Journal of the Meteorological Society of Japan, Vol. 82, No. 2, pp. 761--773, 2004
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Spatial Distribution and Seasonal Variation of Cloud over China
Based on ISCCP Data and Surface Observations
Yunying LI
The State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid dynamics
(LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, P.R. China
Institute of Meteorology, PLA University of Science and Technology, Nanjing, P.R. China
Rucong YU
The State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid dynamics
(LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, P.R. China
Youping XU
The State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid dynamics
(LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, P.R. China
Institute of Applied Meteorology, Beijing, P.R. China
and
Xuehong ZHANG
The State Key Laboratory of Numerical Modelling for Atmospheric Sciences and Geophysical Fluid dynamics
(LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, P.R. China
(Manuscript received 6 May 2003, in final form 5 January 2004)
Abstract
Based on the International Satellite Cloud Climatology Project (ISCCP) data, and the World Meteorological Organization (WMO) surface synoptic observations, spatial distributions and seasonal variations of total cloudiness and fractional cloud amount of high, middle and low clouds over China are examined. It is found that low clouds mainly appear along the southeast coast of China, middle clouds
dominate southern China and high clouds mainly occur over northern China. Seasonal variations of
convective clouds over northern China, southern China and the Tibetan Plateau are similar, reaching a
maximum in summer and a minimum in winter; whereas the seasonal variation of stratiform clouds
displays large spatial variation, with opposite phase in northern and southern China.
1.
Corresponding author and present affiliation: Yunying Li, the State Key Laboratory of Numerical
Modelling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric
Physics, Chinese Academy of Sciences, Beijing
100029, China.
E-mail: [email protected]
( 2004, Meteorological Society of Japan
Introduction
Clouds play an important role in the climate
system. The climatic effect of each type of cloud
depends not only on cloud amount, but also on
cloud height, thickness and other properties
(Qian and Qian 1994). Clouds are involved in
forming regional climates with their distinct
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Journal of the Meteorological Society of Japan
horizontal and vertical distributions, and microphysical properties in different geographical areas. Currently, most general circulation
models cannot satisfactorily simulate climate
features in China, one of the reasons is lack of a
better description of spatial distributions, and
temporal variations of cloud types over China,
let alone an accurate parameterization of these
clouds and the associated cloud radiative interactions. Major regions with large amounts of
stratiform clouds are generally over the oceans
in low and middle latitudes, with Chinese land
as an exception (Klein and Hartmann 1993).
These optical thick stratiform clouds can result
in a large negative cloud radiative forcing (Rajeevan and Srinivasan 2000), thus have great
influence on the local climate. Because of the
frequent occurrence of various cloud types over
China, and their different impacts on local climate, the distribution and seasonal variation of
every type of clouds over China need to be intensively studied.
Much attention has been paid to the clouds
over China. The total cloud amount presented
by the National Centers for Environmental
Prediction (NCEP) reanalyses has been compared with, and found to be in good agreement
with, that by the International Satellite Cloud
Climatology Project (ISCCP) data and surface
observations over the Tibetan Plateau (Wang
et al. 2001). The total cloud amount over the
Tibetan Plateau reaches a maximum in summer, with cirrus and convective clouds dominating the northern and the middle parts of the
Plateau, respectively (Wei and Zhong 1997;
Chen et al. 1999). It is also found that the total
cloud amount maximum occurs over the south
parts in China (Wei et al. 1996; Weng and
Han 1998). However, most of the previous work
mainly focused on the total cloud amount in
some specific regions in China, the distributions and seasonal variations of various cloud
types in different regions in China have not
been paid enough attention. The ISCCP dataset and surface synoptic observations of the
World Meteorological Organization (WMO), are
often used to study clouds. Both datasets have
advantages and deficiencies because of individual limitation of observation methods (Rossow et al. 1993; Brest et al. 1997; Rossow and
Schiffer 1999). In general, the upper cloud
coverage retrieved from infrared and visible
Vol. 82, No. 2
radiances from satellites is relatively accurate,
whereas the lower cloud coverage reported by
surface observers is relatively reliable. If both
kinds of data are used, we can describe the regional cloud climatic features more reasonably.
Our purpose is to combine the ISCCP data, and
surface observations to describe the distributions and seasonal variations of various cloud
types over China. We also examine the largescale circulations associated with cloud formation over China, and analyze the relationships
of cloud distributions and seasonal variations
among three different regions.
The rest of the paper is organized as follows.
The data resources and processing methods
are described in Section 2; the distributions of
various cloud types over China are illustrated
in Section 3; and the seasonal variations are
analyzed in Section 4. Section 5 contains a
short summary.
2.
Data and processing methods
The surface observed cloud is extracted from
the surface synoptic observations on the land of
the WMO. It is based on the visual reports in
China, and its surrounding areas (10 –55 N,
75 –135 E), which is available 4 times a day at
00:00, 06:00, 12:00 and 18:00 UTC, and about
800 station reports from January 1990 to December 1998, including total cloudiness, fractional cloud amount, the base height of low
cloud, cloud type and weather phenomenon.
The cloud types defined by cloud base height
(CBH, i.e., the base height above the ground),
and cloud shape reported by surface observers
are shown in Table 1.
The cloud amount is decoded under the report regulation, which is as follows: when low
clouds occur, low cloud amount is reported;
otherwise, middle cloud amount is reported.
Table 1. The cloud types defined by
surface observations
High cloud
CIRRUS
CIRROSTRATUS
CIRROCUMULUS
ALTOCUMULUS
ALTOSTRATUS
NIMBOSTRATUS
CUMULUS
STRATOCUMULUS
CBH b 4.5 km
Middle cloud
2.5 km a
CBH < 4.5 km
Low cloud
CBH < 2.5 km
CUMULONIMBUS
STRATUS
Y. LI, R. YU, Y. XU and X. ZHANG
Then the middle cloud amount can be estimated in the following two circumstances:
when low clouds occur, the difference between
the total cloud amount and the low cloud
amount is the middle cloud amount, and the
high cloud amount is neglected; when there is
no low cloud, the fractional cloud amount reported is the middle cloud amount. The high
cloud amount can also be estimated in two circumstances: when only high clouds occur, the
total cloud amount is the high cloud amount;
when low or middle clouds occur, the difference
between total cloud amount and the fractional
cloud amount, is the high cloud amount. Of
course, the estimated high cloud amount has
to include unacceptable errors, the high cloud
features are mainly analyzed based on the
ISCCP data in our study.
Nimbostratus and Altostratus are reported
as identical code in the surface observations,
they are separated from each other by different
weather phenomena in our study. If it is raining continuously, the cloud type is specified as
Nimbostratus, otherwise Altostratus.
The decoded clouds are then interpolated to
1 1 horizontal grids. The global mean error
of the surface daily observed cloud is about 15%
(Rossow et al. 1993). However, the daily data
is averaged to the monthly mean data in our
study, and the temporal and special averages
quite minimize the errors.
There are only a few observation stations on
islands of the oceans and fewer stations on
the Tibetan Plateau, thus the clouds over the
oceans are not analyzed, and conclusions about
the coverage and variation of different types of
clouds over the Plateau need a further verification.
Only the base height of low clouds is reported in the surface synoptic observations of
the WMO. In order to estimate the cloud base
height of the middle and high cloud types,
a special dataset is used in this paper. This
dataset includes the cloud amount, the base
height of various cloud types, at 196 stations in
China, and covers the period from January to
December in 2001.
Upper air meteorological bulletins of the
WMO are also used in this paper. Temperature
ðTÞ and dew point ðTd Þ on significant levels,
which are available 2 times a day at 00:00 and
12:00 UTC from January 1990 to December
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Table 2. The cloud types defined by the
ISCCP data (Doutriaux and Seze
1998)
ISCCP CLOUD CLASSIFICATION
CLOUD TOP PRESSURE (MB)
April 2004
CLOUD OPTICAL THICKNESS
1998 in China, are used to display clearly inversion layers in our study.
The satellite data used is the ISCCP dataset,
which is one of the most objective datasets at
present time. In the mid-latitude, its total cloud
coverage agrees well with that of the surface
observations (Rossow et al. 1993; Hahn et al.
2001). The ISCCP D2 global data archive (Rossow and Schiffer 1991; Rossow and Schiffer
1999; Doutriaux and Seze 1998) includes
monthly mean cloud top pressure, and fractional cloud coverage of nine cloud types defined by cloud top pressure and optical thickness, as shown in Table 2 (Doutriaux and Seze
1998) with 2.5 2:5 spatial resolution. The
original dataset covers the period from June
1983 to September 2001. In order to be consistent with the period of the surface observations
over China, the ISCCP D2 data in China and
its surrounding areas, from January 1990 to
December 1998, are only used.
Because of the high elevation, there is almost
no low cloud coverage over the Tibetan Plateau
according to the ISCCP definition, the low and
middle clouds defined by Table 1 might be high
clouds defined by Table 2, which would result
in serious confusion. So high clouds off the Tibetan Plateau are mainly analyzed. Furthermore, low and middle clouds are mainly analyzed by surface observations, because they are
often hidden by high clouds when being observed from space.
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Fig. 1. The spatial distribution of annual mean (1990 to 1998) total cloud coverage (in %) over China
and its surrounding areas (10 N–55 N, 75 E–135 E) derived from (a) the ISCCP D2 data and (b)
the surface observation data. Values greater than 65% are shaded.
3.
Cloud distribution over China
3.1 Distribution of total cloud coverage
The global mean total cloud coverage is 67%
according to the ISCCP D2 dataset, and the
coverage over the oceans is larger than that
over the land. But southern China (20 –35 N,
103 –120 E) is the largest cloudy subtropical
continental region (Yu et al. 2001), while
northern China (35 –50 N, 103 –120 E) and
the Tibetan Plateau (25 –40 N, 75 –103 E)
have relatively small cloud coverage because of
a deficiency of water vapor. Figure 1a and Fig.
1b show the spatial distributions of the annual
mean total cloud coverage over China and its
surrounding areas, derived from the ISCCP
data and the surface observations, respectively. Both figures clearly reveal the regions of
smaller cloud coverage over northern China,
Mongolia, and the western Tibetan Plateau,
and the regions of larger cloud coverage over
the southeastern Tibetan Plateau, southern
China and surrounding seas. The patterns
shown in Fig. 1a and Fig. 1b are similar, but
the major difference in magnitude appears in
northern China, where the cloud amount estimated by the ISCCP data can be 35% greater
than that given by the surface observations in
winter. A possible reason for such differences is
that satellites could not completely distinguish
low clouds from a snow surface. It seems that
the low cloud amount reported by surface ob-
servers is reliable than that by satellites over
northern China and the Tibetan Plateau.
3.2 Distribution of low clouds
Low clouds include Stratus (St), Stratocumulus (Sc), Cumulus (Cu) and Cumulonimbus (Cb)
(see Table 1). Over the area where topography
is greater than 2.5 km, almost no low cloud
coverage is reported by the ISCCP data. Then
the distribution of annual mean low cloud coverage (StþScþCuþCb) by surface observations
is illustrated in Fig. 2a, and convective cloud
coverage (CuþCb) is illustrated in Fig. 2b. In
Fig. 2a, only a little low cloud coverage occurs
over northern China, as well as over western
and northern Tibetan Plateau. Southern China
and southeastern Tibetan Plateau are regions
with larger low cloud coverage. The comparison
between Fig. 2a and Fig. 2b shows that the
convective cloud coverage over the Tibetan Plateau is about half of the low cloud coverage.
Figure 2b also shows that convective cloud coverage over the Plateau is larger than that over
other regions in China. The reason of the large
convective cloud coverage is that this highest
mountain in the world acts like a huge thermal
machine. Even before the rainy season, the
occurrence probability of convective cloud is
probably greater than 70% around noon (Li
2002). However, over southern China, where
relative humidity is often greater than 70%
in the lower troposphere, the stratiform cloud
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Y. LI, R. YU, Y. XU and X. ZHANG
a
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Fig. 2. The spatial distributions of annual mean (1990 to 1998) (a) low cloud coverage and (b) convective cloud coverage over China and its surrounding areas derived from the surface observations.
Values greater than 20% are shaded.
coverage is larger than the convective cloud
coverage. Large-scale ascending flows maintain
in southern China during winter and spring
due to the influence of the persistent Southwest
China vortex, the South China quasi-stationary
front, and the Kunming quasi-stationary front,
which provides the necessary condition to develop stratiform clouds. Because clouds cannot
penetrate the boundary layer inversion in cold
seasons, the typical cloud type along the southeast coastline of China, is the Sc. The approximate thickness of stratiform cloud could be estimated, based on the cloud base height (CBH)
of the surface observations and the cloud top
pressure (CTP) of the ISCCP data. Table 3
shows the annual mean CBH of various cloud
types, at three typical stations in China. Guiyang (26.35 N, 106.43 E, 1.074 km above the
sea level), Tangshan (39.40 N, 118.09 E, 0.028
km), and Lhasa (29.40 N, 91.08 E, 3.65 km)
are representative of southern China, northern
China and the Tibetan Plateau, respectively.
Compared to the lower elevation areas, the
clouds over the Tibetan Plateau can only form
at greater heights, due to the lack of water
vapor. Meanwhile, water vapor can deposit directly in the middle troposphere, because the
temperature is very low (Guo 1985). That is
why among the three typical regions, the CBH
of the low clouds over the Tibetan Plateau is
maximum, whereas that of the high clouds is
Table 3. The mean base heights for
various cloud types at Guiyang,
Tangshan and Lhasa
Station name
Guiyang
(km)
Tangshan
(km)
Lhasa
(km)
Cirrcus and
Cirrostratus
6.190
6.068
4.219
Altocumulus and
Altostratus
3.277
3.366
2.988
Nimbostratus
0.979
1.250
2.000
Stratocumulus
0.821
1.271
2.080
Stratus
0.386
0.435
1.140
Cumulus
0.431
1.218
1.766
Cumulonimbus
0.786
1.011
1.547
Cloud type
minimum. In other regions off the Plateau, the
CBH of Sc is about 0.8–1.3 km and of Cu is
0.4–1.2 km. The CBH is higher over northern
China and lower over southern China, largely
depending on the content of water vapor. According to the ISCCP data, the CTP of Sc over
northern China is about 750 hPa and that of
Cu is about 840 hPa (not shown). Relative to
the ground, the cloud top height of Sc and Cu is
about 2.4 km and 1.5 km, respectively. If there
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Fig. 3. Vertical profiles of temperature (T, solid line) and dew point (Td , dashed line) as a function of
pressure, observed by radiosonde at Guiyang station at 00:00 UTC on (a) 26 February 1997 and (b)
19 February 1996.
is only one cloud layer, the thickness of Sc can
be estimated as about 1.1 km, and that of cumulus as 0.3 km over northern China.
The more accurate cloud thickness can be estimated from the vertical profiles of the temperature ðTÞ and dew point ðTd Þ based on the
radiosonde data shown in Fig. 3. For example,
at Guiyang station, when it is cloudy, the occurrence probability of Sc is generally greater
than 80% in winter and spring, and the CBH
is usually about 0.8 km. Meanwhile, the CTP
could be judged from the difference between the
temperature and dew point ðT Td Þ computed
from Fig. 3. In Fig. 3a, there is an inversion
layer whose bottom is located at 765 hPa.
Above the bottom, the T Td increases rapidly.
Thus the bottom of the inversion layer is also
the top height of the cloud. Relative to the
ground, the cloud thickness is 0.5 km, and the
cloud type belongs to low cloud even if based on
Table 2. Figure 3b shows another case. Two inversion layers coexist in the atmosphere. For
the first layer, the bottom is located at 801 hPa
and the top at 700 hPa. T Td is very small
below the inversion layer. Above the first inversion layer, T Td does not increase, and the
environmental relative humidity maintains a
value of nearly 100%. That is to say, the cloud
penetrates the first inversion layer. The second
inversion layer appears between 632 hPa and
619 hPa, and T Td increases rapidly above
the layer, which shows no cloud there. Relative
to the ground, the thickness of Sc is 2.0 km.
According to the CTP, it can be classified as
middle cloud type. It is found that when Sc
produces no or light rain over southern China,
its top height is lower, or Fig. 3a is generally
the case; whereas it rains continuously, the top
height is higher, or Fig. 3b is generally the
case. The distribution (Fig. 4b) and seasonal
change (not shown) of Sc are consistent with
those of the Nimbostratus (Fig. 4a) defined by
the ISCCP data, though the magnitude of the
former is slightly smaller than the latter. We
presume that Sc with continuous rain should
be thicker and contain abundant liquid water,
and can be classified as middle cloud according
to the CTP. That is to say, the Sc with continuous rain defined by surface observations should
be identified as Nimbostratus defined by the
ISCCP data.
3.3 Distribution of middle clouds
Middle clouds include Altocumulus (Ac), Altostratus (As) and Nimbostratus (Ns) according to Table 1. The distribution pattern of
April 2004
Y. LI, R. YU, Y. XU and X. ZHANG
a
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b
Fig. 4. The 9 year (1990–1998) average spatial distributions of (a) Nimbostratus cloud coverage derived from the ISCCP data and (b) Stratocumulus (with continuous rain) cloud coverage derived
from the surface observations in January over China and its surrounding areas. Values greater
than 4% are shaded. The dashed line is the 3000-m topographic contour line.
Fig. 5. The 9 year (1990–1998) average
spatial distribution of middle cloud coverage in China and its surrounding
areas in January derived from the surface observations. Values greater than
15% are shaded. The dashed line is the
3000-m topographic contour line.
middle cloud coverage based on the surface observations (Fig. 5) is similar to that of the
ISCCP data (not shown), but the magnitude
is smaller because parts of the low clouds reported by surface observers are classified as
middle clouds according to Table 2. The maxi-
mum occurring over the downstream of the Tibetan Plateau shows that the formation of the
middle cloud may be related to the topography
of the Plateau (Yu et al. 2001). In winter and
spring, the westerlies in the lower troposphere
move around the Plateau from the north and
the south side, and converge over the downstream, southern China. Meanwhile, the westerlies in the middle troposphere climb up over
the Plateau and subside on the downstream.
An inversion layer usually occurs between
500 hPa and 600 hPa over southern China,
because the air temperature decreases as the
lower air ascends, and increases as the upper
air descends. Thus, clouds are suppressed beneath the inversion layer. This may be the reason why there is larger middle cloud coverage
over southern China than over other Chinese
regions.
The mid-cloud thickness can also be estimated from the CBH and CTP. Table 3 shows
that the CBH of the middle cloud over southern
China is similar to that over northern China.
Ac and As have the same averaged CBH as
3.2–3.3 km, while the CBH of Ns is only 1.0–
1.2 km. In January, the mean CTP of the
middle clouds, based on the ISCCP data over
northern China, is 580 hPa, or about 4.5 km
above sea level. Thus the thickness of Ac is
about 1.2 km, and that of Ns is about 3.4 km
over northern China. The thickness of middle
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Fig. 6. The spatial distribution of annual mean (1990 to 1998) CiþCs cloud coverage over China and
its surrounding areas derived from (a) the ISCCP data and (b) the surface observations. Values
greater than 30% in Fig. 6a and 15% in Fig. 6b are shaded.
clouds over southern China is similar to that
over northern China.
3.4 Distribution of high clouds
High clouds include Cirrus (Ci), Cirrocumulus (Cc) and Cirrostratus (Cs) according to
Table 1. Figure 6a and Fig. 6b show the distributions of annual mean high cloud coverage,
based on the ISCCP data and surface observations, respectively. In Fig. 6a, we can see the
high cloud coverage is larger over northern
China than over southern China. The maximum is located over the south part of northern
China, with magnitude up to 32%, while the
coverage does not exceed 20% over southern
China. In Fig. 6b, although the high clouds are
usually hidden by the low or middle clouds, its
annual mean coverage still exceeds 10% over
northern China, but less than 5% over southern
China. Based on the both datasets, it is doubtless that the high cloud coverage over northern
China is larger than that over southern China.
However, it is difficult to determine the high
cloud coverage over the Tibetan Plateau. From
Fig. 6b, the coverage is about 20% over the
northwestern Plateau, while it exceeds 30%
over the whole Plateau in Fig. 6a, which shows,
at least, the high clouds dominate the northern
Plateau. But the case is different over the
southern Plateau. The high cloud coverage is
less than 10% according to the surface observations, but more than 30% according to the
ISCCP data. Based on the limited data, we are
not sure whether high clouds are hidden by low
and middle clouds, or the high cloud coverage
itself is small.
4.
Seasonal variation of clouds over
China
4.1 Seasonal variation of total cloud coverage
Figure 7a and Fig. 7b show the seasonal
variations of the total cloud coverage based
on the ISCCP data and surface observations,
which are averaged over 20 N–35 N latitude.
The seasonal phases in these two figures are
consistent. Seasonal variations over the Tibetan Plateau and southern China are both clear,
but have different phases. Figure 7b shows that
the maximum total cloud coverage occurs in
summer over the Plateau, yet in spring over
southern China. Over the Tibetan Plateau,
when the Indian monsoon controls the Plateau,
the relative humidity increases rapidly, and so
does the total cloud coverage, which can exceed
60%. When the westerlies control the Plateau,
the air is dry and the total cloud coverage is
small. Over northern China, the seasonal variation of the total cloud coverage has the same
behavior as that over the Tibetan Plateau, yet
opposite behavior as that over southern China.
To a certain degree, the total cloud coverage is
controlled by the amount of water vapor in the
atmosphere. The water vapor over the Plateau
April 2004
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a
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b
Fig. 7. The 9 year (1990–1998) mean seasonal variations of total cloud coverage averaged over
20 N–35 N latitude derived from (a) the ISCCP data and (b) the surface observations. Values
greater than 70% are shaded.
is mainly transported from the Indian monsoon
region, while it is mainly transported from the
western tropical Pacific over southern China.
Zhang (2001) has shown that the transport of
water vapor over the Indian monsoon region
has a distinct positive relationship with that
over northern China, and a negative relationship with that over southern China, which is
affected by the East Asian Monsoon. When
more water vapor is transported by the Indian
summer monsoon, there is less transported
from the western tropical Pacific and vice versa.
The total cloud coverage is also controled by the
large-scale circulation. The seasonal variation
of divergence calculated from the radiosonde
data in northern China (Fig. 8) is similar to
that in the Indian monsoon region (not shown).
The air converges in the lower layer and diverges in the upper layer in summer. In other
seasons, it behaves in the opposite manner.
This is the possible reason why the largest total
cloud coverage mainly occurs in summer over
northern China and the Tibetan Plateau. The
seasonal variation of relative humidity (Fig. 9)
also shows similar phase patterns over northern China and the Tibetan Plateau.
The peak of the total cloud coverage over
southern China occurs in spring, but the seasonal variability is not very large, and the
difference between the maximum and the minimum is only 15%, which also might be affected by the large-scale circulation. The seasonal variation of the horizontal divergence
over southern China is significantly different
Fig. 8. The 9 year (1990–1998) mean
time-pressure cross section of horizontal divergence (in units of 107 s1 ) over
northern China (averaged over 35 –
50 N, 103 –120 E) calculated from radiosonde data. Values greater than
5 107 s1 are shaded.
from that over the Indian monsoon region. In
summer, they have similar circulation structures, i.e., the air converges in the lower troposphere and diverges in the upper troposphere.
In other seasons, two convergence layers occur
in the higher and the lower troposphere respectively, with a divergence layer in the middle troposphere (Yu et al. 2001). The forcing effect of the Tibetan Plateau topography could be
a factor to maintain the circulation over southern China. As mentioned in Section 3.3, when
the westerlies dominate across China, the cloud
type over southern China is mainly stratiform cloud. After the onset of the East Asian
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Fig. 9. The 9 year (1990–1998) mean
seasonal variation of surface relative
humidity over northern China (averaged over 35 –50 N, 103 –120 E,
dotted-dashed line), southern China
(averaged over 20 –35 N, 103 –120 E,
dashed line) and the Tibetan Plateau
(averaged over 25 –40 N, 75 –103 E,
solid line) calculated from the radiosonde data.
Fig. 10. The 9 year (1990–1998) mean
seasonal variation of convective cloud
coverage over northern China (averaged over 35 –50 N, 103 –120 E,
dotted-dashed line), southern China
(averaged over 20 –35 N, 103 –120 E,
dashed line) and the Tibetan Plateau
(averaged over 25 –40 N, 75 –103 E,
solid line) derived from the surface observations.
Monsoon, the westerlies weaken and stratiform
cloud coverage decreases. In the same time, the
stability of low-level air decreases, and convective cloud coverage increases. Because of the
compensation, the difference of total cloud coverage, between summer and winter, is not very
large over southern China.
northern China, there is almost no convection
in winter because of the cold and dry air dominated by the winter monsoon circulation. In
summer, the coverage of convective clouds over
the Plateau is also the largest among the three
regions. Over the Tibetan Plateau, one of the
most vigorous convective regions in the world,
the convection is easy to develop in the baroclinic boundary layer formed on the uneven
surface, and can be strengthened by the strong
vertical wind shear, forced by the complex topography (Li 2002).
Since there are fewer surface observation
stations on the Plateau, the above explanation
is tentative, and should be verified using more
data over the Tibetan Plateau.
4.2 Seasonal variation of convective clouds
Figure 10 shows the seasonal variations of
the convective cloud (CuþCb) coverage over
northern China, southern China and the Tibetan Plateau. Over all of the three regions, the
maximum occurs in summer, because convection is a product of thermal forcing. It is understandable that the mean convective cloud coverage over northern China is less than that
over southern China in all seasons. But for the
Tibetan Plateau, compared to that in other
areas in the same latitude and altitude, the
surface air temperature is higher and the convection occurs frequently (Li 2002). It is possible that the convective cloud coverage over the
Plateau is a little larger than that over the
other two areas in winter. Convective clouds
can also occur over southern China in winter,
but the magnitude is only about 3%. Over
4.3
Seasonal variation of low and middle
stratiform clouds
Since the low stratiform clouds and middle
clouds have similar seasonal variations, we
add them as one to study. The seasonal variations of stratiform cloud coverage over northern
China, southern China and the Tibetan Plateau are illustrated in Fig. 11. Over northern
China and the Tibetan Plateau, they are in
phase and have a peak in summer. As men-
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a
b
Fig. 11. Same as Fig. 10 except for the
low and middle stratiform cloud coverage.
tioned in Section 4.1, the variations in these
two regions have positive relationship, and
both of them are related to the Indian monsoon.
Besides, stratiform clouds can also form in the
low vortex, or near the shear line, which all
mainly occur from May to September over the
Plateau. During this period, stratiform clouds
appear frequently.
Over southern China, the seasonal variation
of low and middle stratiform clouds is almost
out of phase with that over northern China and
the Plateau, the maximum occurs in March and
the minimum occurs in August. In cold seasons,
the stronger divergence in the middle troposphere limits the clouds towering to a high
level, and the middle clouds take the large part
of the total cloud coverage. In sumer, with the
decreasing westerlies and near surface air stability, the convective clouds increase, and the
middle cloud coverage decreases correspondingly.
4.4 Seasonal variation of high clouds
Because high clouds are often hidden by
lower clouds from the surface sight, the seasonal variations of Ci and Cs in northern
and southern China, are studied based on the
ISCCP data as shown in Fig. 12a and Fig. 12b,
respectively. The cloud amount of Cc is not
analyzed, because its mean value is less than
2% over China. In order to analyze the relationship between Ci, Cs and deep convection,
we examine the seasonal variation of the deep
Fig. 12. The 9 year (1990–1998) mean
seasonal variations of Cirrus (solid
line), Cirrostratus (dashed line) and
deep convective cloud (dotted-dashed
line) over (a) northern China (averaged
over 35 –50 N, 103 –120 E) and (b)
southern China (averaged over 20 –
35 N, 103 –120 E) derived from the
ISCCP data.
convective cloud coverage defined by ISCCP
simultaneously. Over northern and southern
China, the seasonal variations of Cs are basically consistent with that of the deep convective
cloud coverage, and the maximum occurs in
summer. This is consistent with the theory that
Cs sometimes originates from the detrainment
of the deep convective updrafts. The conclusion can also be confirmed by similar diurnal
changes between Cs and Cb in summer (not
shown). However, Ci has a somewhat different seasonal variation between northern and
southern China. Its seasonal variation is similar to that of Cb over southern China, while is
different to that of Cb over northern China. The
Ci might mainly belong to the front or cyclone
cloud systems over northern China.
772
5.
Journal of the Meteorological Society of Japan
Conclusions
The distributions and seasonal variations of
various cloud types are quite different over
northern China, southern China and the Tibetan Plateau. The reasons of these differences
among three regions are analyzed. The main
conclusions of our study are as follows.
(1) The large annual mean total cloud coverage
mainly distributes over southern China and
southeastern Tibetan Plateau. The seasonal
variations of the total cloud coverage over
northern China and the Tibetan Plateau
are in phase, reaching a maximum in summer. However, the peak of the total cloud
coverage over southern China occurs in
spring, though the seasonal variability is
not very large;
(2) The large low stratiform clouds mainly occur along the southeast coast of China, and
the prevailing cloud type is Stratocumulus,
limited to the boundary layer in cold seasons. The convective clouds, the product
of the thermal effects, mainly occur over
the Tibetan Plateau. The middle clouds,
resulted from the dynamical force of the
Tibetan Plateau, dominate southern China.
The large Cirrus and Cirrostratus cloud coverage mainly occurs over northern China,
and the northern Tibetan Plateau;
(3) The seasonal variations of the convective
clouds in northern China, and the Tibetan
Plateau are in phase, reaching a maximum
in summer. Low and middle stratiform
clouds over these two regions have the similar seasonal variation as that of the convective clouds affected by the Indian monsoon system. Affected by the East Asian
Monsoon system and the Tibetan Plateau
topography, the convective cloud coverage
maximum over southern China occurs in
summer, and the stratiform cloud coverage
maximum occurs in spring, and the minimum occurs in summer;
(4) Cirrostratus and deep convective cloud have
similar seasonal variations, with a maximum in summer, since the Cirrostratus
might mainly develop from the detrainment
of the deep convective updrafts in warm
seasons. The formation mechanism, and the
seasonal variation of Cirrus, may be more
complicated.
Vol. 82, No. 2
Combining several cloud datasets, we describe the distributions and seasonal variations
of various cloud types over three regions in
China. We hope that this study can help us
to understand the climate characteristics of
various cloud types over China, and provide
the observed facts to verify the capability of
the cloud simulation in the general circulation
models.
Acknowledgments
We are grateful to the reviewers whose constructive comments have greatly improved the
revised manuscript. The ISCCP dataset was
downloaded from the website (http://isccp.giss.
nasa.gov) offered by NASA Langley Research
Center, Atmospheric Sciences Data Center.
The surface dataset was offered by the Computing and Information Science Center, Institute of Atmospheric Physics, Chinese Academy
of Sciences. This work was jointly supported by
the National Natural Science Foundation of
China (Grant No. 40233031) and the ‘‘Innovation Program of CAS’’ (Grant ZKCX2-SW210).
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