the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Apr 2026
Lake Surface Temperature Dynamics as Precursors to Glacial Lake Outburst Floods: A Case Study of Lake Merzbacher, Central Tianshan
Abstract. Glacial lake outburst floods (GLOFs) have become increasingly frequent under climate warming. Yet the links between lake surface temperature (LST) dynamics and GLOF triggers remain poorly understood due to the absence of in situ lake temperature observations. This study investigates the potential of MODIS-derived LST to serve as a precursor for GLOFs at Lake Merzbacher, a frequently outbursting ice-dammed lake. We analyzed LST trends from 2000 to 2022 and examined their short-term dynamics preceding 25 documented GLOF events. Our results reveal a significant summer LST warming trend of 0.06 °C·yr⁻¹, exceeding the regional air temperature rise. We identified a critical LST threshold of 12 °C, with ~90 % of GLOFs occurring above this level. More importantly, we detected distinct thermal precursors: a rapid LST increase (peaking at 0.65 °C·day⁻¹) beginning ~8 days before outburst, and a critical acceleration phase (exceeding a threshold of 1.04 °C·day⁻²) around 9 days pre-GLOF. Furthermore, the peak discharge of floods showed the strongest correlation with the 15-day cumulative LST before outburst (r = 0.77), highlighting the role of integrated thermal energy in controlling flood magnitude. This study establishes LST not merely as a background climate indicator but as a source of diagnostic, short-term warning signals. We propose a multi-parameter framework integrating absolute LST, its rate of change, and acceleration to enhance early-warning systems for ice-dammed lakes under climate warming.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-5867', Adam Emmer, 30 Jun 2026
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RC2: 'Comment on egusphere-2025-5867', Anonymous Referee #2, 03 Jul 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2025-5867/egusphere-2025-5867-RC2-supplement.pdf
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CC2: 'Reply on RC2', Meixia Wang, 09 Jul 2026
We thank the reviewer for constructive comments. We have substantially revised the Study area, Methods, Results, Discussion, and Conclusions sections. In particular, we now explicitly describe the likely drainage mechanism of Lake Merzbacher as lake filling, increasing hydrostatic pressure, partial ice-dam flotation, and the activation, connection, or enlargement of englacial and subglacial drainage pathways. We also clarify that the 12 °C value is an empirical, site-specific LST reference level rather than a physical trigger threshold, and that the 1.04℃·d⁻² acceleration criterion is an empirical diagnostic threshold rather than evidence of imminent failure. Finally, we emphasize that cumulative LST is statistically associated with peak discharge but should be interpreted together with lake volume, lake level, ice-dam geometry, and drainage-pathway development.
lines 65-67: this could be a bit more precise with explaining what are the physical mechanisms that can lead to lake drainage.
Response: We thank the reviewer for this suggestion. We have revised the Introduction to describe the physical mechanisms of lake drainage more explicitly. In the revised text, we clarify that ice-dammed lake drainage may occur through overtopping or overflow into supraglacial channels, mechanical instability or collapse of the dam, or the opening, connection, and enlargement of englacial and subglacial drainage pathways. For Lake Merzbacher, previous studies indicate that drainage is most likely associated with the activation and enlargement of englacial and subglacial channels under increasing lake level, hydrostatic pressure, and partial ice-dam flotation. We have added this distinction to avoid implying that all GLOFs are controlled by the same mechanism.
- section 2: the lake itself should be described more precisely (volume, area, filling and draining periods, evolution over the studied period, etc).
Response: We thank the reviewer for this helpful comment. We have expanded the Study area section to provide a more detailed description of Lake Merzbacher, including its ice-dammed nature, glacial setting, seasonal filling and drainage behavior, lake-area variability, and the likely drainage mechanism. We now clarify that Lake Merzbacher is impounded by the Southern Inylchek Glacier and drains repeatedly through englacial or subglacial pathways. We also added information on the observed lake-area range during 1990–2024 and the shift in GLOF timing over the study period. These additions better place the LST analysis within the glacio-hydrological context of the lake–ice-dam system.
- line 104: and Aizen., 1998). –> and Aizen, 1998).
Response: done.
Figure 1: Not clear what is land, what is glacier and what is water. Panel a and b should be listed in the caption.
Response: We thank the reviewer for this helpful comment. We have revised Figure 1 to improve the distinction between land, glaciers, and water bodies. In the revised figure, glaciers, water bodies, rivers, non-glacier terrain, and basin boundaries are shown using more contrasting colors and clearer outlines, and the legend has been updated accordingly. We have also revised the figure caption to explicitly describe panels (a) and (b).
lines 122-124: what is/are then the mecanism(s) that trigger the 25 observed GLOF?
Response: We thank the reviewer for this important question. We have revised the Study area and Discussion sections to clarify the most likely drainage mechanism for the 25 documented GLOFs. Since no glacier-surge-triggered GLOFs were reported at Lake Merzbacher during the study period, we interpret these outbursts as being primarily associated with seasonal lake filling, increasing hydrostatic pressure, partial flotation or uplift of the Southern Inylchek ice dam, and the activation, connection, or enlargement of englacial and subglacial drainage pathways. We also clarify that LST is not interpreted as an independent trigger of these events, but as a process-related indicator of thermal and hydrological preconditioning and, potentially, of drainage-channel enlargement after drainage has initiated.
line 157: define what is µ and σ
Response: We thank the reviewer for pointing this out. We have revised the Methods section to explicitly define μ and σ. In the revised manuscript, μ denotes the mean value of the LST second derivative, d²LST/dt², and σ denotes the standard deviation of d²LST/dt². These statistics were calculated from the event-aligned second-derivative values within the window from 30 days before to 7 days after the 25 documented GLOF events. The threshold μ + 1.5σ was then used to identify periods of significantly accelerated LST increase.
line 158: LST > the –> LST larger than the
Response: We thank the reviewer for this correction. We have replaced the symbolic expression “LST > the threshold” with the more formal wording “d²LST/dt² exceeding this threshold” in the revised manuscript.
lines 172-174: this part about ablation in a lake is not clear. When LST is bellow the temperature melting point, then the lake (at least its surface) is frozen, when it is above it is free of ice. Not sure ablation and accumulation are the appropriate terms here. Moreover, the LST might not be representative of the melt condition. For temperature lower than 4°C, because of the density inversion, you could measure a LST of 0°C but having higher temperature up to 4°C close to the lake bottom, one can expect some melt. Or not, if the whole lake column is at zero. The representativity of a surface measurement to deduce the melt condition at depth should be discussed.
Response: We thank the reviewer for this important clarification. We agree that the previous wording was misleading because MODIS-derived LST represents the radiometric surface skin temperature of the lake and should not be interpreted as the temperature of the full lake water column, the lake bottom, or the ice–water interface. We have therefore removed the terms “ablation” and “accumulation” from this part of the Methods section and no longer describe LST above 0 °C as an “effective ablation threshold”.
In the revised manuscript, we define the cumulative LST as a positive cumulative surface thermal index rather than a direct measure of melt energy or ice-dam ablation. We also added text clarifying that, due to possible vertical stratification and the density maximum of water near 4 °C, surface temperature may differ from the thermal conditions at depth. Therefore, this index is interpreted only as an empirical indicator of surface thermal conditions and open-water exposure before outburst. We further note that lake-temperature profiles, lake-level observations, and ice-dam measurements would be required to evaluate melt conditions at depth or at the ice–water interface.
line 178: Where –> where
Response: done.
Figure 2 : needs some explanation to be understandable. Are the steps listed at the top also following the time axis? What represent the blue and orange lines? The two intermediate panels are showing temperature, but of what?
Response: We thank the reviewer for pointing out this ambiguity. We have revised Figure 2 and its caption to make the figure easier to understand. In the revised version, Figure 2. Schematic workflow for detecting thermal precursor signals before glacial lake outburst floods (GLOFs). The boxes at the top represent the analytical steps rather than a chronological sequence. Daily MODIS-derived LST is first smoothed and differentiated to calculate the rate of LST change and its acceleration. The x-axis in the time-series panels represents days relative to the outburst date, with negative values indicating days before the GLOF. The blue line denotes the mean or smoothed daily LST, the light-blue shading indicates the variability among historical events, the orange dashed vertical line marks the outburst date, and the orange shaded areas indicate significantly accelerated warming periods where the second derivative exceeds the threshold μ + 1.5σ. These thermal indicators are then combined with cumulative LST before the outburst to evaluate their relationship with GLOF timing and magnitude.
Figure 3: how do you explain that lake temperature is higher than air temperature? Especially if the lake is in contact with the ice which imposes a T=0°C boundary condition? The temperature are averaged over which period on this plot? It is just 3 points /year, mean over June, July and August? Should be specified more explicitly
Response: We agree that the previous version did not sufficiently clarify the physical meaning of the temperature series and the averaging period used in Figure 3. In the revised manuscript, we have changed Figure 3 from absolute temperature to temperature anomalies relative to the 2000–2022 monthly and seasonal mean. This avoids a direct comparison between MODIS-derived radiometric lake surface skin temperature and ERA5-Land 2 m air temperature, which represent different physical quantities.
We also clarified in the caption section that the June, July, and August values represent monthly means calculated from all valid daily values within each month, and that the summer mean represents the mean anomaly for June–August. MODIS LST is now explicitly described as lake surface skin temperature rather than bulk lake-water temperature or temperature at the ice–water interface. Therefore, the ice–water interface may remain close to the pressure-melting point, while the open-water surface observed by MODIS can still exhibit positive and seasonally variable temperature anomalies. In addition, we revised the interpretation of the trends. The revised text now emphasizes that July LST showed the strongest warming tendency, whereas the statistical significance of individual monthly trends is limited and should be interpreted cautiously.
line 221: ice dam failure is a bit vague and may be not appropriate for the type of drainage occurring at Lake Merzbacher (opening of subglacial channels)
Response: We thank the reviewer for this important clarification. We agree that the term “ice-dam failure” was too vague and could incorrectly imply structural collapse of the ice dam. In the revised manuscript, we replaced this expression with more process-specific wording. We now describe Lake Merzbacher outbursts as being related to the activation, connection, or enlargement of englacial and subglacial drainage pathways, probably controlled by lake filling, increasing hydrostatic pressure, and partial ice-dam flotation. We also revised the interpretation of the 12 °C threshold to emphasize that it is an empirical thermal indicator of elevated outburst likelihood, rather than a physical threshold for ice-dam failure.
line 222: If GLOF happens for much larger temperature than 12°C, then it means that it is not really a threshold for the GLOF to happen: i.e. the lake can be stable for higher temperature than 12°C?
Response: We thank the reviewer for this important clarification. We agree that the 12 °C value should not be interpreted as a deterministic threshold above which a GLOF must occur. The previous wording was too strong and may have implied a causal or trigger threshold. In the revised manuscript, we now describe 12 °C as an empirical LST reference level or non-deterministic thermal indicator that marks the seasonal thermal window in which most documented outbursts occurred.
We have clarified that Lake Merzbacher can remain stable when LST is above 12 °C if lake level, hydrostatic pressure, partial ice-dam flotation, and englacial or subglacial drainage connectivity have not yet reached a critical state. Therefore, the 12 °C value is not treated as a physical trigger threshold, but as one component of a multi-parameter early-warning framework. We also revised the text to emphasize that LST should be combined with LST change rate, LST acceleration, lake filling, floating-ice coverage, water level, and ice-dam motion.
Figure 5: I guess it is one particular year? Which one? From the text it seems that it is a mean? Then the envelop of all years should be presented or all the 25 years also plotted with thin curves. What are the units for the x-axis? I guess it is days? There is no a and b on the two panels.
Response: We thank the reviewer for this helpful comment. Figure 5 represents a multi-event composite rather than one particular year. In the revised figure, all 25 documented GLOF events were aligned to their outburst dates, and we present the composite mean together with the interquartile range, 25–75 %, to show the variability among events. We also revised the x-axis label to “Days relative to GLOF date (d)”, where day 0 indicates the outburst date, negative values indicate days before the outburst, and positive values indicate days after the outburst. Clear panel labels “(a)” and “(b)” have also been added to the two panels.
line 250: can you define what is "accelerated warming events"?
Response: We thank the reviewer for pointing out this ambiguity. We have revised the terminology and definition in the manuscript. To avoid confusion with GLOF events, we now use the term “accelerated LST warming occurrences”. In the revised manuscript, an accelerated LST warming occurrence is defined as a daily value within the event-aligned GLOF windows for which the LST acceleration, d²LST/dt², exceeds the empirical acceleration threshold μ + 1.5σ, corresponding to 1.04 °C d⁻² in this study. These occurrences therefore refer to threshold-exceeding LST acceleration days, not to independent GLOF events.
line 267: Here again, I am missing the process that conduct to the lake drainage. I would said that at the first order, the flood magnitude is proportional to the lake volume, then its temperature can play a role especially if the drainage mechanisms involves the melting of a supra, intra or sub glacial channel.
Response: We thank the reviewer for this important comment. We agree that flood magnitude should not be interpreted as being primarily controlled by LST. In the revised manuscript, we now clarify that peak discharge is expected to be controlled at first order by the volume of water available for drainage, lake level, hydraulic head, drainage-pathway geometry, and the rate of enlargement of englacial or subglacial channels.
We have also revised the interpretation of lake area and cumulative LST. Because direct lake-volume observations are not available for all events, we use lake area only as a first-order geometric proxy for lake storage, not as a direct estimate of volume. Cumulative LST is now described as a supplementary surface thermal index that may be statistically associated with peak discharge and may influence drainage efficiency through thermal erosion of supra-, intra-, or subglacial channels once drainage has initiated. We have revised the Results and Discussion accordingly to avoid implying that LST alone controls flood magnitude.
line 318: which mechanisms?
Response: We thank the reviewer for this comment. We agree that the previous version did not specify the relevant drainage mechanisms clearly enough. In the revised Discussion, we now explicitly describe the mechanisms that may link LST to Lake Merzbacher outbursts. We distinguish between three related processes: (1) thermal and hydrological preconditioning, in which elevated LST reflects enhanced energy input, open-water exposure, and meltwater supply; (2) drainage initiation, which is primarily controlled by lake filling, increasing hydrostatic pressure, partial ice-dam flotation, and the activation or connection of englacial and subglacial drainage pathways; and (3) drainage evolution, in which relatively warm lake water may enhance thermal erosion and enlargement of supra-, intra-, englacial, or subglacial channels after drainage has initiated.
We have revised Section 5.1 to make clear that the primary mechanism for drainage initiation is likely hydrostatic-pressure-induced ice-dam flotation and drainage-pathway activation, whereas LST is interpreted as a process-related precursor that reflects the evolving thermal and hydrological state of the lake–ice-dam system.
line 339: factor or indicator?
Response: We thank the reviewer for pointing out this important distinction. We agree that LST should not be presented as a standalone causal factor or physical trigger of GLOFs. In the revised manuscript, we consistently describe LST as a process-related indicator or remotely sensed precursor. Elevated LST may reflect conditions that favor lake filling and drainage-system development, and it may also contribute to conduit enlargement through thermal erosion after drainage has initiated. However, drainage initiation itself is more likely controlled by lake level, lake volume, hydrostatic pressure, ice-dam geometry, partial flotation, and englacial or subglacial drainage connectivity.
Accordingly, we have replaced terms such as “primary driver”, “controlling factor”, and “critical threshold” with more cautious expressions such as “process-related indicator”, “precursor”, “empirical reference level”, and “diagnostic signal”. We now state explicitly that the 12 °C value is not a deterministic trigger threshold, but an empirical LST reference level associated with the warm-season window in which most documented outbursts occurred.
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CC2: 'Reply on RC2', Meixia Wang, 09 Jul 2026
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CC1: 'Reply on RC1', Meixia Wang, 09 Jul 2026
We sincerely thank the reviewer for this constructive and detailed assessment. We agree that the original manuscript did not sufficiently explain the physical link between LST dynamics and the drainage mechanism of Lake Merzbacher, and that the previous wording could be interpreted as implying a causal or deterministic role of LST. In the revised manuscript, we have substantially revised the Introduction, Study area, Results, Discussion, and Conclusions to clarify that MODIS-derived LST should be interpreted as a process-related remotely sensed indicator or precursor, rather than as a standalone causal trigger of GLOFs.
First, we expanded the discussion of possible GLOF mechanisms and embedded Lake Merzbacher more clearly within the existing research landscape. In the revised Introduction, we now explain that GLOF initiation mechanisms vary among lake types, including overtopping or overflow into supraglacial channels, mechanical dam instability, and the opening, connection, or enlargement of englacial and subglacial drainage pathways. For Lake Merzbacher, we now state explicitly that drainage is more likely associated with seasonal lake filling, increasing hydrostatic pressure, partial ice-dam flotation, and the development of englacial or subglacial conduits, rather than simple surface overflow or catastrophic dam collapse. This interpretation is supported by previous studies on Lake Merzbacher and other ice-dammed lake systems, including Kingslake and Ng (2013), Mayr et al. (2014), Häusler et al. (2016), and Shangguan et al. (2017).
Second, we substantially revised Section 5.1 to disentangle the possible roles of hydrostatic forcing and thermal erosion. We now clarify that drainage initiation is probably controlled primarily by lake level, lake volume, hydrostatic pressure, ice-dam thickness, dam geometry, partial flotation, drainage-pathway connectivity, and the activation or enlargement of englacial and subglacial drainage pathways. Within this framework, LST is not treated as the direct trigger of drainage. Instead, elevated LST is interpreted as a process-related precursor that may reflect enhanced summer energy input, reduced floating-ice cover, increased open-water exposure, intensified glacier meltwater supply, and rapid lake filling. These conditions may bring the lake–ice-dam system closer to the hydromechanical state required for flotation-induced drainage.
Third, we now distinguish between the drainage-initiation phase and the drainage-evolution phase. In the revised Discussion, we state that relatively warm lake water may enhance thermal erosion along englacial or subglacial conduits after drainage has initiated, thereby promoting conduit enlargement and potentially affecting the rising limb and magnitude of the outburst hydrograph. However, we also clearly acknowledge that the present dataset does not allow us to determine whether thermal erosion initiates drainage or mainly amplifies discharge after a hydrostatic-pressure threshold has been reached. We therefore emphasize that direct observations of lake level, lake-water temperature profiles, ice-dam thickness, ice-dam displacement, and subglacial drainage evolution would be required to separate the roles of hydrostatic pressure and thermal erosion.
Fourth, we revised the interpretation of the 12 °C value. In the original manuscript, the use of “threshold” may have implied a deterministic trigger condition. In the revised manuscript, we describe 12 °C as an empirical, site-specific, and period-specific LST reference level that marks the warm-season window in which most documented outbursts occurred. We explicitly state that Lake Merzbacher may remain stable above 12 °C if lake level, hydrostatic pressure, partial ice-dam flotation, and englacial or subglacial drainage connectivity have not reached a critical hydromechanical state. Therefore, the 12 °C value is no longer presented as a physical threshold for ice-dam failure, but as one component of a multi-parameter early-warning framework.
Fifth, we added a new discussion of system non-stationarity and future evolution in Section 5.3. We now emphasize that the Lake Merzbacher system is non-static: glacier thinning, ice-dam lowering, changes in lake-basin geometry, variations in maximum lake volume, and evolving subglacial drainage efficiency may modify the lake level required for flotation-induced drainage. As a result, the LST value associated with outburst occurrence may change over time even if the underlying hydrostatic mechanism remains similar. We therefore state that the empirical LST reference level and acceleration criteria proposed in this study should be recalibrated regularly as new GLOF events and updated satellite observations become available.
Finally, we expanded the Study area section to provide more relevant information on the glacial and hydrological setting of Lake Merzbacher. The revised section now describes the lake as a repeatedly draining ice-dammed lake impounded by the Southern Inylchek Glacier, explains its seasonal filling and drainage behavior, and places it within the broader Inylchek–Sary-Dshaz–Kumarik–Aksu–Tarim hydrological system. We also added details on the lake’s relationship with glacier meltwater supply, floating-ice coverage, lake area changes, ice-dam displacement, and subglacial drainage evolution.
Major revisions made in the manuscript include:
- Introduction: Added a process-based explanation of different GLOF drainage mechanisms and clarified that Lake Merzbacher is more likely governed by lake filling, hydrostatic pressure, partial ice-dam flotation, and englacial/subglacial drainage development.
- Study area: Expanded the description of the glacial, hydrological, and ice-dammed setting of Lake Merzbacher, including its seasonal filling–drainage behavior and relevance to LST-based monitoring.
- Results: Revised the interpretation of the 12 °C value as an empirical LST reference level rather than a deterministic threshold. We also clarified that cumulative LST is statistically associated with peak discharge but should be interpreted together with lake storage, lake level, ice-dam geometry, and drainage-pathway development.
- Discussion 5.1: Added a detailed mechanism-based interpretation linking LST to thermal–hydrological preconditioning, hydrostatic-pressure-induced flotation, and possible thermal erosion during conduit enlargement.
- Discussion 5.2: Revised the early-warning implications to emphasize that LST indicators should be incorporated into a multi-parameter framework including LST, LST anomalies, dLST/dt, d²LST/dt², lake area, floating-ice coverage, lake level or volume estimates, ice-dam displacement, and downstream hydrological or seismic observations.
- Discussion 5.3: Added a dedicated section on the non-stationarity of the LST indicator and future system evolution under continued climate warming.
- Conclusions: Revised the conclusions to state explicitly that LST provides supplementary diagnostic information but should not be interpreted as a standalone trigger or primary control of Lake Merzbacher outbursts.
Representative revised text:
Line 388-396: In this hydrostatic-pressure-induced flotation and drainage-initiation mechanism, lake level, lake volume, ice-dam thickness, dam geometry, drainage-pathway connectivity, and heat transfer to conduit walls are the primary controls on drainage initiation. Within this framework, LST is more appropriately regarded as a process-related precursor rather than a standalone causal trigger. Elevated LST may reflect enhanced summer energy input, reduced floating-ice cover, increased open-water exposure, and intensified glacier meltwater supply to the lake. These conditions favor rapid lake filling and may bring the lake closer to the hydrostatic conditions required for flotation-induced drainage.
Representative revised text on non-stationarity:
Line 429-435: The 12 °C LST reference level identified in this study is empirical, site-specific, and period-specific. The Lake Merzbacher system is non-static: glacier thinning, ice-dam lowering, changes in lake-basin geometry, variations in maximum lake volume, and evolving subglacial drainage efficiency may all modify the lake level required for flotation-induced drainage. As a result, the LST value associated with outburst occurrence may change over time even if the underlying hydrostatic mechanism remains similar.
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- 1
This study of Meixia Wang and colleagues explores the potential of the lake surface temperature (LST) in predicting the outburst of ice-dammed Lake Merzbacher, Kyrgyzstan, using MODIS data. The study is clearly structured, accompained by illustrative figures. The analysis primarily focuses on the relationship between LST and GLOF timing, corroborating and extending previous observations that most GLOFs from Lake Marzbacher occur in summer. The study, however, lacks deeper elaboration of the relationship between LST and outburst mechanism. The study leaves the readers wondering whether is there any causality. The discussion of actual outburst mechanism in section 5.1 is very brief and needs deeper elaboration in order to outline implications for monitoring and EWS design. In particular, disentangling whether the outburst is related to thermal erosion and weakening of the dam or whether the mechanis is hydrostatic pressure-induced ice dam flotation (where the water temperature itself is far less important) is of the utmost importance for the prediction purposes. Lake Merzbacher is famous, well-studied lake with number of publications focusing on its repeated outbursts. The findings of this study should better be embedded in exsiting research landscape while existing findings may help to better explain how LST is process-wise linked to outburst mechanism (if so). The use of the 12 °C threshold is defensible but ignores glacier dam evolution over time (ice dam lowering, decreased maximum lake volume, etc.) and needs to be presented carefully and re-evaluated regularly. The non-static nature of the system and possible future trajectories under ongoing climate change should be highlighted and discussed. The study area section would benefit from more details on the study site, especially when it comes to climate, glacial setting and other characteristics relevant for the study. I suggest to consider these points in revised version of the study and recommend moderate to major revisions.