Spatial and temporal characteristics of the site-specific N-factor over the Qinghai-Tibet Plateau

The N-factor is a vital intermediate parameter in simulating the distribution and dynamics of permafrost. To successfully model or map the permafrost distribution, it is necessary to fully understand the spatial and temporal characteristics of the N-factor and its correlation with local factors. In this study, we preliminarily analyzed the N-factor on the Qinghai-Tibet Plateau (QTP) after calculating the freezing and thawing indices with the long-term observed near-surface air temperature (Ta) and ground surface temperature (GST) from 66 national meteorological stations. Results demonstrated that the freezing N-factor (Nf) ranges from 0 (including 11 stations with daily GST always above 0 °C) in the southeast to 1.29 (at Nuomuhong) in the northwest. The thawing N-factor (Nt) at most of the stations ranges from 1.20 to 1.70, which is lowest at Batang (1.14) and highest at Wudaoliang (2.59), respectively. Mean Nf decreased from 0.85 to 0.58 at a rate of −0.03/a in 1998–2007, then fluctuated around 0.64 in 2008–2020; while mean Nt did not change so abruptly as that of mean Nf, which varied around 1.45 and 1.65 for the respective periods. The variability of Nf in the east of the QTP is larger than that in the middle part and the changing rate of Nf is negative at most stations. The changing rate of Nt ranges from −0.10/10a at Wudaoliang to 0.03/10a at Zedang in 1980–2020. The stepwise multi-factor linear regression revealed that latitude is a vital factor influencing Nf (R = 0.16, p < 0.01), while Nt is negatively correlated with mean annual air temperature (MAAT) (R = −0.18, p < 0.01), longitude (R = −0.07, p < 0.01) and altitude (R = −0.03, p < 0.01). In the context of climate warming, the decrease in the ground surface freezing index (GFI) and air freezing index (AFI) and the increase in the ground surface thawing index (GTI) and air thawing index (ATI) with the same amounts are likely to cause the decrease of both Nf and Nt. As the start and end of the freezing periods may vary greatly, the change of Nf is larger than that of Nt. We conclude that more synergistic observations of near-surface air, land surface, and ground surface temperatures and their interactions with local factors are required to clarify the spatial heterogeneity and the parameterization of N-factors on the QTP.