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Changes in Low Cloudiness over China between 1971 and 1996
NOBUHIKO ENDO
Frontier Research Center for Global Change, JAMSTEC, Yokohama, Kanagawa, JapanTETSUZO YASUNARI
Frontier Research Center for Global Change, JAMSTEC, Yokohama, Kanagawa, and Hydrospheric Atmospheric Research Center,
Nagoya University, Nagoya, Aichi, Japan(Manuscript received 13 November 2004, in final form 5 September 2005)
ABSTRACTThe climatology and long-term trends of low-cloud conditions over China were examined using the
Extended Edited Cloud Report Archive data from 1971 to 1996. Linear regression analysis was applied totime series of clear-sky frequencies and low-cloud frequencies, and low-cloud amounts when present. Overthe 26-yr study period, the clear-sky frequency increased over northern China. During summer, the frequency of cumuliform clouds decreased over almost all of China. A significant decrease characterized thetrend in cumulonimbus (Cb) frequency; however, the Cb cloud amount when present increased over the
Yangtze River basin and southern China. Increasing trends in stratocumulus (Sc) cloud amount whenpresent were also observed over much of China.

1. Introduction
The global climate has changed during the last century (Houghton et al. 2001). Clouds are a key component of the global climate system because they stronglyinteract with radiative processes and the hydrologicalcycle. Thus, a thorough description of temporal andspatial variations in cloud conditions will facilitate adeeper understanding of changes in radiative processes
and the hydrological cycle. Conventional surface-based
visual cloud observations have been ongoing for decades worldwide, and cloud datasets based on such visual observations have been compiled in several countries over recent decades. Long-term cloud studies overthe former United Soviet Socialist Republic (Sun and
Groisman 2000), the United States (Angell 1990), Europe (Henderson-Sellers 1986), and Australia (Jonesand Henderson-Sellers 1992) have found evidence ofincreasing total cloud amounts. In contrast, a decreasing trend in the annual-mean total cloud amount has
been found over much of China (Kaiser 1998; 2000).
Total cloud amount is defined as “the amount of sky
estimated to be covered by all cloud types” (WMO
Corresponding author address: Dr. Nobuhiko Endo, Frontier
Research Center for Global Change, JAMSTEC, 3173-25, Syowamachi, Kanazawa-ku, Yokohama City, Kanagawa 236-0001, Japan.
E-mail: nobu@jamstec.go.jp© 2006 American Meteorological Society1975). Thus, changes in several cloud types may constitute or be included in a change in the total cloudamount. Cumuliform, stratiform, and cirriform clouds
can change in different ways. Such variations complicate investigations of relationships between trends intotal cloud amount and other climate parameters. Eachcloud type forms under different dynamic and/or thermodynamic conditions. A change in a specific cloudtype may reflect changes in a specific atmospheric condition. Descriptions of long-term changes for specificcloud types are thus important.
Li et al. (2004) discussed the spatial distribution andseasonal cycle of clouds over China using a visual cloud
observation dataset and a cloud dataset based on satellite observations from 1990 to 1998. However, they
did not analyze long-term changes in low-cloudamounts. The present paper describes the climatological distribution of low clouds over China and presents
trends in low-cloud frequencies and cloud “amounts
when present” (“AWP”). Section 2 describes the data
and methods. Section 3 presents the climatological features of low clouds and trends in low-cloud amount andfrequency at individual stations and regionally. The discussion in Section 4 is followed by a summary.

2. Data and methods
This study used data from the Extended Edited
Cloud Report Archive (EECRA). Hahn and Warren

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FIG. 1. Observation stations and geographic divisions used for the regional time series; N:
northern China, TI: Tibet, NE: northeastern China, S: southern China, NW: northwest China,
and Y: the Yangtze River basin.

(1999) compiled the EECRA based on global accumulated surface visual observations. Surface visual observations made at synoptic times and observational
results were coded in accordance with World Meteorological Organization regulations (World Meteorological Organization 1988) and were sent to the global telecommunication systems. The EECRA includes cloudcodes, as well as total and low-cloud amounts. EECRA
data were collected from land stations from 1971 to
1996. The stations selected for the present study met
the following two conditions: they were between 15°
and 55°N, 70° and 135°E and had at least 100 observations for each month, with the exception of December
and February, for every year. The requirements for December and February were less restrictive because data
were frequently missing in those months during theearly 1970s. There were 415 stations that met the above
two conditions; this station network was fixed throughout the analysis period.
Regional time series of low-cloud conditions revealed regional characteristics for statistically robust
trends. As described in a later section, the cumulonimbus frequency, which was closely associated with heavy
rainfall, was climatologically larger in summer. Basedon total precipitation distributions for summer, we defined six regions and examined trends in low-cloud conditions for each. Using the first-component spatial pattern produced by empirical orthogonal function (EOF)
analysis of the summer rainfall total obtained by Nitta

and Hu (1996), we divided the area east of 105°E into
northern China (34°–42°N, 105°–120°E, including the
Shandong Peninsula; 64 stations), the Yangtze Riverbasin (27°–34°N, east of 105°E; 58 stations), and southern China (21°–27°N, east of 105°E; 47 stations). The
area west of 105°E was divided into northwestern
China (north of 36°N; 33 stations) and Tibet (south of
36°N; 39 stations). Northeastern China (north of 42°N)
included 39 stations (Fig. 1). Stations south of 21°N
were not included in the regional time series analyses.
The arithmetic mean yielded the regional averages for
low-cloud amounts.
The EECRA contains nine cloud codes for low-cloud
conditions (World Meteorological Organization 1975)
and two additional cloud types for obscured sky conditions. This study considered statistics for four types oflow clouds [cumulus (Cu), CL: types 1, 2; cumulonimbus (Cb), CL: types 3, 9; stratocumulus (Sc), CL: types 4,5, 8; and stratus (St), CL: types 6, 7]. The extended
cloud-type code CL ⫽ 10 in the EECRA also indicatedcumulonimbus; however, that code was not included inthis study because the code did not appear at any stations selected in the study. Low-cloud frequencies were
calculated according to Norris (1998). Frequencies ofclear sky (FQclr), sky-obscuring precipitation (FQsop),and sky-obscuring fog (FQfog) were calculated by
FQclr ⫽

Nclr
,
Nall

FQsop ⫽

Nsop
,
Nall

FQfog ⫽

Nfog
,
Nall

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FIG. 2. Seasonal cycle of cloud and clear-sky frequency for the six study regions of China: clear-sky (thick line),
Cu (short-dashed line), Cb (long-dashed line), Sc (long-dash–dotted line), and St (thin line) frequencies.

where Nclr, Nsop, and Nfog were the total number ofobservations reporting clear sky, sky-obscuring precipitation, and sky-obscuring fog, respectively, and Nall was
the total number of all cloud observations, regardless ofwhether they were reported as low clouds. Occurrence
frequencies of low-cloud types 1–9 (FQi where CL 1 ⱕi ⱕ CL 9) were calculated by
FQi ⫽

Ni
共1 ⫺ FQclr ⫺ FQsop ⫺ FQfog兲,
Nlow

where Ni was the total number of observations reporting cloud type i, and Nlow was the total number of observations reporting low-cloud types when the sky was
not clear. This study defined AWP as the sum of the
low-cloud amount (Nh) divided by the number of lowcloud appearances. AWP statistics were prepared foreach season.
Surface visual observations under dim light mayhinder identification of cloud types (Hahn et al. 1995).
The EECRA include data flags denoting sufficient
light. Daytime statistics and nighttime statistics withgood illumination were separately prepared for the four
seasons (DJF, MAM, JJA, SON); daytime and nighttime statistics (hereafter DN) with good illumination
were then averaged. Hahn and Warren (1999) noted
several possibilities for biases, such as “day–night sampling bias” and “monthly sampling error.” We also prepared statistics based on data collected at 0600 UTC(approximately 1400 local time in eastern China), andcompared these results with DN statistics. The comparison showed that cumulus and cumulonimbus frequencies in the DN statistics were smaller than those in
the 0600 UTC statistics. However, the climatologicalseasonal cycle of cloud frequencies was similar. Further
time series of cloud frequencies also resembled eachother. The DN statistics were more stable than the 0600
UTC statistics because of the large number of data used

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FIG. 3. Spatial distribution of trends in clear-sky frequency for each season. Trends with statistical significance at the 0.05 level areplotted by larger marks, trends without statistical significance are indicated by small circles, black and white marks indicate decreasing
and increasing trends, respectively, and “slp” means trend of frequency.

in DN statistics. Therefore, we used the DN statistics inthe following analysis.
Linear trends were calculated using linear regression
techniques. A trend was defined as the linear regression
coefficient. A significance level of 0.05 was usedthroughout the analysis.

3. Resultsa. Seasonal cycles of low-cloud frequencies
Figure 2 shows the seasonal changes in the clear-sky
frequency and the cloud frequencies for four low-cloudtypes over six regions of China. The clear-sky frequency showed great seasonal variation. Clear skies occurred most frequently in winter and were also prevalent in autumn. The amplitude of seasonal change incumulus frequency was large, showing a summer maximum in all regions. The maximum Cu frequency exceeded 30% in summer, except over the Tibetan Pla-

teau. Cumulonimbus activity was also vigorous in summer. The frequency of Cb exceeded 20% in southern
China and the Tibetan Plateau. The seasonal cycle ofthe stratocumulus frequency also exhibited a summer
maximum and a winter minimum over northern, northeastern, and northwestern China and Tibet. In southern
China and the Yangtze River basin, the frequency of Sc
exceeded 20% throughout the year; the minimum Sc
frequency was observed in summer. Regional differences in the seasonal cycle of Sc frequency were linked
to regional differences in both atmospheric water vapor
content and large-scale circulation. Nakata (1991) examined the occurrence of stratiform clouds over southern China during winter and found that stratiform
clouds formed between low-level cold northeasterlies
and warm southwesterlies south of the Tibetan Plateau.
Klein and Hartman (1993) noted that downward motion caused by a local Hadley circulation helped support low-level stratiform cloud over southern China.

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TABLE 1. Regional trends in clear-sky frequency, total cloud amounts, and three low cloud types during summer. Trend values are
expressed as percent per decade. Asterisk indicates statistical significance at the 0.05 level.

Northeast China
Northwest China
North China
Yangtze River
South China
Tibetan Plateau

Clear-skyfrequency

Total
cloud
amountCumulusFrequency

AWP

Frequency

AWP

Frequency

AWP

2.7*
2.5*
3.6*
2.6*
1.6*
0.6

⫺0.6
⫺2.1
⫺1.6
0.4
⫺2.8
⫺0.5

⫺1.7*
⫺2.7*
⫺3.0*
⫺2.4*
1.0
⫺1.8

1.5
0.4*
0.8*
1.3*
⫺0.9
⫺0.9

0.9
0.0
⫺0.3
⫺3.0*
⫺6.3*
⫺1.2*

4.0*
3.9*
5.1*
9.7*
5.7*
1.6

0.5
3.4*
⫺0.5
⫺0.4
3.4*
2.5*

5.0*
1.7
4.3*
5.8*
4.3*
2.3*

b. Trends in clear-sky frequency and low cloudiness
The clear-sky frequency showed an increasing trend
over China throughout the year (Fig. 3). However, thistrend was unequally distributed over the four seasons.
During winter, the clear-sky frequency increased overthe northern Yangtze River basin. The clear-sky frequency increased at a rate of 6.7% per decade in northern China. During summer, the largest increase, 3.6%
per decade, occurred over northern China (Table 1).

Cumulonimbus

Stratocumulus

Although the number of stations with statistically significant trends was small, regional statistics showed increasing trends in clear-sky frequency over most of
China, with the exception of the Tibetan Plateau. These
increasing trends in the clear-sky frequency corresponded well with annual decreasing trends in total
cloud amount over northern China (Kaiser 1998, 2000).
Figure 4 shows trends in the stratocumulus frequency. Statistically significant increasing trends wereobserved in southern China, while statistically signifi-

FIG. 4. As in Fig. 3 but for the trend in stratocumulus frequency in each season.

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FIG. 5. Regional time series of clear-sky frequency, cloud frequency, and cloud amount when present (AWP). Only
time series with statistical significance at the 0.05 level are plotted: N, TI, NE, S, NW, and Y as in Fig. 1.

cant decreasing trends occurred around the Sichuan basin during winter. In addition, the Sc frequency also
increased in northwestern China. Similar spatial patterns in Sc frequency trends were also observed in otherseasons. During summer, Sc frequency tended to increase in northeastern China. Interestingly, the AWP of
Sc increased over China in summer (Figs. 5g–5k).
Trends in cumulus frequency are shown in Fig. 6. The

Cu frequency trends tended to decrease over much of
China. In winter, decreasing trends were observed overTibet. Decreasing trends were dominant in southern
China, the Yangtze River basin, and Tibet in spring.
The trends were generally negative north of the
Yangtze River basin, while the positive trends occurredin southern China during summer. The frequency of Cu
decreased by 3.0% and 2.4% every 10 years in northern

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FIG. 6. As in Fig. 3 but for the trend in cumulus frequency in each season.

China and the Yangtze River basin, respectively, during
summer (Figs. 5c–5d). Concurrently, the Cu AWP increased over northwestern and northern China and the
Tibetan Plateau at a small, but statistically significant,rate (Figs. 5h–5j). In autumn, the trends generally
tended to decrease over almost all of China.
Figure 7 shows trends in the frequency of cumulonimbus and the AWP of Cb in summer. The figure
clearly illustrates a significant decrease in Cb frequency
over southern China. The Cb frequency decreased byapproximately 6.3% every 10 years and was statisticallysignificant at the 0.05 level (Table 1). In contrast, the
AWP of Cb increased at a rate of 5.7% per decade insouthern China. Over southern China, Cb decreased,
Sc increased, and Cu showed small changes. Increasing
trends in Cb frequency and the AWP of Cb were evident over Inner Mongolia and southern Siberia. The
trends in the AWP of Cb were positive over China; thelargest such increase was 9.7% per decade over the
Yangtze River basin (Table 1). A large decrease in the
Cb frequency (⫺3.0% per decade) also occurred overthe Yangtze River basin.

4. Discussion
During summer, the Cb frequency clearly decreased
in southern China, especially along the coast of theSouth China Sea. The seasonal average of the Cb cloudamount (cloud amount ⫽ frequency ⫻ AWP) decreased from 17% in the early 1970s to 12% in the early
1990s, whereas the seasonal average of the Sc cloudamount increased from 17% to 26%. Because the
EECRA included “coded” cloud reports, this change
may indicate only a regional decrease in Cb in thatcumulonimbus-type clouds were reported when Cb appeared, regardless of the existence of Sc (WMO 1975).
Limitations in the coded cloud report must be considered. Recently, Lu and Dong (2001) and Lu (2001)
found that the summer subtropical high over the northwestern Pacific has extended farther westward since
1979, frequently extending westward to cover parts ofsouthern China. Downward motion generally dominates in an anticyclone, so any westward extension of
the subtropical high would result in enhanced downward motion over southern China. In addition, as the1 APRIL 2006

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FIG. 7. Trends in (a) the frequency of Cb, and (b) Cb amount when present. Trends with statistical significance at the 0.05 level areplotted by larger marks, trends without statistical significance are indicated by small circles, black and white marks indicate decreasing
and increasing trends, respectively, and “slp” means trend of frequency.

subtropical high builds to the west, water vapor transport from the Tropics to eastern China along the northwestern rim of the subtropical high would move westward, and the large-scale convergence zone may also
move. Increasing amounts of Cb occurred over Yunnan
Province in southwestern China (Fig. 7a). Thesechanges in Cb may be related to the westward extension of a subtropical high over the northwestern Pacific.
The frequency of Cb in the Yangtze River basin decreased from about 13% in the early 1970s to 6% in the
1990s during summer. The frequency of Cu also tended
to decrease over the Yangtze River basin. Thesechanges suggest more stable atmospheric stratification
over the Yangtze River basin. However, heavy rainfall
events in summer have become more frequent in the
Yangtze River basin in recent decades (Endo et al.
2005). The decrease in the Cb frequency, in conjunction
with an increase in heavy rainfall events, at first seemedcounterintuitive. However, a decrease of the number ofdays with precipitation (figure not shown) and an increase of the AWP of Cb (Table 1) were also found in
the Yangtze River basin. Therefore, while Cb frequency declined, an increasing probability of heavy
rainfall with larger AWP of Cb appeared during thestudy period.
Trenberth (1998) found that an increase in surface
temperature increased the atmospheric water holding
capacity and the atmospheric moisture content in acoupled atmosphere–ocean general climate model(AOGCM) climate change simulation. These changes
imply that the number of stronger rainfall events will

increase. In fact, researchers have found increased
amounts of convective clouds (Sun et al. 2001), heavy
rainfall events (Karl and Knight 1998; Groisman et al.
1999), and tropospheric water vapor content (Ross andElliott 2001) in the United States. Although the tropospheric water vapor content also increased over eastern
China (Zhai and Eskridge 1997; Ross and Elliott 2001),
a different relationship between Cb and heavy rainfall
trends is evident in the Yangtze River basin.
It is useful to consider differing relationships between trends in convective cloud frequency and heavy
rainfall events for the Yangtze River basin and theGreat Plains of the United States. Heavy rainfall is generally brought by organized convective clouds that appear near the mei-yu front over the Yangtze River basin (e.g., Akiyama 1973; Ninomiya 1984); such clouds
also appear over the Great Plains (e.g., Carlson et al.
1983). Convective development over the Great Plains istypically accompanied by a strong capping inversion
and a dry, warm midtroposphere above a mixed layer(e.g., Emanuel and Raymond 1993). The developmentof deep convective clouds in eastern China during summer is characterized by large evapotranspiration frompaddy fields and a moist midtroposphere environment(Shinoda and Uyeda 2002). The environment that supports deep convective development is quite different
for the two regions. The different relationships between
convective cloud trends and heavy rainfall events in thetwo regions may be attributed, therefore, to the different environmental factors accompanying the development of deep convection.

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5. Summary
The climatology and long-term trends of low-cloud
conditions over China were examined based on the 26yr Extended Edited Cloud Report Archive. The following observations are reported:
1) Clear-sky frequency increased over northern China
throughout the year. Clear-sky frequency increased
in northeastern China, except in summer. Clear-sky
frequency also rose in northwestern and southern
China in autumn.
2) Cumulus frequency decreased over eastern China inspring, summer, and autumn. A significant decreasein cumulonimbus frequency occurred, while the
AWP of Cb showed a statistically significant increase over southern China and the Yangtze Riverbasin during summer. Convective cloud amounts decreased over most of China. However, an increase ofCb frequency and an increase of the AWP of Cb
occurred over Inner Mongolia during summer.
3) Stratocumulus frequency increased during summer
over southern and northwestern China and the Tibetan Plateau. The AWP of Sc also increased overmost of China. Stratocumulus frequency increased
over southern China and Sc frequency declined
north of the Yangtze River in other seasons.
This study used “coded” cloud reports. Surface observers can log several cloud types in the observation
records, but the coded cloud report can describe a cloudtype only for a single layer. In addition, data for land
stations in EECRA covered only a limited period. Further analyses using surface-level visual observation data
for longer periods, archived at meteorological agencies,
are warranted.
Acknowledgments. The Extended Edited Cloud Report Archive was obtained from the Carbon Dioxide
Information Analysis Center, Oak Ridge National
Laboratory, U.S. Department of Energy.
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