Comparison of Surface Solar Irradiance from Ground Observations and Satellite Data (1990–2016) over a Complex Orography Region (Piedmont—Northwest Italy)

Round 1

Reviewer 1 Report

Two different Surface Solar Irradiance (SSI) products were compared with ground-based observations over a complex orography region. The differences between the two SSI products and ground-based observations were analyzed and discussed in order to investigate the representativeness of satellite SSI data, especially in high-elevation areas. The idea is great. Comparison of surface solar irradiance from satellite data to ground-based observations is crucial to evaluate the quality of the datasets. Researchers rely on the comparison or the so-called validation procedures to assess the ability of the satellite dataset to capture variability of ground data. The problems in complex orography regions (or mountainous areas) have not been intensively investigated or fully discussed yet. Both satellite data products and ground-based observations are subject to some complicated problems and become less reliable in these areas. It makes the comparisons and interpretations much harder.

The manuscript did successful work in the comparisons, i.e., SSI averages, SSI anomalies, and the temporal trends were compared thoroughly, and some preliminary conclusions were derived from the results. The structure and writing of the manuscript are excellent. There are two major concerns:

1. The authors did find different patterns in high-elevation areas in the results. But I would not call it “a new contribution” since it does not answer the vital question: which one should we trust more, the satellite product or the ground observation? They all become problematic and complex in high-elevation areas, which is not surprise to anyone. But if you do not try to answer the question, you will get more confused by the results. Of course, it’s unreasonable and unfair to anticipate all mysteries being solved in one single paper or by one team, but at least there are some tiny steps should have been taken given all the data, model and information that the authors have in their pocket. For example, in the introduction section the authors mentioned: “it includes stations with highly different levels of aerosol concentrations, ranging from the highly-polluted Po Plain to unpolluted Alpine high elevation sites. Moreover, in this area the emission of pollutants has strongly decreased over the last decades, causing a significant trend in all variables connected to atmospheric transparency with the only exception of high-level stations that fall outside the atmospheric layer directly influenced by ground-level pollutant emissions.” Unfortunately, there are no follow-up investigations based on these precious data. The author just cited several references to explain the results instead of rigorous data experiments and in-depth analysis, which has clearly lessened the contribution of this work.

In this circumstance, there are at least these important and interesting issues should be analyzed and discussed:

• How to identify “the underestimation of SSI at the high elevation sites due to snow cover and high albedo in the satellite products”, and measure its impact? Or at least, where and when should we be cautious about it? Can we just know it by comparing to ground-based observation?
• “Trends are not consistent with that of ground data because of the constant aerosol scheme”. Are there any specific numbers and proofs to explain this?
• It would be helpful to give more examples about “shading problem at some sites”.
• “The spatial representativity of the station measurements” should be taken into account in the complex orography region all the time. Actually, it should be the emphasis and baseline of validation of satellite products at mountainous areas, given the coarse resolution of such products (e.g. 25km² and 625km² in this case). Maybe only with the answer of this question, we will be able to study the representativeness of satellite SSI data in a more reliable and convincing context.

 

2. The authors clearly have very good experience and publishing history in this area, hence are very familiar with all the details of the data and other related issues. That may explain why the manuscript is lack of some important information and details that are crucial to fully understand this work and follow the lead to its conclusion. It is not fair to assume all the readers have the same background knowledge and information as the authors do, so it’s better to provide some vital information about the satellite data, the method and procedure of the two SSI products (at least those will be discussed in the latter sections, i.e., cloud, aerosol, etc.), processing of the ground observation in section 2, and also some equations of the error indexes and trend analysis in section2.4. It would be very helpful for better understanding of the theory and conclusion of the study.

 

Some minor issues:

• References are missing on multiple occasions, i.e., P3 L134-135, P6 L175, P6 L198, P11 L333, P12 L371, etc.
• Figures (5 to 8) are not clear, try different lines and colors.
• In P20 L545-550, about the better performance of the lower resolution CLARA-A2 at high-elevation SSI station sites, the explanation here is misleading by ignoring the fact that the spatial representativity of the station measurements become even smaller at these sites, hence logically the comparison to lower resolution products faces more challenging at these sites. The real reasons behind this results should be much more complex than that in the paragraph.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presented good comparison of solar surface irradiance (SSI) and Orography complexity index (OCI) of ground based and satellite data SARAH-2.1 and CLARA-A2. 

 

But data analysis for both defining ground based and satellite data refining / filtering has not been provided. 

 

Conclusion was made CLARA-A2 does not capture well SSI, have you compared with other studies its response in other areas.  

 

It was concluded that atmospheric aerosol affect the satellite data , but this affect has not been calculated / quantified. 

 

 

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The manuscript has been improved.