rshas been attempted in
China (Wang et al., 2015; Tang et al., 2011; Yang et
al., 2018), Italy (Manara et al., 2016), Spain (Sanchez-
Lorenzo et al., 2013), and Europe (Sanchez-Lorenzo et
al., 2015). It is essential to find a homogeneous reference
station to compare with the possible inhomogeneous sta-
tion to test and adjust the inhomogeneity in the observed
time series, as done for the homogenization of air temper-
ature (Du et al., 2020; Zhou et al., 2021). However, this pro-
cess is difficult for Rsbecause the instrument replacement
of Rsgenerally occurs nearly simultaneously throughout a
country. Therefore, the sunshine-duration-derived (SunDu-
derived) Rs(Yang et al., 2006) has been used as a homo-
geneous reference dataset to detect and adjust the inhomo-
geneity in Rsin China (Wang et al., 2015).
SunDu records the hours of surface direct solar radiation
exceeding 120 W m−2and provides an alternative way to es-
timate Rs(Yang et al., 2006; Stanhill and Cohen, 2008).
SunDu-derived Rsis capable of capturing the variability in
Rs. He et al. (2018) use the SunDu-derived Rsat ∼2600
stations to revisit the global dimming and brightening over
different continents and restate the dimming over China and
Europe is consistent with the increasing trend of clouds and
aerosols. Feng and Wang (2021a, b) merge the satellite re-
trievals with SunDu-derived Rsto produce high-resolution
long-term solar radiation over China and indicate the cloud
fraction could explain approximately 86 %–97% of Rsvaria-
tion. Zeng et al. (2020) demonstrate that SunDu plays a dom-
inant role in determining Rsbased on a random forest model
framework across China. Stanhill and Cohen (2005) indicate
the high correlation between SunDu and Rsat the 26 stations
in the United States. Sanchez-Lorenzo et al. (2008) show the
variation in SunDu is consistent with that in Rsover west-
ern Europe for 1938–2004 and the SunDu time evolution in
spring can partly be explained by clouds and that in win-
ter can be related to the anthropogenic aerosol emissions.
Stanhill and Cohen (2008) establish a simple linear relation-
ship between Rsand SunDu to determine the long-term vari-
ation in Rsover Japan. Manara et al. (2017) highlight that at-
mospheric turbidity should be considered when using SunDu
for investigating multidecadal evolution of Rs.
Artificial shifts in SunDu observations may come from
the replacement of instruments. It has been revealed that the
Jordan recorder is 10 % more sensitive than the Campbell–
Stokes recorder for SunDu measurements (Noguchi, 1981).
The homogenization of SunDu has been carried out in the
Iberian Peninsula (Sanchez-Lorenzo et al., 2007), Switzer-
land (Sanchez-Lorenzo and Wild, 2012), and Italy (Manara
et al., 2015).
The measurement of Rs, which started in 1961 in Japan,
has a long history (Tanaka et al., 2016), and a data record of
more than half a century has been accumulated. The dataset
has been widely used to study decadal variability (Wild et
al., 2005; Stanhill and Cohen, 2008) and to evaluate model
simulations (Allen et al., 2013; Dwyer et al., 2010). The Ep-
pley and Robitzsch pyranometers used to measure Rsover
Japan were replaced by the Moll–Gorczynski thermopile
pyranometers in the early 1970s (Tanaka et al., 2016). How-
ever, the possible inhomogeneity of the observed Rsover
Japan has not been well quantified, and most existing stud-
ies directly used raw Rsdata (Wild et al., 2005; Tanaka et
al., 2016; Tsutsumi and Murakami, 2012; Allen et al., 2013;
Wild and Schmucki, 2011; Kudo et al., 2012; Ohmura, 2009).
Some studies have had to abandon data from the early years
and focused on only Rsdata collected after 1975 (Tsutsumi
and Murakami, 2012; Dwyer et al., 2010). Therefore, the ob-
served decadal variability in Rsover Japan is questionable,
especially for the 1961–1975 time period.
In Japan, SunDu observations started in 1890, and more
than a century of data have been recorded. They cannot
be too precious for the climate change detection on a cen-
tury scale. It is reported that the Jordan recorders used
to measure SunDu were replaced by EKO rotating-mirror
recorders in approximately 1986 (Inoue and Matsumoto,
2003; Stanhill and Cohen, 2008). Therefore, SunDu obser-
vations over Japan themselves may suffer inhomogeneity is-
sues.
Non-climatic shifts in the observations may severely in-
fluence the climate assessment; therefore rigorous homog-
enization is required. The World Meteorological Organiza-
tion (WMO) guidelines on climate metadata and homoge-
nization list 14 data homogenization assessment techniques
developed and applied by different groups/authors (Aguilar
et al., 2003). Reeves et al. (2007) compared eight represen-
tative homogenization methods and provided guidelines for
which procedures work best in different situations; for exam-
ple the standard normal homogeneity (SNH) test (Alexander-
sson, 1986) works best if good reference series are available,
and two-phase regressions of the Wang procedure (Wang,
2003) are optimal for an unavailable condition of good ref-
erence series. Based on the comparison work, the RHtests
method was improved by detecting multiple change points in
the climate data no matter if the reference series are avail-
able (Wang, 2008b; Wang et al., 2010, 2007; Wang, 2008a).
This method, which first detects the change points in a series
using penalized maximal tests and then tunes the inhomoge-
neous data segments to be consistent with other segments in
empirical distributions, has been widely used in homogeniz-
Earth Syst. Sci. Data, 14, 463–477, 2022 https://doi.org/10.5194/essd-14-463-2022
Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 465
ing climate variables (Dai et al., 2011; Wang et al., 2010; Du
et al., 2020; Zhou et al., 2021).
Discontinuities inevitably occurred in the long-term ob-
servation system which are required to be checked out and
adjusted in the raw data. The homogenized series pose a sig-
nificant role in the realistic and reliable assessment of cli-
mate trend and variability. The main objective of this study
is to detect and adjust the inhomogeneity in Rsestimates over
Japan. The metadata were first extracted from website infor-
mation and related records at each site. The SunDu observa-
tions were converted into Rs. The RHtests method was ap-
plied to homogenize the observed Rsand SunDu-derived Rs,
and finally, the century-long homogenized Rsdata were pro-
duced over Japan. Furthermore, the impacts of cloud cover
and aerosols on Rsvariation over Japan in recent decades
were explored.
2 Data and methods
2.1 Surface incident solar radiation and sunshine
duration
The monthly observed Rsat 105 stations and SunDu at
156 stations were downloaded from the Japan Meteorology
Agency (JMA) website (see Table S1 in the Supplement
and Fig. 1). Rsrecords were available from 1961. During
the 1960s, two Rsmeasurements were conducted in paral-
lel by both Eppley and Robitzsch pyranometers. In the early
1970s (see Fig. 2 and Table S2), these instruments were re-
placed by Moll–Gorczynski thermopile pyranometers. This
replacement occurred at approximately 12.4 % of Rsstations
in 1971, followed by 22.9 %, 24.8 %, 3.8 %, and 30.5 % in
the next 4 years, which may have caused severe data discon-
tinuity problems (Tanaka et al., 2016).
SunDu has been routinely measured since 1890. Jordan
recorders were replaced by EKO rotating-mirror recorders at
49.4 % of SunDu stations in 1986. Until 1990, nearly all of
the SunDu stations used new instruments for observations;
4.5 % of SunDu stations before 1985 and 9.0 % of SunDu
stations after 2000 were moved away from the original sites
(see Fig. 2 and Table S2) (Stanhill and Cohen, 2008).
In this study, SunDu was used to derive Rsbased on the
following equation (Yang et al., 2006):
Rs/Rc=a0+a1·n/N +a2·(n/N)2,(1)
where nis sunshine duration hours; Nis the maximum pos-
sible sunshine duration; Rcis surface solar radiation under
clear skies; and a0,a1, and a2are coefficients. This method
was recommended in many studies (Wang et al., 2015; Tang
et al., 2011).
2.2 Homogenization method
Both Rsand SunDu measurements over Japan suffer
severe inhomogeneity problems, which require rigorous
data homogenization. RHtests (http://etccdi.pacificclimate.
org/software.shtml, last access: 21 January 2022) is a widely
used method to detect and adjust multiple change points in
a climate data series, such as in surface temperature (Du et
al., 2020), radiosonde temperature (Zhou et al., 2021), pre-
cipitation (Wang et al., 2010), and surface incident solar ra-
diation (Yang et al., 2018). RHtests provides two algorithms,
the penalized maximal t(PMT) test (Wang et al., 2007) and
the penalized maximal F(PMF) test (Wang, 2008b), to de-
tect change points. The problem of lag-1 autocorrelation in
detecting mean shifts in time series was also resolved (Wang,
2008a). The PMT algorithm requires the base time series to
have no trend, and hence a reference series is needed. It is
invalid when a reference series is not often available or its
homogeneity is not sure; also the trend in the base and refer-
ence series are probably different. The PMF algorithm allows
the time series to be in a constant trend and thus is applicable
without a reference series. Both algorithms have higher de-
tection power, and the false-alarm rate can be reduced by an
empirically constructed penalty function.
As the instrument changes for Rsand SunDu observation
happened nearly nationwide and simultaneously, it is difficult
to find reference data series to match the base data series,
and hence the PMF algorithm was used to detect the change
points in this study. Multiple change points were detected
including climate signals and artificial shifts, and only the
ones confirmed by discontinuity information from metadata
in Table S2 were left to be adjusted. Then two homogenized
series based on the direct measurement of Rsand SunDu-
derived Rswere obtained.
Large uncertainties may still exist in both homogenized
data series, as the discontinuities in the raw observations may
not be sufficiently and correctly recorded in the metadata.
Further change points can be detected by considering the im-
pact of the variation in independent climate variables such as
clouds and aerosols on the Rsvariation. If these uncertain-
ties were found, further change point detections were needed
based on the PMT or PMF algorithm.
To diminish all significant artificial shifts caused by the
change points, newly developed quantile-matching (QM) ad-
justments in RHtests (Vincent et al., 2012; Wang et al., 2010)
were performed to adjust the series so that the empirical dis-
tributions of all segments of the detrended base series agree
with each other. The corrected values are all based on the
empirical frequency of the datum to be adjusted.
Another independent homogenization method proposed
by Katsuyama (1987), which was developed due to the
replacement of the Jordan recorders with EKO rotating-
mirror recorders during the late 1980s, is denoted as follows:
SR=0.8SJSJ<2.5hd−1,(2)
https://doi.org/10.5194/essd-14-463-2022 Earth Syst. Sci. Data, 14, 463–477, 2022
466 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
Figure 1. The spatial distribution of stations over Japan with observed sunshine duration (SunDu, 156 stations) and surface incident solar
radiation (Rs, 105 stations) data. The colours indicate the data length of the SunDu records from 1890 to 2015 and Rsrecords from 1961 to
2015. Unit: month.
Figure 2. The fraction of stations that suffer from data inhomo-
geneity due to site relocation, the change of instruments, and the
measurement method for sunshine duration (SunDu) records, cloud
amount (CA) records, and surface incident solar radiation (Rs)
records. In total, there were 156 stations with SunDu records, 105
of which had Rsrecords and 155 of which had CA records. The
inhomogeneity information shown here was derived from meta-
data from https://www.data.jma.go.jp/obd/stats/data/en/smp/index.
html (last access: 21 January 2022) and was used as primary in-
formation to perform the inhomogeneity adjustment in the RHtests
method detailed in Sect. 2.2.
SR=SJ−0.5SJ≥2.5hd−1,(3)
where SJis the daily SunDu observed by the Jordan recorders
before replacement and SRis the daily SunDu adjusted to be
consistent with the values observed with the EKO rotating-
mirror recorders.
These two homogenization methods were compared in this
study and yielded nearly the same SunDu-derived Rsvaria-
tion, as shown in Fig. 3. Although the second method pro-
posed by Katsuyama (1987) is simple and efficient, we just
Figure 3. The anomalies of surface incident solar radiation (Rs) de-
rived from homogenized sunshine duration (SunDu) data (red line)
by the RHtests QM method and other independent data (blue line)
adjusted by the method in Katsuyama (1987). Both of the homoge-
nized datasets yield nearly the same Rsvariation.
use it to cross-validate the accuracy of the RHtests method.
Since most artificial shifts in the observation system were
undocumented worldwide, the statistical methods including
RHtests are optimal to identify these non-climatic signals and
reduce the discontinuities in the data series. As RHtests can
detect the change points in the raw data series when the meta-
data are unavailable, while Katsuyama (1987) cannot, RHt-
ests was selected in this study.
2.3 Clouds
Clouds play an important role in Rsvariation (Norris and
Wild, 2009). Monthly cloud cover observations at 155 sta-
tions were also available on the JMA website. The observa-
tion time for cloud amount has been 08:00–19:00 LT since
Earth Syst. Sci. Data, 14, 463–477, 2022 https://doi.org/10.5194/essd-14-463-2022
Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 467
1981 at 9.0 % of cloud amount stations and 08:30–17:00 LT
from 1990 to 1995 at another 15.4 % of cloud amount sta-
tions (see Fig. 2 and Table S2). However, the difference be-
tween annual raw and homogenized cloud data is trivial, as
cloud data are relatively homogeneous in space compared
with Rsand SunDu observations. A site observation of cloud
amount can represent the value over a large spatial scale,
likely leading to a few inhomogeneity issues for cloud data.
To explore the impact of the cloud cover anomaly on the
Rsvariation, the cloud cover radiative effect (CCRE), defined
as the change in Rsproduced by a change in cloud cover, was
proposed by the following (Norris and Wild, 2009):
CCRE0(lat,long, y, m)=CC0(lat,long, y , m)
×CRE(lat,long, m)/CC(lat,long, m),(4)
where lat is the latitude, long is the longitude, yis the year,
mis the month, CCRE0is the cloud cover radiative effect
anomaly, CC0is the cloud cover anomaly, CC is the clima-
tology of cloud cover in 12 months, and CRE is the cloud
radiative effect calculated by the Rsdifference under all-sky
and clear-sky conditions.
The residual radiative effect was determined by remov-
ing the CCRE anomalies from the Rsanomalies. It is noted
that a part of the cloud albedo radiative effect proportional
to the cloud amount was contained in the CCRE, as a large
cloud amount tends to yield enhanced cloud albedo, whereas
another part of the cloud albedo radiative effect due to the
aerosol first indirect effect (more aerosols facilitating more
cloud condensation nuclei may enhance cloud albedo) may
be included in the residual radiative effect, which mainly
contains the aerosol radiative effect.
Clouds and the Earth’s Radiant Energy System (CERES)
provides a reliable surface incident solar radiation (Ma et
al., 2015) primarily based on the Moderate Resolution Imag-
ing Spectroradiometer (MODIS) cloud and aerosol products
(Kato et al., 2012). The cloud amount in CERES agrees well
with the observations, and the annual CRE in CERES is well
correlated with the annual cloud amount in Fig. 10. The re-
gional average cloud amount over Japan in Fig. 10 (blue line)
increases at a rate of 0.7 % per decade from 1960 to 2015,
which is consistent with the previous results (Fig. 4 in Tsut-
sumi and Murakami, 2012).
In this study, long-term observations of cloud amount and
monthly cloud radiative effect (CRE) data in the CERES
EBAF (Energy Balanced and Filled) edition were used fol-
lowing Eq. (4) to distinguish the cloud cover radiative effect
from Rsvariation.
2.4 Data processing
We first interpolated the monthly observational data at sites
into 1◦×1◦grid data and then calculated the area average of
the climate variables. As the brightening and dimming over
Japan were the main concern in this study, monthly values
were converted into annual values for calculation. If there
are missing values in any month in a specific year, the an-
nual value for that year is set to a missing value. The linear
regression was used for trend calculation.
3 Results
In this section, we first compared the observed Rsand
sunshine-duration-derived Rsbefore and after adjustment to
demonstrate the necessity and feasibility of the homogeniza-
tion procedure in Sect. 3.1. As artificial shifts may not be suf-
ficiently and correctly documented by metadata, uncertain-
ties may still exist in the homogenized series. We then tried
to explore these uncertainties by considering the influence
of other independent climate variables such as clouds and
aerosols on the Rsvariation and ultimately created a more
reasonable homogenized Rsseries in Sect. 3.2. In Sect. 3.3,
we claimed the significant correction in trend analysis of Rs
in Japan and quantified the influence of clouds and aerosols
on the Rsvariation.
3.1 Homogenization of observed Rsand
sunshine-duration-derived Rs
The comparisons between raw data and homogenized data at
each site were shown in Fig. 4, and their differences were
illustrated in Fig. 5. Compared with raw data, the absolute
values of biases between Rsand SunDu-derived Rsat 74 sta-
tions decrease after homogenization, of which the absolute
values of biases decrease by more than 4 W m−2at 42 sta-
tions and more than 10 W m−2at 8 stations. The root mean
square errors at 80 stations were reduced after homogeniza-
tion, of which reductions are more than 4 W m−2at 40 sta-
tions. After adjustments, the correlation coefficients between
the annual observed Rsand annual SunDu-derived Rsare
improved at 68 stations, including an improvement of more
than 0.2 at 31 stations. There are 41 stations (marked with red
in Table S1, Fig. 6) at which the correlation coefficients were
greater than 0.5, and the biases and the root mean square er-
rors generally decrease after homogenization.
Figure 7, as an example, shows the time series of surface
incident solar radiation (Rsand SunDu-derived Rs) at the
Hamada site (WMO ID: 47755; lat 34.9, long 132.07) be-
fore and after homogenization. Details in the improvements
after homogenization at most stations can be traced back to
Figs. 4, 5, and 6. The improved patterns of time series of
surface incident solar radiation after homogenization high-
light the necessity and feasibility of the RHtests method.
The SunDu-derived Rsvariation over Japan during recent
decades inferred from these “perfect” data at 41 sites (Fig. 8)
was nearly identical to that from all available data at 156 sites
(as shown in Table 1 and Fig. 9).
https://doi.org/10.5194/essd-14-463-2022 Earth Syst. Sci. Data, 14, 463–477, 2022
468 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
Figure 4. The spatial distribution of bias, root mean square error, and the correlation coefficient between SunDu-derived surface incident
solar radiation (Rs) and observed Rsbefore (a, c, e) and after (b, d, f) homogenization. Improvements were made at most sites after
homogenization.
3.2 Uncertainties in Rsobservations
Figure 9 displays the change in Rsduring the last 5 decades,
while Fig. 10 shows the variation in observed clouds over
Japan. The sharp decrease in Rsin 1963 caused by the vol-
canic eruption of Mount Agung in Indonesia (Witham, 2005)
can be clearly found. The sharp decreases in Rsin 1991 and
1993 are due to the combined effect of the volcanic eruption
of Mount Pinatubo in the Philippines in 1991 (Robock, 2000)
and the simultaneous significant increases in clouds (Fig. 8 in
Tsutsumi and Murakami, 2012). The volcanic eruption of El
Chichón in Mexico in 1982 exerted little impact on the de-
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Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 469
Table 1. Trends of surface incident solar radiation (Rs) in Japan during specific time periods for different types of datasetsa. Unit:
W m−2per decade.
CasebDatasetsc1961–1980 1981–1995 1996–2014 1961–2014
Selected 41 stations OBS-raw −12.0∗∗ −2.1 2.4 −0.3
OBS_HM −4.8∗−2.1 2.4 1.5∗∗
OBS_2HM −0.8∗−2.1 2.4∗0.9∗∗
SunDu-derived 1.4 −11.3∗∗ 1.4 −2.1∗∗
SunDu-derived_HM 1.4 −1.3∗1.5 0.9∗∗
All stations OBS-raw −11.2∗∗ −1.3 2.2 0.2
OBS_HM −8.4∗∗ −1.3 2.2 0.8
OBS_2HM 0.7 −1.3 2.2 1.6∗∗
SunDu-derived 2.3∗−10.6∗∗ 1.2 −1.9∗∗
SunDu-derived_HM 1.6 −1.2 1.4 0.9∗
Radiative effect CCRE series −1.1−1.4−0.0−1.4∗∗
Residual series 2.4∗∗ −0.1 1.2∗2.2∗∗
aThe trend calculations were based on the linear regression method. Values with two asterisks (∗∗) imply p < 0.01, and those with one
asterisk (∗) imply 0.01 < p < 0.1.
bRstrends were calculated by different numbers of observations, including all stations that are available on the JMA website and 41
stations (marked with red in Table S1, detailed in Sect. 3.1) that are significantly improved after homogenization. This implies that the
sample number has a subtle impact on the trend calculation over Japan. Radiative effects from clouds and aerosols were also explored.
cTrend calculations were based on the raw (observational) measurements of surface incident solar radiation (OBS-raw), their
homogenized series (OBS_HM), derived incident solar radiation from sunshine duration hours (SunDu-derived), and their
homogenized series (SunDu-derived_HM). OBS_HM from 1961 to 1970 was further homogenized by using SunDu-derived_HM as
reference data, termed OBS_2HM. It is found that homogenized SunDu-derived Rsvalues have the lowest uncertainties among these
five datasets in Sect. 3.1. The cloud cover radiative effect (CCRE) was denoted as the change in Rsproduced by a change in cloud
cover, and the CCRE calculations were performed following Eq. (4) by observed cloud amounts and the cloud radiative effect (CRE)
from CERES satellite retrieval. Residual effect series were obtained by removing the CCRE from homogenized SunDu-derived Rs
anomalies.
cline in Rsand may have been compensated by the decrease
in clouds, as shown in Fig. 10. The pronounced Rsdecline in
1980 coincides with the significant increase in clouds, while
the lightening of Rsin 1978 and 1994 encounters abrupt de-
creases in cloud covers.
As shown in Fig. 9, no major modifications were found in
Rsobservations before and after homogenization (compar-
ison between the light-blue and dark-blue lines). However,
the SunDu-derived Rsseries are smoother after adjustment
by the QM method, as the sharp decrease from 1983 to 1993
caused by the replacement of sunshine duration instruments
(Jordan recorders were replaced with EKO rotating-mirror
recorders) (Stanhill and Cohen, 2008) was repaired (compar-
ison between the light-red line and dark-red lines). Despite
the identical increase in Rsvia both the homogenized direct
measurements of Rsand the homogenized SunDu-derived
Rsduring the 1995–2014 period, their variations in Rsfrom
1961 to 1994 are different (dark-red line and dark-blue line).
Large discrepancies in Rsvariation were found during the
time period of 1961–1970, although homogenization was
performed on the direct measurements of Rsand SunDu-
derived Rs(dark-blue line and dark-red line in Fig. 9). An
existing study noted the inaccurate instruments used at the
beginning of operation in the Rsobservation network in ap-
proximately 1961, and the parallel use of two different types
of instruments during the 1960s may result in the large vari-
ability in observed Rs(Tanaka et al., 2016). At this time, the
clouds fluctuated gently, as shown in Fig. 10, and the change
in volcanic aerosols from 1965 to 1966 was nearly the same
as that from 1962 to 1963 (Table 2 in Sato et al., 1993), so
the sudden decline in the direct observations of Rsfrom 1965
to 1966, which was twice as large as that from 1962 to 1963,
is suspicious. It is inferred that anthropogenic aerosols play
a subtle role in the significant reduction in Rs, as this type of
phenomenon is common for both polluted and pristine sta-
tions in Japan (Fig. 22 in Tanaka et al., 2016).
Figure 11 shows the correlation coefficients between ho-
mogenized Rs(observed and SunDu-derived) and cloud
amount. In general, the observed Rs(−0.45) is less corre-
lated than the SunDu-derived Rs(−0.67), particularly from
1961 to 1970, −0.21 compared with −0.64. This in turn sup-
ports the reliability of homogenized SunDu-derived Rs, espe-
cially during the time period of 1961–1970. The false vari-
ability of the observed Rsfrom 1961 to 1970 was modified by
the RHtests method against the homogenized SunDu-derived
Rsas shown in Fig. 12.
https://doi.org/10.5194/essd-14-463-2022 Earth Syst. Sci. Data, 14, 463–477, 2022
470 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
Figure 5. Histograms of the difference of (a) absolute values of
bias, (b) root mean square error, and (c) correlation coefficient be-
tween SunDu-derived surface incident solar radiation (Rs) and ob-
served Rsbefore and after homogenization. Their differences de-
crease after homogenization.
General decreases in stratospheric aerosol optical depth
(AOD) were reported in Sato et al. (1993) from 1965 to 1980,
and clouds fluctuated slightly, as shown in Fig. 10; both of
these factors contributed to a brightening of Rs. This is in
agreement with the SunDu-derived Rsand contrasts with the
direct measurements of Rs.
During the 1985–1990 period, clouds varied slightly, as
shown in Fig. 10, and the observed atmospheric transmission
under cloud-free conditions increased (Wild et al., 2005),
which suggests that the large declines in directly observed
Rsand SunDu-derived Rsare defective and reinforce the re-
Figure 6. Taylor diagram describing the relative biases, standard-
ized deviations, and correlation coefficients between the annual
observed surface incident short-wave radiation (Rs) and annual
sunshine-duration-derived (SunDu-derived) Rsbefore and after ho-
mogenization at 41 selected stations (numbered 1–41 here). “REF”
can be treated as the perfect point, where values the closer to this
point indicate a better evaluation. The size and direction of the trian-
gles denote the magnitude and negativity or positivity of biases, re-
spectively. The boxes indicate the smaller bias in raw (black colour)
or HM (red colour) series. This figure shows that biases decrease at
most sites (in red boxes) after homogenization, except for the five
stations numbered 4, 25, 32, 36, and 41 (in black boxes). Three sta-
tions (numbered 1, 8, and 40 in black colour) listed below the panel
are beyond the scope of the figure, with the bias (triangle), ratio of
the standardized deviation (above the — line), and correlation coef-
ficient (below the — line) shown. In addition to the improvements in
the correlation coefficients after homogenization, the biases and the
standard deviations generally become small in this Taylor diagram.
liability of the adjusted SunDu-derived Rs(dark-red line in
Fig. 9).
From the above analysis, it can be inferred that fewer un-
certainties exist in homogenized SunDu-derived Rs, which
was confirmed by another work that utilized a different data-
adjusted method (Stanhill and Cohen, 2008).
3.3 Trends of Rsover Japan
The trends of Rsduring specific time periods for different
types of datasets are listed in Table 1. Direct measurements
of Rsand SunDu-derived Rsfrom 41 selected stations and all
available stations reveal similar variations in Rsover Japan,
which demonstrates that the sample number has a subtle im-
pact on the estimation of global brightening and dimming
over Japan.
Earth Syst. Sci. Data, 14, 463–477, 2022 https://doi.org/10.5194/essd-14-463-2022
Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 471
Figure 7. Time series of annual anomalies of observed surface inci-
dent solar radiation (Rs) and SunDu-derived Rsat the Hamada site
(WMO ID: 47755; lat 34.9, long 132.07) before and after homoge-
nization.
Figure 8. Time series of annual anomalies of surface incident solar
radiation (Rs) based on direct Rsobservations (light-blue line) and
their homogenized series (dark-blue line) and sunshine-duration-
derived (SunDu-derived) Rs(light-red line) and their homogenized
series (dark-red line). All of the lines were calculated based on ob-
servations at 41 sites. Details on how these 41 sites were selected
are given in Sect. 3.1. The Rsvariations are nearly the same as those
shown in Fig. 7, which were calculated based on all available ob-
servations.
A revisit of global dimming and brightening was listed
in Table 1. Major differences were found in the time
period of 1961–1980, ranging from −11.2 (−12.0) to
−8.4 (−4.8) W m−2per decade before and after Rsho-
mogenization for all available stations (41 selected sta-
tions) over Japan; significant repairs occurred during the
1981–1995 period, ranging from −10.6 (−11.3) to −1.2
(−1.3) W m−2per decade before and after SunDu-derived
Rshomogenization for all available stations (41 selected sta-
tions) over Japan. Both corrections were mainly attributed
to the homogenization of corrupted raw data caused by the
replacement of instruments for Rsand SunDu measurements.
After careful checking and adjustment of the SunDu-derived
Figure 9. Time series of annual anomalies of the surface inci-
dent solar radiation (Rs) based on direct observations (light-blue
line) and their homogenized series (dark-blue line) and sunshine-
duration-derived (SunDu-derived) Rs(light-red line) and their ho-
mogenized series (dark-red line). All of the lines were calculated
based on as many observations as possible. The light-blue line and
dark-blue line were calculated from the Rsobservations at 105 sites,
while the light-red line and dark-red line were derived from the
SunDu-derived Rsat 156 sites. The Rsvariations are nearly the
same as those shown in Fig. 6, which were calculated based on the
41 selected sites in Sect. 3.1. Large discrepancies were found in the
homogenized data series (dark-blue and dark-red lines).
Figure 10. The cloud amount (CA) from CERES (blue line) agrees
well with that derived from surface observations (red line) over
Japan. At the annual timescale, the negative cloud radiative effect
(-CRE, grey line) in CERES correlated well with the cloud amount.
Rsseries, the decadal variation in Rsover Japan, which was
totally different from former studies (Wild et al., 2005; Nor-
ris and Wild, 2009), was remedied. Direct measurements of
Rsdisplay a trend of nearly zero from 1961 to 2014 over
Japan, while their homogenization series report a positive
change of 0.8–1.6 W m−2per decade; SunDu-derived Rsde-
creases at a rate of 1.9 W m−2per decade, while its homoge-
nized series reveals a brightening of 0.9 W m−2per decade.
The combined effects of clouds and aerosols on Rs
make the global dimming and brightening complicated.
https://doi.org/10.5194/essd-14-463-2022 Earth Syst. Sci. Data, 14, 463–477, 2022
472 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
Figure 11. Scatter plot of homogenized monthly surface incident solar radiation (Rs) (observed and SunDu-derived solar radiation) as a
function of ground-based observations of cloud amount over Japan at all stations only when both cloud amount data and observed Rsdata
are available: (a, c) for 1961–2015 and (b, d) for 1961–1970. The smallest correlation coefficient in (b) indicates that the observed Rsdata
are spurious for 1961–1970 and that SunDu-derived Rsare more convincing.
Figure 12. Time series of annual anomalies of the surface incident
solar radiation (Rs) based on Rsobservations after two rounds of
homogenization (dark-blue line). The homogenized series of ob-
served Rsfrom 1961 to 1970 shown in Fig. 7 was tuned by the
RHtests method again using the homogenized series of SunDu-
derived Rs(dark-red line in Figs. 7 and 10) as a reference.
The CCRE can explain 70 % of global brightening from
1961 to 2014 at monthly and interannual timescales, while
the residual radiative effect dominates the decadal varia-
tion in Rs, as shown in Fig. 13 and Table 1, which is in
agreement with Wang et al. (2012). Homogenized SunDu-
derived Rsshows an increase of 1.6W m−2per decade
from 1961 to 1980; however, a persistent increase in cloud
amount yields a CCRE decrease of 1.1 W m−2per decade.
The residual radiative effect accounts for an increase of
Figure 13. Area-averaged anomalies of homogenized SunDu-
derived Rs(red line) over Japan. The cloud cover radiative effect
(CCRE, blue line) was denoted as the change in Rsproduced by
a change in cloud cover and calculated following Eq. (4) by ob-
served cloud amounts and the cloud radiative effect (CRE) from the
CERES satellite retrieval. The residual effect (grey line) was ob-
tained by removing the cloud cover radiative effect (CCRE) from
the homogenized SunDu-derived Rsanomalies.
2.4 W m−2per decade for this time period. The cloud radia-
tive effect (−1.4 W m−2per decade) modulates Rsvariation
of −1.2 W m−2per decade for the 1981–1995 period, while
the residual radiative effect (1.2W m−2per decade) domi-
nates Rsvariation of 1.4 W m−2per decade from 1996 to
2014.
Homogenized SunDu-derived Rsshows a slight increase
of 0.9 W m−2per decade from 1961 to 2014 with a 90 % con-
fidence interval. However, the CCRE accounts for a deceased
Earth Syst. Sci. Data, 14, 463–477, 2022 https://doi.org/10.5194/essd-14-463-2022
Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 473
Rsof 1.4 W m−2per decade, which implies that cloud cover
changes are not the primary driving forces for the Rstrend
over Japan. Meanwhile, the residual radiative effect exhibits
an increase of 2.2 W m−2per decade, which surpasses the
negative CCRE.
Several studies demonstrate a generally cleaner sky over
Japan from the 1960s to the 2000s (except for the years im-
pacted by volcanic eruptions) based on atmospheric trans-
parency and aerosol optical properties (Wild et al., 2005;
Kudo et al., 2012), which supports the dominant role of
aerosols in Rsbrightening over Japan, as revealed by the
residual radiative effect here. Furthermore, the residual ra-
diative effect in this study is stronger than that in Norris
and Wild (2009), as raw data were remedied and more ac-
curate satellite data from CERES were adopted to quantify
the radiative effect. Tsutsumi and Murakami (2012) demon-
strate that cloud amount categories exert an important effect
on Rsvariation. Rsenhancement by the increased appear-
ance of large cloud amounts is superior to Rsdecline by the
decreased appearance of small cloud amounts during 1961–
2014, which yields increased Rswith increasing total cloud
amount. They also pointed out that the decrease in cloud op-
tical thickness due to the large emissions of SO2and black
carbon from East Asia through the aerosol semi-direct effect
(absorption of more energy by aerosols results in the evapo-
ration or suppression of clouds) may have facilitated the in-
creased Rsover Japan.
The decrease in spring dust storms in March–May dur-
ing the last 5 decades from China (Qian et al., 2002; Zhu et
al., 2008), which may travel to neighbouring countries (Uno
et al., 2008; Choi et al., 2001), could also have triggered the
increase in Rsover Japan. The Rsvariation and radiative ef-
fect in different seasons are categorized in Fig. 14 and Ta-
ble 2, in which an increasing trend of 1.5 W m−2per decade
in the homogenized SunDu-derived Rsprevails in spring for
the whole time period, dominated by a dramatic increase
of 2.8 W m−2per decade in the residual effect and an even
larger increase for 1961–1980 (3.1 W m−2per decade) and
1996–2014 (3.4 W m−2per decade).
4 Data availability
Monthly observed surface incident solar radiation, sun-
shine duration, and cloud amount data were provided by
Japan Meteorological Agency (2022; https://www.data.jma.
go.jp/obd/stats/data/en/smp/index.html, last access: 21 Jan-
uary 2022), and monthly cloud radiative effect (CRE)
data were derived from Clouds and the Earth’s Radi-
ant Energy System for CERES EBAF data (https://ceres.
larc.nasa.gov/order_data.php, last access: 21 January 2022;
CERES Science Team, 2022). The homogenized observed
Rsand SunDu-derived Rsused in this study are avail-
able at https://doi.org/10.11888/Meteoro.tpdc.271524 (Ma et
al., 2021).
Figure 14. Same as Fig. 12 but for the four seasons. The decrease in
Asian spring dust may have triggered the brightening over Japan for
1961–2015, as the Rsin spring increases most among the seasons.
5 Conclusions
The homogenization of raw observations related to Rscan
significantly improve the accuracy of global dimming and
brightening estimation and provide a reliable assessment of
climate trends and variability. In this study, we for the first
time homogenized the raw Rsobservations and obtained a
more reliable Rsdata series over Japan for a century.
Documented artificial shifts in metadata play an important
role in regulating the raw observations. If change points were
confirmed by metadata or other independent climate vari-
ables, the RHtests method was applied to remove the dis-
https://doi.org/10.5194/essd-14-463-2022 Earth Syst. Sci. Data, 14, 463–477, 2022
474 Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan
Table 2. Trends of surface incident solar radiation (Rs) in Japan during specific time periods for different types of datasets for all seasons.
Unit: W m−2per decade.
Season Datasets 1961–1980 1981–1995 1996–2014 1961–2014
Spring SunDu-derived_HM 3.1 −1.5 3.4∗1.5
CCRE series −0.7−1.6−1.6−0.9
Residual series 4.9∗∗ −0.5∗∗ 2.2∗∗ 2.8∗
Summer SunDu-derived_HM 1.4 −3.4 0.6 0.4
CCRE series −1.9−2.1−4.4∗∗ −2.7
Residual series 2.0∗∗ −1.8 1.5∗∗ 2.8
Autumn SunDu-derived_HM 0.6 1.5 3.3∗∗ 1.0∗
CCRE series −1.3∗∗ 1.6 1.6 −0.9
Residual series 1.8∗∗ 0.8∗∗ 2.1∗∗ 2.0∗
Winter SunDu-derived_HM 0.6 −1.5−1.6 0.5
CCRE series −0.6−3.3−0.6−0.7
Residual series 1.1∗∗ 0.9∗∗ −0.9∗∗ 1.2∗∗
∗0.01 < p < 0.1.∗∗ p < 0.01.
continuities. In this study, shifts in the homogenized raw Rs
were further checked by exploring the relationship with the
ground-based cloud amount and tuned again using homoge-
nized SunDu-derived Rsas the reference data. By comparing
the variations in independent climate variables of clouds and
aerosols, the homogenized SunDu-derived Rsproved to be
more reliable in detecting Rsvariability over Japan.
A revisit of global dimming and brightening is made based
on the homogenized Rsseries. Rsover Japan increases at
a rate of 1.6 W m−2per decade for 1961–1980, which is
contrary to the trend (−4.8 to −12.0 W m−2per decade)
in the unreasonable Rsobservation. A slight decrease
of 1.2 W m−2per decade for 1981–1995 in homogenized
SunDu-derived Rsaccounts for only 1/10 of the trend in its
unadjusted series. This directly contributes a brightening of
0.9 W m−2per decade (with a 99 % confidence interval) for
the last 5 decades in homogenized series, which is totally
contrary to the variation in its original series. Global bright-
ening since 1961 over Japan is consistent with that in Stanhill
and Cohen (2008), except that the magnitude is not as large.
We also explored how the clouds and aerosols mediate the
transformation of Rs. The brightening in Japan for 1961–
1980 was the combined effect of cloud cover (negative effect)
and aerosols (positive effect). The dimming for 1981–1995
was governed by reduced cloud amounts, while the increase
in Rsfor 1996–2014 was controlled by decreased aerosols.
These results are different from those in Norris and Wild
(2009), as homogenization was performed on the raw data,
and more accurate cloud radiative effect data series from
CERES were utilized in our study. During the entire period of
1961–2014, cloud amounts dominated seasonal and interan-
nual Rsvariations, while aerosols (including aerosol–cloud
interactions) drove decadal Rsvariations over Japan, noted
by other studies, in response to general cleaner skies and a re-
duction in spring Asian dust storms (Wang et al., 2012; Kudo
et al., 2012).
Supplement. The supplement related to this article is available
online at: https://doi.org/10.5194/essd-14-463-2022-supplement.
Author contributions. QM and KW designed the research and
wrote the paper. LS collected the raw data. YH homogenized the
raw data. QW provided the technical support. YZ and HL checked
the data.
Competing interests. The contact author has declared that nei-
ther they nor their co-authors have any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. This study is funded by the National Key
R&D Program of China (grant no. 2017YFA0603601) and the Na-
tional Science Foundation of China (grant no. 41930970), and the
project is supported by the State Key Laboratory of Earth Surface
Processes and Resource Ecology (grant no. 2017-KF-03). We thank
Earth Syst. Sci. Data, 14, 463–477, 2022 https://doi.org/10.5194/essd-14-463-2022
Q. Ma et al.: Homogenized century-long surface incident solar radiation over Japan 475
many institutions for sharing their data, including the Japan Meteo-
rological Agency for observation data over Japan (https://www.data.
jma.go.jp/obd/stats/data/en/smp/index.html, last access: 21 January
2022) and Clouds and the Earth’s Radiant Energy System for
CERES EBAF data (https://ceres.larc.nasa.gov/order_data.php, last
access: 21 January 2022). We thank the Expert Team on Climate
Change Detection and Indices (ETCCDI) for providing the RHt-
estsV4 homogenization package (http://etccdi.pacificclimate.org/
software.shtml, last access: 21 January 2022).
Financial support. This research has been supported by the Na-
tional Key R&D Program of China (grant no. 2017YFA0603601)
and the National Science Foundation of China (grant
no. 41930970), and the project is supported by the State Key
Laboratory of Earth Surface Processes and Resource Ecology
(grant no. 2017-KF-03).
Review statement. This paper was edited by Bo Zheng and re-
viewed by two anonymous referees.
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