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

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.