General comments

The authors present a research on solar radiation conditions over Europe, based on EUMETSAT’s CMSAF CLARA-A3 satellite data record. Their analysis includes also influencing factors and trends, and they introduce a new concept called climate regimes, which may be helpful for a variety of applications including future planning of solar energy. Overall, the paper is interesting, well-written, and interesting for the readers of Egusphere.

In the abstract and elsewhere, the authors state the conclusion that their results suggest an increasingly important role of clouds for solar radiation conditions. I have difficulties understanding which part of the presented results would really show that this is true. From general knowledge about aerosol trends, climate change and so on, it would seem possible (or perhaps even plausible), but from the results presented in this paper, I do not see how they would confirm that clouds today are more strongly regulating solar radiation conditions than, for example, 20 years ago. I suggest revising and being careful about putting forward only such conclusions that are confirmed by the results of this paper.   

I also have some hesitation to accept the term climate regimes, which as used here more or less can be understood to be the composite of trends in SIS, cloud fraction, and cloud optical depth. Somehow, I find it a bit contradicting to call a trend composite climate regime. For me, it would be more natural to call it climate trend regime or something similar. Maybe the authors have already given this a lot of thought, but I would anyhow careful consideration once more on what term to use. 

I recommend minor revisions. 

Specific comments and suggestions

• L21-22 (“Net efficacy…”): a bit difficult to understand, perhaps a bit sloppy sentence. How would that knowledge influence the net efficacy?
• L56: Hammer et al., 2023, in reference list there is only one Hammer et al., with the year 2003.
• L71-72: I believe correlation between clouds and SIS is something that has been studied before. There is probably a wealth of publications based on ground-based measurements. Also, there is one recent satellite-based study (Post and Aun, 2024; https://doi.org/10.1016/j.oceano.2023.11.004) that is of particular interest here, worthwhile including in the references and discussing in conjunction with the results of the present paper. Post and Aun look at changes in cloudiness and SIS and how they correlate, and, in addition, they also evaluate the trends in the light of atmospheric circulation patterns.
• Figure 1 (and other figures): I suggest adding a label and unit to the color bar, and a title to the plot, so that the figure is more easily accessible stand-alone
• L148 (and elsewhere as needed): Oxford Languages (online resource) explains that bias is “a systematic distortion of a statistical result“. This is also how I understand bias: it is a systematic feature, that is the aggregated result of individual errors in a set of data. Thus, it does not make sense in my mind to calculate the standard deviation of the bias, but the standard deviation should be calculated from the errors (or differences). Same goes for the root-mean-squared error.
• Figure 2 and L156: figure uses RMSD while text says root mean squared bias.
• L191-192: Here, as a reader, I would like to have a bit more explained about what these studies found, without having the read them. Post & Aun (2024) mentioned above is relevant also here.
• L221: shouldn’t water vapour column also be mentioned here?
• L222: Please provide a reference for this sentence
• L236: Check sentence
• L240-241: Which studies? Please provide references.
• L250 (and elsewhere throughout the paper): Definition of the term climate regime, as discussed already above.
• L262: check sentence (in-cloud?)
• L342: How does the temperature and the moisture influence solar energy systems? References?
• L352-354: Related to the comment above: How does increasing temperature help increase net performance of a solar energy system? It is known that for photovoltaics, increasing cell temperature decreases the relative efficiency of the system.
• L356: spelling of water vapour. Also, maybe it would be helpful for readers to clarify whether the effect of water vapour is included in the CLARA SIS product (I guess it is).
• L401: please provide references

In this manuscript, an innovative approach is presented to analyze the long-term trend of SSI. The proposed concept is based on the synthesis of trends of SSI, cloud cover, cloud optical depth and humidity into a single quantity called “climate regime”. The methodology is applied to satellite derived cloud properties and SSI from CLARA-A3 over Europe. The paper is very interesting, well-written and represents a very relevant contribution to readers of EGU sphere as well as the solar energy community to better quantify the effect of climate change on the solar resource.

I suggest the following minor revisions. 

Line 173: “covering the 1982-2020 period » Please precise that according to recent works on SSI trends, a steady brightening is expected over this period.

The term “climate regimes” can be misleading in the context of the paper as it can lead to confusion with large-scale circulation regimes. I would suggest using a different term such as e.g. “multi-variable trend class”.

Since aerosol and water vapor are affecting SSI, it would be important to clearly explain how these are treated in the retrieval of the cloud property (atmospheric correction) in section 2.1.

In lines 219-220, the authors wrote “The shortwave solar radiation reaching the surface is regulated by a number of atmospheric components and their feedbacks in the backdrop of increasing greenhouse gases. Among them, the most important are clouds and aerosols,… ». The absorption by water vapor is very important for SSI calculation but it is not mentioned in this paragraph and not treated in section 4. In contrast, it is addressed in section 5. This is confusing and I think that reconsidering the organisation of these two sections would improve the flow of the text.

SSI trends are assessed but it is not clear from the text if a statistical significance test has been applied. (it is briefly mentioned in L174-175). More details on this aspect would be helpful.

Figure 5 and Figure 6 show the trend considering liquid and ice water COD respectively. It is unclear whether the dataset have been split or not for generating these two figures. This makes the maps not easy to interpret.