An Assessment of Using Remote Sensing-Based Models to Estimate Ground Surface Soil Heat Flux on the Tibetan Plateau during the Freeze-Thaw Process

The ground surface soil heat flux (G(0)) is very important to simulate the changes of frozen ground and the active layer thickness; in addition, the freeze-thaw cycle will also affect G(0) on the Tibetan Plateau (TP). As G(0) could not be measured directly and soil heat flux is difficult to be observed on the TP in situ due to its high altitude and cold environment, most of previous studies have directly applied existing remote sensing-based models to estimate G(0) without assessing whether the selected model is the best one of those models for those study regions. We use in-situ observation data collected at 12 sites combined with Moderate Resolution Imaging Spectroradiometer (MODIS) data (MOD13Q1, MODLT1D, MOD09CMG, and MCD15A2H) and the China meteorological forcing dataset (CMFD-SRad and CMFD-LRad) to validate the main models during the freeze-thaw process. The results show that during the three stages (complete freezing (CF), daily freeze-thaw cycle (DFT), and complete thawing (CT)) of the freeze-thaw cycle, the root mean square error (RMSE) between the models' G(0) simulated value and the corresponding G(0) "measured value" is the largest in the CT phase and smallest in the CF phase. The simulated results of the second group schemes (SEBAL, Ma, SEBAL(adj), and Ma(adj)) were slightly underestimated, more stable, and closer to the measured values than the first group schemes (Choudhury, Clawson, SEBS, Choudhury(adj), Clawson(adj), and SEBSadj). The Ma(adj) scheme is the one with the smallest RMSE among all the schemes and could be directly applied across the entire TP. Then, four possible reasons leading to the errors of the main schemes were analyzed. The soil moisture affecting the ratio G(0)/R-n and the phase shift between G(0) and net radiation R-n are not considered in the schemes directly; the scheme cannot completely and correctly capture the direction of G(0); and the input data of the schemes to estimate the regional G(0) maybe bring some errors into the simulated results. The results are expected to provide a basis for selecting remote sensing-based models to simulate G(0) in frozen ground dynamics and to calculate evapotranspiration on the TP during the freeze-thaw process. The scheme Ma(adj) suitable for the TP was also offered in the study. We proposed several improvement directions of remote sensing-based models in order to enhance understanding of the energy exchange between the ground surface and the atmosphere.