| In the background of global warming,the cryosphere,as an indicator of climate change,has undergone significant changes in the past few years.Snow is one of the most active elements of the cryosphere.Because of its high sensitivity to climate change and its important feedback on climate change,it has attracted more and more attention.The Karakoram-West Kunlun area is most covered by snow,which is a typical area for snow cover research in high Asia.Based on the GEE cloud platform,this paper analized the temporal and spatial distribution and change characteristics of the Snow Cover Fraction(SCF),Snow Cover Duration(SCD),First Snow Day(FSD)and Last Snow Day(LSD)as well as regional lower limit of snow cover from 2003 to 2018 by using the improved Terra–Aqua MODIS daily cloud-free snow and Randolph Glacier Inventory 6.0 combined product(M*D10A1GL06).By combined with terrain factors such as elevation,aspect,and slope,as well as climate factors such as temperature and precipitation,we also explored the influencing factors.The main conclusions are as follows:(1)The annual average snow cover fraction of the study area is about 40%.The snow cover fraction shows a slow decreasing trend between the 16 years.From the perspective of elevation,the snow cover fraction changes by most activity mode in the 5000 m to 6000 m elevation zone;and the snow cover fraction has a severe interannual fluctuations in the 30° to 40° slope range.The range of snow cover of the shaded slope is larger than that of the sunny slope;The change of snow cover fraction during one year showed a "single peak" characteristic.The highest snow cover fraction appeared in February,accounting for an average of about 54%,and the minimum snow cover rate was in August,with an average coverage rate of 24%.(2)The Snow Cover Duration in the study area showed the distribution characteristics of "higher in the west and lower in the east".The areas with significantly reduced Snow Cover Duration were mainly distributed in the western part of the West Kunlun Mountains and the northern part of the Karakoram Mountains,and the northwestern part of Ali.The areas with significant increasing rate are mainly located in the eastern part of West Kunlun and the northeastern part of Karakorum.(3)The spatial distribution and altitude of the First Snow Day and the Last Snow Day in the study area are highly consistent with the elevation.Except for the premanent snow covered areas,the First Snow Day in the transitional areas of mountains and basins was from early October to mid-December,and the Last Snow Day was at the end of April.The area where the First Snow Day was significantly delayed is located in the western part of the Western Kunlun Mountains and the northern part of the Karakoram Mountains,where on the east and west sides of the Yarkant River Basin,the delayed rate of the First Snow Day is greater than 2 days/year,and the area where the First Snow Day was significantly advanced includes the parts of Aksai Chin Basin and the eastern part of West Kunlun.The Last Snow Day showed an early trend in most areas,and the overall average rate of change in the region is-0.267 days/year.(4)The regional lower limit of snow cover of the study area is a single valley distribution during the year.It was the lowest in February,with an average of 4359 meters,and the highest in August,with an average of 5196 meters.In terms of interannual changes,the height of the regional lower limit of snow cover changed most drastically in autumn,while it is relatively stable in summer.Compared with autumn and spring,the increasing trend of the regional lower limit of snow cover in winter and summer was more obvious,and the rising rate was 3.46m/year and 2.91m/year,respectively.In terms of spatial distribution,the overall regional lower limit of snow cover was higher in the west and lower in the east.The high-value area has a concave spatial distribution characteristic from the southeast to the northwest,except for the significant increasing rate of western Kunlun Mountains and parts of the Karakoram northwest(i.e.some areas of Yarkant River),the height of the lower boundary of the snow cover in other regions did not changed significantly.(5)The regional lower limit of snow cover was closely related to temperature and precipitation,and their correlations are significantly different in space.Since the precipitation in autumn,winter and spring is mostly being the solid precipitation,the lower boundary of snow in most parts of the study area has a significant negative correlation with precipitation in autumn,winter and spring,and the areas with significant negative correlation(P<0.05)account for 35.9%,26.4% and 23.6% of each,while the relationship between them was not significant in summer.The temperature was positively correlated with the regional lower limit of snow cover in most of the study areas throughout the year,and the significant positive correlation(P<0.05)accounts for about 57.3%,18.9%,43.7% and 69.4% in autumn,winter,spring and summer.While the temperature in the fall,winter and spring have an insignificant negative correlation with the regional lower limit of snow cover in some parts of the Karakoram,may be due to the warm and humid air currents bringing snowfall,resulting the decreasing of the regional lower limit of snow cover. |
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