| 19 Jun 2026
From sub-daily to multi-year: permafrost ground surface deformation processes revealed by collocated multi-sensor observations at a supersite on the Tibetan Plateau
Abstract. Ground surface deformation in permafrost terrain provides critical information on heat and mass transfer during soil freezing and thawing, and serves as a key indicator of permafrost dynamics. However, continuous observations at minute-scale resolution remain extremely rare, sub-daily deformation processes are still poorly documented, and the consistency among remote sensing, contact, and non-contact in situ measurements has not been systematically evaluated. To address these gaps, we established an intensively instrumented permafrost deformation monitoring supersite in an alpine meadow on the central Tibetan Plateau. Ground surface deformation was measured using collocated linear variable differential transformer (LVDT) sensors, ultrasonic ranging, GNSS interferometric reflectometry (GNSS-IR), and Sentinel-1 SBAS-InSAR, together with multi-layer soil temperature and moisture observations. Automated LVDT-based observations provided continuous 5 min deformation records with sub-millimetre precision from 2022 to 2026, thereby resolving, for the first time, sub-daily deformation processes across different freeze–thaw stages. Results show that active layer thaw settlement commonly develops in a stepwise manner within a day, whereas late-thaw-season subsidence associated with excess ground ice melt is more continuous, reflecting sustained drainage and compression. This process also gives rise to a breakpoint-style acceleration in subsidence during the late thaw season. The sub-daily record further clarifies the origin of short-lived late-winter heave events, which are most consistent with infiltration and rapid refreezing of liquid water in shallow soil. The four deformation datasets show broadly consistent temporal patterns. LVDT and InSAR agree closely (r = 0.91), whereas GNSS-IR and ultrasonic ranging show even stronger agreement (r = 0.96). InSAR maintains strong agreement with the other observation methods in both the freezing and thawing seasons, with correlation coefficients consistently exceeding 0.86. GNSS-IR and ultrasonic ranging are more affected by snow and vegetation and require substantial filtering, making them more suitable for seasonal- to multi-year monitoring than for resolving subtle sub-daily signals. At our site, settlement during the active layer thaw stage is strongly correlated with the square root of thawing degree days across all four thaw seasons (R2 > 0.96), and ice–water phase change within the active layer can explain more than 78 % of the observed seasonal deformation. These results provide a valuable observational benchmark for permafrost deformation from sub-daily to multi-year timescales and support improved process-based modelling and interpretation of GNSS-IR and InSAR observations in permafrost regions.
This manuscript presents a valuable multi-year, high-temporal-resolution dataset of permafrost ground-surface deformation from the central Tibetan Plateau. The combination of 5-min LVDT measurements, ultrasonic ranging, GNSS-IR, Sentinel-1 InSAR, and soil hydrothermal observations provides a relatively unique opportunity to examine ground deformation across sub-daily to multi-year timescales. In particular, the high-frequency LVDT observations have the potential to improve our understanding of deformation processes that are difficult to resolve using conventional field measurements or satellite observations. However, several methodological and interpretational issues need to be addressed before the main conclusions can be adequately supported.
Major comments:
1. The uncertainty of the LVDT measurements needs to be evaluated much more carefully, particularly because some of the sub-daily deformation signals interpreted in the manuscript are very small. The stated sensor resolution does not directly represent measurement accuracy, and possible effects from instrumental noise, thermal expansion/contraction, and benchmark stability should be quantified before sub-millimetre deformation is interpreted physically.
2. The representativeness of the LVDT measurements also needs further consideration. The LVDT measures deformation over an approximately 8-cm footprint, while vegetation conditions around the footplate appear substantially different between summer and winter, and the manuscript does not clearly demonstrate whether vegetation growth, surface contact, or local microtopography can affect the measurement. More importantly, it is unclear whether deformation measured at such a small footprint can reasonably represent the much larger footprints of GNSS-IR and particularly the 40–80 m scale represented by InSAR. This scale mismatch should be explicitly evaluated rather than treating the four measurements as directly comparable.
3. The multi-sensor comparison needs to be redesigned more rigorously. As shown in Figure 7, the deformation series can be referenced to a common starting date, whereas some of the subsequent comparisons appear to retain different offsets or reference conditions. More importantly, the very high correlations among the four datasets are somewhat misleading because all records contain a strong common seasonal cycle and long-term trend. A high Pearson correlation therefore does not necessarily indicate measurement agreement. The authors should compare the datasets over exactly the same time period and reference date and evaluate differences in magnitude, seasonal amplitude, trend, and residual deformation in addition to correlation.
4. The statement that changes in active-layer water content can explain more than 78% of the seasonal deformation is not physically clear. The manuscript appears to relate changes in measured volumetric liquid water content to surface deformation, but liquid-water-content changes during freezing and thawing cannot directly be converted into vertical ground deformation without considering soil porosity, ice segregation, water migration, soil compressibility, and the volume change associated with phase transition. The calculation and its physical assumptions need to be fully explained and justified; otherwise, this quantitative conclusion should be removed or substantially weakened.
5. There is currently no clear evidence demonstrating that the observed late-season breakpoint acceleration is caused by excess ground-ice melt. The increase in soil moisture at depth and the simultaneous acceleration in subsidence are consistent with deeper thaw, but they do not distinguish thaw of excess ground ice from thaw of ordinary pore ice or subsequent consolidation and drainage processes. Unless independent borehole or cryostratigraphic observations demonstrate excess ice at the corresponding depth, the interpretation should be presented as a possible mechanism rather than direct evidence.
6. More generally, several process interpretations in the manuscript are stronger than the observations support. The sub-daily observations are interesting and potentially very useful, but the manuscript should more clearly distinguish what is directly observed from what is inferred from temporal coincidence between deformation, temperature, and soil moisture. A more cautious interpretation would strengthen rather than weaken the contribution of this dataset.