4.1.1.

Eight-day composite snow cover changes

Figure 2 shows the time series of SCA changes from January 2001 to December 2013 over the TP using MOD10A2 products. The general patterns present a great seasonal variation of snow cover with a large area in the 8-day composites during winter and spring and a small area in the 8-day composites during summer. The maximum SCA for 8-day composites occurs from October 31 to November 7 in 2008 with an area of $111.16\times {10}^4{\mathrm{km}}^2$ and 44.32% of the total TP area, followed by November 9 to 16 in 2002 and December 11 to 18 in 2006 with around 43% of the total TP area, whereas the minimum SCA is found in an 8-day composite MODIS image from August 13 to 20 in 2010 with only $5.51\times {10}^4{\mathrm{km}}^2$ and 2.20% of the total TP area, followed by July 28 to August 4 in 2013 with $5.82\times {10}^4{\mathrm{km}}^2$ (2.32%). Then SCA in 8-day composites from August 5 to 12 both in 2010 and 2003 and July 4 to 11 in 2013 is also around $5.94\times {10}^4{\mathrm{km}}^2$ and 2.37% of the total TP area.

Fig. 2

The time series of 8-day snow cover over the TP from 2001 to 2013 based on MOD10A2.

If an SCA above 25% of the TP area in 8-day composites is used as a threshold for more snow cover, all of these occur from October to May, with the top three months in autumn and spring instead of the winter months. In detail, the most frequent months for snow cover above 25% are November with 23 times, the second is March with 21 times, followed October with 19 times, and the least frequency happens in May with seven times. Although based on the threshold for less snow cover, less than 5% of the TP area, the most frequent months are July with 51 times, followed by August with 32 times, and the least frequency occurs in September with six times (Table 1). Overall, mainly affected by atmospheric circulations and its seasonal variations over the TP, the larger snow cover extent in an 8-day period is more frequent in November and March during months of the transition seasons and the least snow cover extent in an 8-day composite occurs in July.

Table 1

The frequencies of SCA above 25% and below 5% of the TP area in MOD10A2 from 2001 to 2013.

In addition to prominent seasonal cycles of snow cover with a larger extent in 8-day composites in autumn, winter, and spring and a smaller extent in summer, the linear trend analysis for MODIS 8-day composite products indicates that there is no obvious interannual change trend in SCA on the TP over the past 13 years and general snow cover extent remains a relatively stable.

4.1.2.

Monthly changes

The monthly SCA is shown in Fig. 3, indicating that the minimum is reached in July in most years followed by August, while with an increase in SCA from September, the maximum is reached in November or March in most years from 2001 to 2013. The increasing trend from December to February in the following years is not obviously clear, although the SCA remains relatively high in these three winter months due to the early snow accumulation and low temperatures, which are very favorable for snow cover. The monthly SCA from 2001 to 2013 also shows that there is a slightly declining trend in SCA from 2001 to 2013, especially in November 2008 the decrease in SCA is more obvious but statistically insignificant. As described above, in addition to the strong seasonal variations, the SCA on the TP over the last 13 years remains relatively stable.

Fig. 3

Monthly snow-covered area (SCA) over the TP from 2001 to 2013 based on MOD10A2.

The monthly mean SCA from 2001 to 2013 in Fig. 4 shows that the mean minimum SCA occurs in July with $8.75\times {10}^4{\mathrm{km}}^2$, which is only 3.49% of the total TP area, followed by August with $10.03\times {10}^4{\mathrm{km}}^2$ and 4.00% of the TP area. The increasing trend of SCA is rapid from September to November and the second monthly largest extent of snow cover is reached in November, which is next to March (to $60.79\times {10}^4{\mathrm{km}}^2$) in spring. In three months of winter, the increase of SCA is not so much, but due to the early snow accumulation and favorable low temperatures, the SCA maintains a relatively high status with above $47\times {10}^4{\mathrm{km}}^2$. Along with a gradually increase in temperature after March, the SCA decreases due to melting.

Fig. 4

Monthly mean SCA over the TP from 2001 to 2013 based on MOD10A2.

4.1.3.

Annual changes

The annual mean SCA for the TP is shown in Fig. 5. The average annual mean SCA from 2001 to 2013 is $41.93\times {10}^4{\mathrm{km}}^2$, which is 16.72% of the total TP area. Thus, for the TP, the SCA of $42\times {10}^4{\mathrm{km}}^2$ or 17% of the TP area covered by snow at an annual mean level can be considered as a snow cover indicator for a normal year. Figure 5 shows that 2002, 2005, and 2008 are snowy years with 46.92, 47.84, and $47.05\times {10}^4{\mathrm{km}}^2$, and 18.70%, 19.07%, and 18.76% of the total TP area, respectively. Snow cover extent in these three years is larger by 5 to $6\times {10}^4{\mathrm{km}}^2$ than the average from 2001 to 2013.

Fig. 5

Annual mean SCA over the TP from 2001 to 2013 from MOD10A2.

In contrast, the SCA for 2010 is the least with $35.36\times {10}^4{\mathrm{km}}^2$ and 14% of the TP area, followed by 2001 and 2003 with 15% of the total TP area. As another less-snow year, the SCA in 2007 is $39.75\times {10}^4{\mathrm{km}}^2$ (15.85%), which is lower by $2.19\times {10}^4{\mathrm{km}}^2$ than the average SCA from 2001 to 2013. In other years, the SCA ranges from 15.96% to 17.90% of the study area and the differences in SCA from 13 years’ average are lower than $2.0\times {10}^4{\mathrm{km}}^2$, presenting these years as normal for snow cover extent. The standard deviation of SCA from 2001 to 2013 is $3.96\times {10}^4{\mathrm{km}}^2$ and the average absolute error is $3.07\times {10}^4{\mathrm{km}}^2$, ranging from 0.09 to $6.58\times {10}^4{\mathrm{km}}^2$. The relative error ranges from $-15.69\%$ to 14.09%. Therefore, interannual differences and variations of SCA over the TP from 2000 to 2013 are not significant. As shown in Fig. 5, a very slight declining trend exists from 2001 to 2013 and the linear equation is $y=-0.0866x+42.54$ ($R^2=0.0072$), which means it is not statistically significant.

At a seasonal level, the maximum SCA is reached in spring with $54.28\times {10}^4{\mathrm{km}}^2$ and 22% of the total TP area and the second is in winter with $53.80\times {10}^4{\mathrm{km}}^2$ (21%), followed by autumn with $44.90\times {10}^4{\mathrm{km}}^2$ (18%). The minimum occurs in summer with only 6% of the total TP area. The main patterns of SCA seasonal changes from March 2001 to December 2013 are that there is a slight decreasing trend in SCA during the spring and winter and a more obvious decrease at 0.05 level of significance during the summer (figure not shown here), while an increasing trend in the SCA is found in autumn, which is not statistically significant based on the linear trend analysis.

4.2.1.

Annual snow cover distribution

The spatial distribution of snow cover on the TP is shown in Fig. 6 using the snow cover fraction (SCF) derived from MOD10A2. Owing to high topography and complex terrain in combination with the atmospheric circulation, the spatial distribution of snow cover represented by the annual mean SCF from 2001 to 2013 over the TP, as shown in Fig. 6(a), is spatially very uneven. Overall, snow cover is rich and its duration is long with more permanent snow in the high mountains while snow cover and duration are limited in the vast interior of the TP except for some high mountains. In detail, there are two highest snow cover concentrated areas in the TP. One is Nyainqentanglha Mountain ranges and the surrounding high mountains in the southeastern TAR; another is the west Himalayans, Pamirs, and Karakoram Mountains to the western Kunlun Mountains. In addition to these two regions, more areas covered by snow are the Himalayan regions in the south and Kunlun Mountain in the north due to favorable high terrain conditions and low temperatures for snow accumulation. The largest area and the longest duration of snow cover in the interior of the TP occurs in Tanggula, Animaqing, Bayan Har, Hengduan, and Qilian Mountain ranges, although snow cover in the vast interior of the TP is limited. Snow cover in Qaidam Basin located in the northern TP and the river valleys of southern TP is very rare and the duration is $<10$ days. Moreover, the snow cover in wide areas in the north of Gangdise Mountain, Qinghai Lake valley, and east regions of Mt. Animaqing is also very small.

Fig. 6

Annual mean snow cover fraction (SCF) (a), mean SCF in spring (b), summer (c), autumn (d), and winter (e) from 2001 to 2013 over the TP.

4.2.2.

Seasonal snow cover distribution

The seasonal snow cover distribution is further analyzed on the basis of seasonal average SCF over the TP from 2001 to 2013, as shown in Figs. 6(b)–6(e). As for the TP, spring is the most snow-covered season and the area of $\mathrm{SCF}\ge 50\%$ accounts for 12% of the total TP area. During this season, as shown in Fig. 6(b), the widespread snow cover is distributed in the Nyainqentanglha Mountains, Himalayas, and Karakoram in the northwest, the Kunlun and Altun Mountain ranges in the north, and the eastern plateau and Qilan Mountains in the northeast, whereas the region where the SCF is $<10\%$ is mainly situated in Qaidam Basin, the southern valley of Lake Qinghai, the Yarlung Zangbo River valley, and the broad region in the north of the Gangdise Mountains, which is in accord with the annual mean distribution of snow cover over the TP.

In summer, as shown in Fig. 6(c), the TP is least covered by snow and the area of SCF above 50% only accounts for 2% of the TP area. Since it is rainy season for the TP and the temperature is above 0°C, there is no snow cover in most areas on the TP except for some high mountain ridges where the temperature is lower than 0°C. Relatively speaking, more snow cover can be found at the higher latitudes in the northern high mountain ridges such as Karakoram, Kunlun, and Qilan Mountains compared to the lower-latitude mountain ridges such as the Nyainqentanglha Mountains and Himalayas.

In autumn, the increase of snow cover over the TP represented by the SCF is obvious and the area of SCF above 50% occupies 7% of the total TP area. In addition to the apparent increase of snow cover in the high mountains such as the Nyainqentanglha, Karakoram, Kunlun, Tanggula, and Qilan, the apparent feature of the snow cover distribution in autumn compared to other seasons is that the increase of snow cover and SCF in the eastern TP, especially around Tanggula Mountain and broad regions where Bayan Har and Anyemaqen Mountain ranges are located, are significant, as shown in Fig. 6(d). The increase of snow cover and SCF is more obvious in the northern, central, and eastern TP. The location of SCF below 10% is similar to that in spring or the annual average.

In winter, the snow cover extent over the TP is second only to the spring. The area of $\mathrm{SCF}\ge 50\%$ is 11% of the TP area, which is slightly smaller than the 12% of spring. The main characteristics of snow cover distribution in winter are that due to the decrease in temperature in the whole TP, particularly at lower latitudes compared to the previous season, which is very favorable for snow accumulation and maintenance, the increase in snow cover in the southeastern TP such as the Nyainqentanglha Mountain ranges and surrounding area is the most significant, followed by the Bayan Har and Anyemaqen Mountain ranges and surrounding areas. On the other hand, an increase in the snow cover extent in the west Himalayas is more obvious. However, the increase in the snow cover extent in the northwest and north of the TP is not apparent, as shown in Fig. 6(e). The location where the SCF is lower than 10% is in agreement with other seasons like spring, autumn, and the annual average.

These spatial distributions of snow cover on the TP above are controlled by a combination of spatially and temporally variable atmospheric forcing conditions and how those conditions interact with relatively static local topography and vegetation distributions. Snowfall is a solid form of precipitation and the only material source of snow cover. There is no snow cover on the surface without snowfall. Snow cover is also very sensitive to temperature. Higher temperature will cause precipitation falling as rain instead of snow or will make terrestrial snow cover melt and make difficulty to form snow cover on the ground. The increase in temperature will lead to a shortening of the duration of the snow cover and is not favorable to the maintenance of the snow cover. The high topography of the TP is also an important condition for the spatial distribution of snow cover. At the higher altitude regions, air temperature is lower and precipitation is more, which is more favorable for the formation and maintenance of snow cover. This is why the most persistent snow cover is located at the regions where the elevation is above 6000 m, whereas the lower latitudes and elevations, because of higher temperatures, are not conducive to snowfall and snow accumulation and there is no snow covered in many places. In the TP, the high mountains and low lands are the most consistent with more and less snow cover distributions.²⁶