Gaoyuan qixiang (Jun 2023)
The Spatio-temporal Distribution of the Start-end Date and Freezing Depth and their Relationships with Air Temperature over the Western China
Abstract
Permafrost have a critical impact on the infrastructure construction， hydrology and ecology in the cold region.Under the background of global warming， it is of great significance to explore soil freeze-thawing cycle over the Western China.Based on the natural geographical and permafrost characteristics， the western region of China is divided into four sub-regions as the study region of this investigation.The ERA-5 surface temperature， soil volumetric water content and monthly air temperature data from January 1981 to June 2020 are to be deployed in analyzing the spatio-temporal distribution of soil freeze-thawing status， the active layer thickness and maximum freezing depth over the western China in the past 40 years.the correlationship between variables of freeze-thawing cycles and air temperature and altitude are discussed.The results show that： the spatio-temporal distribution of the start date of freezing and thawing has the characteristics of delayed freezing and early thawing from high-altitude areas to low-altitude areas over the western region.The high-altitude northern Tibetan plateau freezes the earliest， thaws the last， and freezes the longest.The sporadic areas on the Kunlun Mountains can last for more than 300 days.The Tarim Basin in the west-northwest with low altitude and low soil moisture content freezes the latest， melts the earliest， and lasts the longest.The melting in the Taklimakan Desert can last for more than 280 days.The thickness of the permafrost active layer is basically more than 2.0 m， only the area near the Karakoram Mountains has a large area with a thickness of less than 2 meters.the seasonal frozen soil of the Qinghai-Tibet Plateau has the largest freezing depth， and the thickness can reach more than two meters， and the Tarim Basin shows a shallowest freezing depth， and the thickness is less than 0.6 m.The start date of freezing was delayed over the western region， and the start date of thawing was advanced.The date of start freezing and the date of completely frozen increased at a rate of 0.089 d·a-1and 0.061 d·a-1.The date of start thawing and the date of completely thawed decreased at a rate of 0.102 d·a-1 and 0.156 d·a-1.When the freezing trend is rising and the thawing trend is declining， the duration of completely thawed increased by about 12 days at a rate of 0.256 d·a-1， and the duration of completely frozen is shortened by about 11 days with a rate of 0.164 d·a-1， during the study period， the start date of completely thawed was mutated in 1996.The date of start freezing and the duration of completely thawed were abrupted changed in 1997.The overall change trend of the start date of the freezing and thawing is the same over the western region， but there are locally slight differences.The duration of start freezing and completely frozen of the Loess Plateau decreased the fastest at the rate of 0.166 d·a-1 and 0.405 d·a-1.The duration of start freezing and the duration of completely frozen were shortened by around 7 and 16 days.The trends in the duration of completely melt period in all four regions showed an upward trend.However， the duration of completely frozen rises rapidly at a rate of 0.435 d·a-1 over the Loess Plateau， and increase about 17 days during 1981 -2020.There is a significant correlation between the start dates of freezing and thawing and the annual average temperature and altitude.The correlation between the annual average temperature and all freeze-thaw times exceeded 0.79， and the negative correlation between the date of completely thawed and the annual average temperature was as high as 0.963.The start date of freezing was delayed by 2.03 days per 1.0 ℃ increase， the start date of complete freezing was delayed by 2.12 days， and the start dates of completely thawed and frozen were advanced by 5.10 and 5.17 days， respectively.The start dates of freezing and completely frozen are advanced by 6.1 days and 4.5 days per 1000 m rise in altitude， respectively， and the start dates of thawing and freezing are delayed by 14.4 days and 19.9 days， respectively.This research clarifies the situation of soil freezing and thawing and its variation in the past years over the western China， and provides a scientific supports for the ecological system and infrastructure construction over the western region.
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