SPATIAL AND TEMPORAL DISTRIBUTION OF THE CLOUD HEIGHT AND CLOUD THICKNESS OVER CHINA AND THE ADJACENT AREAS BASED ON CALIOP

Based on CALIOP data, spatial and temporal distribution of cloud height and thickness over China as well as the adjacent areas have been analysed in this paper. The results show significant regional differences. The heights of clouds that lie to the south of 27.6°N are greater than those to the north of 27.6°N. The highest and thickest clouds are located above the Bay of Bengal and the Western Pacific, while the lowest values distribute in the northwest of the Tibetan Plateau and Sichuan Basin. The clouds ranging from 0.3 km to 0.6 km thickness account for a large proportion of total clouds. And the probability of occurrence of clouds decreases as cloud thickness increases. Overall, within the area under study, the thick clouds are higher than the thin clouds. Besides, low and thin clouds occur more frequently than high and thick clouds. As for their seasonal variation, the height of clouds peaks in summer. In addition, the mean of cloud thickness to the south of 27.6°N is 2.4 km thicker in summer than in any other season.


INTRODUCTION
The energy distribution of the earth-atmosphere system is regulated and influenced by cloud radiation.When covered by clouds, the earth's albedo is twice as high as that under clear-sky conditions (Ramanathan et al., 1989;Curry et al., 1996).This effect is closely related to the microphysical properties, macroscopic properties, and optical properties of the cloud (Charlson et al., 1987;Albrecht et al., 1988;Kiehl, 1994).In the analysis of cloud radiation characteristics, CVS (Cloud Vertical Structure) has been receiving much attention.Its essential components, namely, Cloud height (incl.Cloud Top Height, CTH, and Cloud Base Height, CBH) and thickness, are also significant for the study (Wang et al., 2000).
On the premise of considering the cloud as a uniform single layer, some studies have discussed the distribution characteristics of cloud amount and types, as well as their effects on the radiation balance of the earth-atmosphere system based on data detected via traditional methods.(Wang et al., 1998;Rossow et al., 1999;Chen et al., 2000;Warren et al., 2007).There are also preliminary studies that analyze the characteristics of the CVS distribution.(Wang et al., 2000;Mace et al., 2009;Bourgeois et al., 2016;Yan, 2016).However, studies have shown that multi-layer clouds (N>1) play a vital role in the radiation balance of the earth-atmosphere system when clouds overlap.(Stephens et al., 2004;Huang et al., 2006;Pan et al., 2015).Therefore, in order to obtain more accurate distribution characteristics of CVS, this paper analyses the distribution characteristics and variation laws of single-layer and multi-layer clouds at different heights in China and the adjacent areas during the past ten years by using satellite data, which may provide references for climate change research.The cloud top pressure in the Tibetan Plateau and the Mongolian Plateau is less than 440hPa, and the altitude is above 7km, both of which are dominated by single-layer high clouds (Cai et al., 2017).Therefore, in this study, the characteristics of the relative heights of the clouds are smaller than those of other regions.

Horizontal Distribution of Cloud Thickness:
Overall, the cloud thickness varies from 0.3km to 3.6km, with the range of 1.5~1.8kmaccounting for 37%, which concentrates in the area north of 27.6°N, especially the main body of the Tibetan Plateau, the Tarim Basin and the Mongolian Plateau.
The clouds thickness in the western part of the Tibet Plateau, the Sichuan Basin and the East China Sea is 1.2 to 1.5 km, and the PDF (Probability Density Function) is 21%.The thickness of the clouds between 2.1 and 2.4km accounts for 12%, mainly in the low-latitude region to the south of 27.6°N.
In order to further analyse the regional characteristics of cloud

Relationship between Cloud Height and Cloud
Thickness: By calculating the joint probability density of cloud height and thickness, their corresponding relationship in different regions is discussed (Fig. 6).Within the areas under study, the thick clouds are higher than the thin clouds, and low and thin clouds occur more frequently than high and thick clouds.
In Area1, the CTH with a high probability density is no more than 3km and the thickness is less than 1.5km.High and thick clouds have a maximum probability density of 5%.There are about 4% of the CTH in the 13~16km and the thickness is 4km.
There are also some clouds in between, with a probability of around 5%.The clouds in Area2 are relatively higher and the thickness also increases, but the highest CTH does not exceed    Mongolian Plateau area exceeds 1.6km, and even 2.4km in some areas, and decreases in autumn.The thinnest cloud occurs in summer, mainly varying from 1.2km to 1.6km.

Seasonal Variation in the Relationship between
Cloud Height and Cloud Thickness: In the area under study, the relationship between cloud height and thickness of single-layer clouds and multi-layer clouds in different regions and seasons is shown as follows (see Figure 11).In general, the seasonal variations of CTH and CBH in different levels in each region are basically the highest in summer and the lowest in winter.In winter, the cloud height of Area2 is significantly higher than that of the other three seasons.It is worth mentioning that, the CTH and CBH of single-layer clouds, and CTH of double-layer clouds, are higher in spring than in summer.In Area4, the CTH of the third layer of three-layer clouds is higher in winter than in summer.(1) There is a clear regional difference between CTH and CBH in different levels of clouds.The heights of clouds that lie to the south of 27.6°N are greater than those to the north of 27.6°N.
The highest and thickest clouds are located on the Bay of Bengal and the Western Pacific, as the CTH and CBH of the first layer clouds can be as high as 14km and 11km, respectively.
The CTH in the western part of the Tibetan Plateau is 3~4km, while the CBH is less than 3km.Values of cloud heights in the southwest region, the southeast part of China's mainland, and over the eastern seas are similar to that in the Tarim Basin, but all in the middle of high and low limits.The cloud height in the Mongolian Plateau is lower than that in the Tarim Basin and its eastern regions.Compared with the first level clouds, the second level clouds and the third level clouds have the same distribution characteristics, but the height changes prominently.
Among them, the range of 1.5~1.8kmaccounts for 37% of the total.The thickness of the clouds in the western part of the Tibet Plateau, the Sichuan Basin and the East China Sea is 1.2 to 1.5 km, and the PDF is 21%.The thickness of the clouds between 2.1 and 2.4km accounts for 12%, mainly in the low-latitude area to the south of 27.6°N.
China and the adjacent areas (0°N~55°N, 70°E~140°E) as research subjects, and divides the areas into 1.2°×1.2°latitude and longitude grids according to the coverage of CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) orbits (Figure 1).Using the Standard-V4-10) from June 2006 to May 2016, the height and thickness distribution characteristics of different level in the areas under study were analysed.

Figure 1 .
Figure 1.Track coverage of CLIPSO satellites over China and the adjacent areas within one year In this study, the height of clouds is the relative height after subtracting the DEM_Surface_Elevation.
CTH and CBH: There is a significant regional difference between CTH and CBH in different level of clouds.Based on the detection method of CALIOP, the first layer contains the single layer clouds, the upper layer clouds of the double-layer clouds and the top layer clouds of the three-layer clouds.With 27.6°N as the boundary, the CTH is over 9km to the south of 27.6°N.In the Bay of Bengal, the Indochina Peninsula and parts of the Malay Peninsula, the CTH of the first layer clouds can be as high as 14km.27.6°N ~36°N , in Tibetan Plateau with an altitude over 3.1km, the CTH in the western part of the plateau is 3~4km, which is 1~2km lower than the eastern part of the plateau.At the same latitude, in the eastern part of China, in the eastern seas and in the Tarim Basin, the CTH of the first level clouds is 7~8 km.To the north of 46.8°N, the CTH of the Mongolian Plateau is about 6km (Figure 2).As shown in Figure 2b, the CBH distribution characteristics of the first layer of clouds are consistent with CTH.In the low latitude tropics to the south of 27.6°N, CBH is more than 10km, except for the northern part of the Indian Ocean, and the Tibetan Plateau is less than 4km.The CBH in the Sichuan Basin and its surrounding areas are lower than that in the eastern part of China.The variation range of the Mongolian plateau CBH is 3~6km.

Figure 3 .
Figure 3. Horizontal distribution (a) and PDF (b) of the cloud thickness over China and the adjacent areas.The shaded part indicates that the surface elevation is above 3.1km (680hPa)

Figure 4 .
Figure 4. Six sub-areas of China and the adjacent areas.
Figure 6.Joint probability density distribution of cloud thickness and CTH in sub-regions of China and the adjacent areas.

Figure 7 .
Figure 7.The height of different levels (first, second, third) clouds in sub-regions over China and the adjacent areas, km.
CTH and CBH: The distribution characteristics of CTH and CBH in different layers are basically the same as their horizontal distribution, but the seasonal variations are obvious (Figure 8 and Figure 9).Summer has the highest cloud height, followed by spring, autumn and winter.In summer, the CTH of the uppermost clouds in the area south of the Tibetan Plateau is above 11km, and the CBH is no less than 9km.The CTH and CBH in the eastern coastal areas of China are 8~10km and 6~9km, respectively.In the majority of other regions, CTH also exceeds 6km and CBH exceeds 5km.The CTH and CBH of the uppermost cloud of the Tibetan Plateau are 3~6km and 3~5km.In spring, the CTH of the first layer of clouds in the east of the Tarim Basin increases, while CTH of the remaining areas decreases by 1 to 2 km.The CBH changes are small.In autumn and winter, the CTH of the first layer of clouds does not change much in the Tibetan Plateau.In winter, the CBH of the Mongolian Plateau reduces by about 1km, and the CTH and CBH of the first level clouds in the eastern China decrease to 5km and 4km, respectively.

Figure 8 .
Figure 8. Distribution of CTH in different levels (first, second, third) over China and the adjacent areas in different seasons (MAM, JJA, SON, and DJF), km.The shaded part indicates that the surface elevation is above 3.1km (680hPa)

Figure 10 .
Figure 10.Distribution of cloud thickness in different seasons (MAM, JJA, SON, and DJF), km.The shaded part indicates that the surface elevation is above 3.1km (680hPa) data, spatial and temporal distribution of cloud height and thickness over China as well as the adjacent areas have been analysed in this paper.According to the characteristics of CTH and CBH distribution in different regions, the research scope has been divided into six sub-regions.By discussing the horizontal distribution and seasonal variations of cloud height and cloud thickness in different regions, we have a more detailed understanding of CSV in China and the adjacent areas.The analysis results are as follows:

( 3 )
The distribution characteristics of CTH and CBH in different layers are basically the same as their horizontal distribution, but the seasonal variations are obvious.Summer has the highest cloud height, followed by spring, autumn and winter.The seasonal variation of cloud thickness is very prominent.The clouds in the east of the Tarim Basin and the Mongolian Plateau are thicker in spring and winter, and the thinnest in summer.In addition, the cloud thickness in area to the south of 27.6°N is more than 2.4 km in summer.(4) The cloud in Area1 is highest overall, followed by Area5, and Area6 is the lowest.The clouds develop higher in the low-latitude Area1 than mid-latitude Area5, and the clouds in the high-latitude plateau Area6 are lower than in the mid-latitude Area2.Overall, the seasonal variations of CTH and CBH in different levels in each region are basically the highest in summer and the lowest in winter.Overall, within the area understudy, the thick clouds are higher than the thin clouds, and low and thin clouds occur more frequently than high and thick clouds.Nevertheless, single-layer clouds are both lower and thicker than the first layer of three-layer clouds and the upper layer of double-layer clouds.