Temporal And Spatial Distribution Characteristics And Influencing Factors Of Soil Freezing In Northeast China

China is the world third frozen soil country,permafrost and seasonal frozen soil account for about 70% of the total land area,mainly distributed in the Qinghai-Tibet Plateau and the northeast region.In recent years,the temperature increase in Northeast permafrost area is more obvious,and the temperature increase rate is higher than the national level.Therefore,it is necessary to study the freezing-thawing status of nearsurface soil in Northeast permafrost region.Soil freezing-thawing conditions have an impact on global and regional air and energy exchange,vegetation growth,agricultural production,surface runoff,carbon cycle and terrestrial ecosystem.Soil freeze-thaw status is an important indicator of climate change,and in-depth study of soil freeze-thaw status is a prerequisite for better understanding of carbon cycle,land-surface hydrological process,vegetation growth cycle and climate change.Based on the bright temperature data of AMSRE and AMSR-2,the freezing-thawing discriminant function algorithm was used to judge the freezing-thawing status of surface soil in the northeast permafrost region,and the accuracy of the algorithm was verified by0 cm ground temperature data in the northeast permafrost region.The characteristics of soil freezing in Northeast China from 2002 to 2021 were analyzed from the perspectives of annual scale,different permafrost types,different cover types and monthly scale,and the effects of different influencing factors on the days of soil freezing were discussed.The main conclusions of this paper are as follows:(1)Verify the accuracy of the freeze-thaw discriminant function algorithm based on0 cm ground temperature data in the northeast permafrost region.The results show that the overall discriminant accuracy of the algorithm based on freeze-thaw discriminant function is higher than 88%,which proves that the proposed method has good feasibility.In the freeze-thaw transition period(spring melting transition period and autumn freezing transition period),the overall accuracy of discrimination is low,but in the thawing period,the overall accuracy of discrimination is higher,and the accuracy of discrimination in the orbit ascending period is more stable than that in the orbit descending period.(2)The number of frozen days in 2012 was the highest in 19 years.The days of soil freezing were higher in the north than in the south and higher in the east than in the west.The area with the highest value of soil freezing days could reach more than 180 days,which was distributed in the northernmost part of the study area and along the Greater Khingan Mountains.The area decreased from 2003 to 2007 and from 2014 to 2021.The soil freezing days in the range of 90～180 days account for more than 70% of the total area of the northeast frozen soil area,and the distribution range is mainly concentrated in the Xing ’anling Mountains,Songnen Plain,Hulun Buir Plateau and Changbai Mountain,showing a dustpan shape with a southward opening.The region with the soil freezing days in the range of 150～180 days has a decreasing trend.The low freezing days were concentrated within 90 days,mainly in the Liaodong Peninsula,Liaohe Plain and the Inner Mongolia Plateau in the southwest of the study area.(3)In the past 19 years,the number of days of soil freezing for the four types of frozen soils was as follows: large continuous permafrost area > island thaw permafrost area > island permafrost area > seasonal permafrost area.In terms of different cover types,the land cover type with freezing days from high to low is forest land > Wetland >Grassland > Cultivated land > Construction land > Bare ground.(4)At the monthly scale,soil freezing occurred first in the northern Greater Khingan Mountains in September,accounting for 4.4% of the total area of the study area.The soil gradually froze from September to December until the whole study area froze,and lasted until January of the next year.From February to May,the soil freezing area gradually decreased from the south to the north.In May,the soil freezing phenomenon occurred only in the areas along the Greater Khingan Mountains.In June,the soil freezing state ended completely.(5)Among the influencing factors,air temperature has a significant negative correlation with the number of days of soil freezing,and has the strongest explanatory power for the spatial differentiation of the number of days of soil freezing,followed by latitude and solar radiation.In the interactive factor detection,temperature ∩ precipitation has the strongest explanatory power of 0.949 for the soil freezing days,followed by temperature ∩ soil water content and temperature ∩ altitude,both of which have a strong explanatory power of 0.939.