the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Dec 2025
Aerosol Optical Depth over the Sichuan Basin: An Intercomparison of CALIPSO and MODIS Observations (2006–2022)
Abstract. As the first satellite equipped with lidar for vertical atmospheric profiling, the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) provides unique insights into aerosol vertical distribution. In contrast, the Moderate Resolution Imaging Spectroradiometer (MODIS), a representative passive remote sensing instrument, excels in broad horizontal coverage for aerosol detection. This study systematically compares Level 2 aerosol optical depth (AOD) products from CALIPSO and MODIS over the Sichuan Basin, China, from 2006 to 2022, after harmonizing their spatial resolution and temporal windows. Through comprehensive statistical analyses of annual, seasonal, monthly, and regional averages, key findings emerge. While both datasets capture a consistent long-term temporal trend in AOD over the region, significant systematic biases are observed. Differences are more pronounced in winter and spring than in summer and autumn. The absolute AOD difference between the two sensors remains relatively stable, with nearly 60 % of discrepancies falling within the 0–0.2 range. MODIS demonstrates greater sensitivity to both high and low AOD extremes compared to CALIPSO. Spatially, regional deviations are most apparent in areas of aerosol extremes: MODIS tends to overestimate in high-AOD regions, whereas CALIPSO often reports higher values in low-AOD areas. These findings highlight the distinct methodological characteristics of each sensor and underscore the need for careful dataset selection or synergistic use, depending on specific research objectives in this complex geographical region.
Status: open (until 10 Feb 2026)
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CC1: 'Comment on egusphere-2025-6149', Chakradhar Rao Tandule, 28 Dec 2025
reply
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AC1: 'Reply on CC1', Chengyu Hu, 01 Jan 2026
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We sincerely thank you for their constructive and insightful comments on our manuscript titled “Aerosol Optical Depth over the Sichuan Basin: An Intercomparison of CALIPSO and MODIS Observations (2006-2022).” The feedback has helped us better recognize the limitations of our current work and identify areas for improvement. Below we provide a point-by-point response to the comments.
General CommentsReviewer: The manuscript presents a long-term intercomparison of MODIS and CALIPSO aerosol optical depth (AOD) over the Sichuan Basin. The topic is of general interest, and the authors have compiled an extensive multi-year dataset. However, the study primarily documents statistical differences between two existing satellite products without sufficiently advancing measurement methodology, retrieval validation, or uncertainty characterization.
Response: We appreciate your acknowledgment of the relevance of our study and the effort in compiling a long-term dataset. We agree that the current work is largely descriptive and focused on documenting inter-sensor differences. Our intention was to provide a regional climatological perspective on the consistency and discrepancies between two widely used satellite AOD products in a topographically complex region like the Sichuan Basin, and this study aims to provide a reference for scholars wishing to utilise these two datasets for regional AOD research.We acknowledge that the study does not introduce new retrieval methods or a comprehensive uncertainty framework. In the revised manuscript, we will more explicitly position the work as a regional intercomparison study and clarify its scope and limitations in the Introduction and Discussion sections.
Specific Comments
1. Lack of Measurement Validation FrameworkReviewer: The intercomparison is conducted exclusively between MODIS and CALIPSO AOD products, without the use of any independent ground-based reference data (e.g., AERONET or ground-based lidar). Consequently, the analysis evaluates only the relative consistency between the two satellite products and does not allow assessment of their absolute accuracy. Statements regarding overestimation or underestimation by either sensor should therefore be interpreted cautiously. Inclusion of even limited ground-based validation, or clearer acknowledgement of this limitation, would strengthen the study.
Response: We fully agree with this important point. In the original manuscript, we did not incorporate ground-based validation data such as AERONET, which limits our ability to assess absolute accuracy.As our original intention was to conduct a broad-scale data comparison, and Aeronet or other ground-based lidar systems can only provide individual point data, so we did not incorporate these data. In the revised version, we will:
- Explicitly acknowledge this limitation in the Methodology and Discussion sections.
- Clarify that statements regarding “overestimation” or “underestimation” are relative between the two sensors and not absolute.
- Include a brief discussion on the availability of AERONET sites in the Sichuan Basin and the challenges in collocating sparse ground measurements with satellite overpasses, suggesting this as a direction for future work.
2. Methodological Simplicity of the Collocation Approach
Reviewer: MODIS and CALIPSO AOD are collocated by resampling both datasets to a common 1° × 1° grid and comparing monthly mean values. While this approach facilitates long-term climatological analysis, it does not account for differences in overpass times, pixel-level spatial matching, or the sparse sampling of CALIPSO due to its narrow nadir-only track. Given that the distance between consecutive CALIPSO ground tracks (~280–300 km at the study latitude) is comparable to or larger than the chosen grid size, the representativeness of monthly mean CALIPSO AOD within each grid cell is limited. This aspect should be more explicitly discussed, as it likely contributes to the weak correlations reported.
Response: This is a valid methodological concern. In the revised manuscript, we will:
- Expand the Methodology section to explicitly discuss the limitations of our collocation approach, including the sparse sampling of CALIPSO and its impact on representativeness.
- Acknowledge that the coarse grid (1°×1°) and monthly averaging likely smooth out fine-scale variability and contribute to the weak correlations observed.
- Note that this approach was chosen to facilitate basin-wide climatological analysis over a long period, but we will clarify that it is not suitable for pixel-level or diurnal comparison.
3. Incomplete Description of Retrieval Parameters and Processing Choices
Reviewer: The manuscript does not explicitly specify the exact scientific dataset variables used from the MODIS and CALIPSO products, nor does it clearly describe quality assurance thresholds or algorithm pathways (e.g., MODIS Dark Target vs. Deep Blue). Whether both TERRA and AQUA are considered for collocating with CALIOP. Such details are essential for reproducibility and for interpreting whether observed differences arise from measurement physics or processing choices.
Response: We apologize for the lack of clarity. In the revised manuscript, we will provide the following details:
- Specify the exact datasets used: CALIPSO Level 2 5-km AOD at 532 nm, and MODIS MOD04/MYD04 3-km AOD at 550 nm from both Terra and Aqua.
- Clarify that we used the combined Dark Target and Deep Blue AOD product for MODIS over land, with standard quality flags applied.
- State that only daytime CALIPSO data were used to match MODIS daytime overpasses.
- Include a brief note on data filtering criteria (e.g., confidence level, cloud mask) to enhance reproducibility.
4. Spectral and Physical Consistency Not Addressed
Reviewer: The direct comparison of CALIPSO AOD at 532 nm with MODIS AOD at 550 nm is performed without spectral adjustment or discussion of potential impacts. Although the wavelength difference is small, neglecting this aspect introduces an additional source of systematic uncertainty that remains unquantified. Similarly, the lack of supporting analysis using meteorological or aerosol-type information limits the physical interpretation of seasonal differences. A brief quantitative estimate or justification for neglecting this effect would improve methodological rigor.
Response: We acknowledge this oversight. In the revision, we will:
- Discuss the wavelength difference between 532 nm and 550 nm and note that, while small, it may introduce a minor systematic bias, especially under certain aerosol types.
- Cite relevant literature that supports the comparability of AOD at these wavelengths in the absence of strong spectral dependence.
- Briefly discuss the potential influence of aerosol type and meteorological conditions on seasonal differences, referencing related studies in the Sichuan Basin.
5. Primarily Descriptive Nature of the Results
Reviewer: The study focuses on documenting spatiotemporal differences between two datasets, but does not introduce new retrieval methods, measurement concepts, or uncertainty analyses. The conclusions largely reiterate known characteristics of passive versus active aerosol observations, without substantially advancing methodological understanding.
Response: We agree that the study is descriptive in nature. Our primary goal was to provide a regional, long-term perspective on the consistency between MODIS and CALIPSO in a special geographical setting. In the revised Discussion and Conclusion, we will:
- More clearly frame the study as a regional intercomparison aimed at informing data selection for basin-scale aerosol research.
- Avoid overstating the methodological novelty and instead emphasize the practical implications for users of these datasets in topographically complex regions.
- Suggest specific follow-up studies that could build on our findings, such as multi-sensor fusion, uncertainty quantification, or integration with ground-based and model data.
Conclusion
We are grateful for the thorough and thoughtful review, which has helped us improve the clarity, rigor, and positioning of our work. We believe the suggested revisions will strengthen the manuscript and provide a more balanced and transparent presentation of our intercomparison study. We are committed to addressing all points raised and will submit a revised version accordingly.
Thank you for your time and comments.
Happy new year!Sincerely,
Chengyu Hu & Hwee San LimCitation: https://doi.org/10.5194/egusphere-2025-6149-AC1
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AC1: 'Reply on CC1', Chengyu Hu, 01 Jan 2026
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General Comments
The manuscript presents a long-term intercomparison of MODIS and CALIPSO aerosol optical depth (AOD) over the Sichuan Basin. The topic is of general interest, and the authors have compiled an extensive multi-year dataset. However, the study primarily documents statistical differences between two existing satellite products without sufficiently advancing measurement methodology, retrieval validation, or uncertainty characterization.
While the results may be informative at a regional or descriptive level, several fundamental methodological limitations restrict the depth of insight that can be drawn regarding measurement performance. Some statements imply relative superiority of one dataset over the other; given the lack of external validation, such statements may be misleading the independent datasets. As a result, the study remains primarily descriptive rather than advancing measurement methodology or validation.
Specific Comments
1. Lack of Measurement Validation Framework
The intercomparison is conducted exclusively between MODIS and CALIPSO AOD products, without the use of any independent ground-based reference data (e.g., AERONET or ground-based lidar). Consequently, the analysis evaluates only the relative consistency between the two satellite products and does not allow assessment of their absolute accuracy. Statements regarding overestimation or underestimation by either sensor should therefore be interpreted cautiously. Inclusion of even limited ground-based validation, or clearer acknowledgement of this limitation, would strengthen the study.
2. Methodological Simplicity of the Collocation Approach
MODIS and CALIPSO AOD are collocated by resampling both datasets to a common 1° × 1° grid and comparing monthly mean values. While this approach facilitates long-term climatological analysis, it does not account for:
Given that the distance between consecutive CALIPSO ground tracks (~280–300 km at the study latitude) is comparable to or larger than the chosen grid size, the representativeness of monthly mean CALIPSO AOD within each grid cell is limited. This aspect should be more explicitly discussed, as it likely contributes to the weak correlations reported.
3. Incomplete Description of Retrieval Parameters and Processing Choices
The manuscript does not explicitly specify the exact scientific dataset variables used from the MODIS and CALIPSO products, nor does it clearly describe quality assurance thresholds or algorithm pathways (e.g., MODIS Dark Target vs. Deep Blue). Whether both TERRA and AQUA are considered for collocating with CALIOP. Such details are essential for reproducibility and for interpreting whether observed differences arise from measurement physics or processing choices.
4. Spectral and Physical Consistency Not Addressed
The direct comparison of CALIPSO AOD at 532 nm with MODIS AOD at 550 nm is performed without spectral adjustment or discussion of potential impacts. Although the wavelength difference is small, neglecting this aspect introduces an additional source of systematic uncertainty that remains unquantified. Similarly, the lack of supporting analysis using meteorological or aerosol-type information limits the physical interpretation of seasonal differences. A brief quantitative estimate or justification for neglecting this effect would improve methodological rigor.
5. Primarily Descriptive Nature of the Results
The study focuses on documenting spatiotemporal differences between two datasets, but does not introduce new retrieval methods, measurement concepts, or uncertainty analyses. The conclusions largely reiterate known characteristics of passive versus active aerosol observations, without substantially advancing methodological understanding.