30 Aug 2021

30 Aug 2021

Review status: this preprint is currently under review for the journal TC.

Convective Heat Transfer of Spring Meltwater Accelerates Active Layer Phase Change in Tibetan Permafrost Areas

Yi Zhao1, Zhuotong Nan1,2, Hailong Ji1, and Lin Zhao3 Yi Zhao et al.
  • 1Key Laboratory of Ministry of Education on Virtual Geographic Environment, Nanjing Normal University, Nanjing, 210023, China
  • 2Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing, 210023, China
  • 3School of Geographical Sciences, Nanjing University of Information Science & Technology, Nanjing 210044, China

Abstract. Convective heat transfer (CHT) is one of the important processes that controls the near ground surface heat transfer in permafrost areas. However, this process has often not been considered in most permafrost simulation studies and its influence on the freeze-thaw processes of the active layer lacks quantitative investigation. The Simultaneous Heat and Water (SHAW) model is one of the few land surface models in which the CHT process is well incorporated in the soil heat-mass transport processes. We applied the SHAW model to investigate the impacts of CHT on active layer thermal dynamics on the Tanggula station, a typical permafrost site located at the eastern Qinghai-Tibetan Plateau with abundant meteorological and soil temperature/moisture observation data. The 2008–2009 observed hourly data were used to calibrate the model parameters and those of 2010 for validation. A control experiment was carried out to quantify the changes in active layer thermal regime affected by vertical advection of liquid water, consisting of three setups: using (1) the original SHAW model with full consideration of CHT; (2) a modified SHAW model ignoring the CHT due to infiltration from the surface, and (3) a modified SHAW model ignoring complete CHT processes in the system. The impacts of vapor convection are not considered in this experiment. The results show that the CHT events mainly happened during thawing periods when the active layer melted at shallow (0–0.2 m) and middle (0.4–1.3 m) soil depths, and its impact on soil thermal regime at shallow depths was significantly greater in spring melting periods than in summer. The impact was minimal in freezing periods and in deep soil layers. During melting periods, temperatures in the shallow and middle soil depths simulated under the scenario considering CHT were higher by up to 10.0 and 1.5 °C, respectively, than those under the scenarios ignoring CHT. The ending dates of zero-curtain effect were considerably advanced with CHT considered, due to the warming effect of CHT associated with infiltration. However, the opposite cooling effect also existed due to presence of upward liquid fluxes and thermal differences between the soil layers. In some certain period, the advection flow including partial return flow reduced the temperatures in the shallow and middle depths by as much as −5.0 and −1.0 °C, respectively. The overall annual effect of CHT by liquid flux is to increase soil temperature in the active layer and favors thawing of frozen ground at the study site.

Yi Zhao et al.

Status: open (until 25 Oct 2021)

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Yi Zhao et al.

Data sets

SHAW_CHT_Results Yi Zhao

Yi Zhao et al.


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Short summary
Convective heat transfer (CHT) is important in affecting thermal regimes in permafrost regions. We quantified its thermal impacts by contrasting the simulation results from three scenarios in which the SHAW model is modified to include full, partial and no consideration of CHT. The results show the CHT commonly happens in shallow and middle soil depths during thawing periods and it has greater impacts in spring than in summer. The CHT has both heating and cooling effects on the active layer.