Spatio-temporal variations in air/ground freezing–thawing indices along the China–Nepal Highway during 1987–2017

Freezing–thawing indices serve as a comprehensive indicator of both the duration of the freezing/thawing periods and the degree of cold and heat in a given region. In-depth analysis of the freezing-thawing indices not only enables the prediction of permafrost distribution and its dynamic changes, but also facilitates the assessment of damage risk to infrastructure under freeze-thaw action. In this paper, the air/ground freezing–thawing indices from 1987 to 2017, based on daily temperature observations from meteorological stations along the China–Nepal Highway (CNH), were calculated, and their spatial and temporal variation patterns were analyzed. The results showed that: (1) Both mean annual air temperature and mean annual ground surface temperature along the CNH fluctuated upward, with climate tendency rates of 0.43 and 0.52℃·(10a)^-1, respectively; (2) The number of days with negative air temperature and ground temperature showed fluctuated downward, with change rates of -8.6 and -8.3 d·(10a)^-1, respectively; (3) The ranges of air freezing index, air thawing index, ground freezing index, and ground thawing index over the years were 157.05-458.88℃·d, 2034.20-2560.73℃·d, 108.78-396.83℃·d, and 3515.25-4288.67℃·d, respectively. The climate tendency rates were -5.42, 10.22, -6.79, and 12.14℃·d·a^-1, respectively, showing a general warming trend; (4) The air freezing index, ground freezing index, and ground thawing index changed abruptly in 1999, 2000, and 2002, respectively, evincing significant changes after 2002. The research results can provide a basis for the risk assessment of freezing–thawing erosion and the prevention and control of permafrost engineering diseases along the CNH.