Submitted to: Journal of Geophysical Research Atmospheres
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/6/2024
Publication Date: 10/22/2024
Citation: Li, N., Cuo, L., Zhang, Y., Flerchinger, G.N. 2024. Diurnal soil freeze-thaw cycles and the factors determining their changes in warming climate in the upper Brahmaputra basin of the Tibetan Plateau. Journal of Geophysical Research Atmospheres. 129(20). Article e2023JD040369. https://doi.org/10.1029/2023JD040369.
DOI: https://doi.org/10.1029/2023JD040369

Interpretive Summary: Diurnal freeze–thaw cycles (DFTC) have a profound influence on hydrological process, runoff/erosion, vegetation phenology, biogeochemical cycling and regional climate. The spatiotemporal pattern of DFTC and its response to climate change remain poorly understood. This study conducted a comprehensive assessment of the spatiotemporal variations of DFTCs in response to climate change on the Tibetan Plateau (TB), which is characterized by the largest areas of freeze–thaw terrain in the mid– and low–latitudes of the world. We found that DFTC area and DFTC frequency increase insignificantly over 1980–2018 within the TB. Air temperature effect on changes in DFTC frequency concentrates around the freezing point. Precipitation, snow depth and seasonal warming/cooling rates are the top three factors decoupling of the response of DFTC to air temperature change. Observation– and simulation–based results corroborate each other concerning the effects of rainfall and snowfall on soil thermal regime. However, snowfall plays a more important role in the variability of DFTC frequency than rainfall.

Technical Abstract: Soil freeze–thaw cycles play a critical role in ecosystem, hydrological and biogeochemical processes, and climate. The Tibetan Plateau (TP) has the largest area of frozen soil that undergoes freeze–thaw cycles in the low–mid latitudes. Evidence suggests ongoing changes in seasonal freeze–thaw cycles during the past several decades on the TP. However, the status of diurnal freeze–thaw cycle (DFTC) of shallow soil and its response to climate change remain less well known. In this study, using in–situ observations, latest reanalysis, machine learning and physics–based modelling, we conduct a comprehensive assessment of the spatiotemporal variations of DFTC and DFTC response to climate change in upper Brahmaputra (UB) basin. About 24 ± 8% of the basin is subjected to DFTC with mean frequency of 87 ± 55 days during 1980–2018. DFTC area and DFTC frequency show little long–term changes during 1980–2018. Air temperature impacts on DFTC frequency changes center mainly around the freezing point. Precipitation (27.4 ± 1.6%), snow depth (23.4 ± 1.1%) and seasonal warming/cooling rates (14.9 ± 0.9%) are the top three factors to account for the spatial variations of the response of DFTC to air temperature change. Both rainfall and snowfall reduce diurnal fluctuations of soil temperature subsequently reducing DFTC frequency, primarily by decreasing daytime temperature through evaporation–cooling and albedo–cooling effects, respectively. Snowfall plays a more important role in the variability of DFTC than rainfall. These results provide an in–depth understanding of diurnal soil freeze–thaw status and its response to climate change.