In this paper, the authors validate two corrections to satellite-based ground radiation

retrievals using ground measurements: parallax correction and shadow correction.  This

is more involved than it looks at first glance and the paper contains some results that

many might overlook.  It presents some evidence that the parallax error and shadow error

partially cancel out each other for geostationary-based radiation measurements, and that the combined

error may therefore increase if only parallax correction is applied.  The paper also shows

that results improve when using a value smaller than the retrieved CTH, at least in some

cloud regimes.  These result should be stressed more clearly, including in the abstract.

The science in this paper is good, but some conclusions should be presented more clearly.

I have just one significant worry on the validation of the empirical collocation shift correction.

Below is a list of suggestions before this paper can be accepted with minor revisions.

Abstract

This abstract undersells the paper.  It should prominently mention that

applying parallax correction alone can worsen validation results (as

shown in Figure 4) and that using a height smaller than the retrieved

CTH may be better than the full height.

Page 1, line 7: This line is a bit confusing.  The geolocation of the shadow (on the ground),

as visible from the satellite, should not be affected by parallax.  The 1-D assumption

of "shadow is straight below the cloud" should be mentioned here, an assumption that is of

course erroneous in most cases.

Introduction

Page 3, line 64: same here, the shadow location would be retrieved

accurately by just measuring the radiance from the shadowy pixel?

But that's not what is done?

3.2 Shadow correction

Page 6, line 160: this 1-D assumption should be pointed out earlier (introduction and perhaps abstract)

as otherwise the reader may be confused why there is an error in the shadow location, as a simple radiance

retrieval without any assumptions will have the correct geolocation for the shadowed pixel (if the shadow

is on the ground and not on a lower cloud).

3.4 Empirical collocation shift correction

Page 8, line 195: variations in CTH are also not accounted for.

Page 9, Figure 3, latitude shift: why is there no latitude shift (south/north) around 12 noon?

Shouldn't there be a shadow in the north?

Page 10/11, Figures 3 (both panels) and 4(right panel): please add a thin grid line at 0, like you have in figures 5, 7, 8.

4.1.3 Pixel-based and area-based corrections

Page 12, line 258: I think "uncertainties" here should be "errors", shouldn't it?

4.1.5 Resolution sensitivity

Page 12, line 283: You have derived the emipirical collocation shift

correction using pyranometer measurements and now you are using

comparisons with pyranometer measurements to validate them.  It would

seem your validation is not independent of your reference.

4.2 Separation into cloud regimes

Page 13, figure 13: bit surprised by seeing the clear sky regime here, as

there will be no parallax or shadow error.  But that actually provides a

baseline for improved analysis of the rest of the results.  If you have

ca. 50 W/m² RMSE just from other sources, couldn't you subtract that

error estimate from all the other figures so you get an estimate that

covers only parallax and shadow effects?  Or at least indicate this in

the other figures for context.

Page 13, figure 13 caption: Note that the "natural color RGB" is known as

the "day land cloud RGB" in the USA and perhaps some other communities.

Adding this name (in addition to "natural colour" common in europe")

may help some readers.

4.3 Diurnal Cycle

Line 19, page 398: "growing importance at increasing spatial resolutions", here one could mention FCI.

5.1 Generalizability of results

Page 19, 414: this could be tested with IODC.  Not suggesting you need to do it for this study, but you could mention it at least.

5.2 Remaining mismatch errors

Page 21, line 473: this could be tested with LEO, such as Sentinel data or even Landsat (if calibration is good enough).

Page 21, line 479: the comment on 3D is a bit oddly formulated.

Typos:

page 19, line 391, missing space in "theassymmetric".

page 24, line 563: Github → GitHub

General comments:

The manuscript "Evaluating parallax and shadow correction methods for global horizontal irradiance retrievals from Meteosat SEVIRI" by Wiltink et al. presents correction methods for satellite-based GHI retrievals.

Two approaches are used here: purely geometric (parallax and shadow corrections) and empirical collocation shifts.

The paper is overall well written and the scientific topic investigated is justified and matches well with AMT.

The structure makes sense overall, although some main findings could be stressed a bit more clearly (e.g. the need to lower the cloud height to achieve the lowest RMSE and the counter-acting or opposing effects of two correction methods).

In the "Conclusions and outlook" section I miss a bit to bring the findings of this study into a broader context. E.g. what are the implications/suggestions for the usage of satellite derived GHI in predictions for PV yields? Is the difference of the satellite retrievals of about 100 W/m2 w.r.t. the pyranometers considered critical in view of typical total solar irradiance values? The presented correction methods seem to be able to reduce the errors between the satellite retrievals and the ground based retrievals of GHI by about 10-15 W/m2. This could be emphasized more clearly.

If the authors could expand a bit more regarding these points mentioned above it would be appreciated, however, I will not insist on that.

I recommend publication of the manuscript after the following minor revisons:

Specific comments:

Line 33:

I think what is meant here is "Geostationary Operational Environmental Satellites (GOES)" and not Global Earth Observing System (GEOSS)?

Line 189:

What does "any direction" mean here? Fixed along either N, S, W, E direction or along the SEVIRI footprints or along any polar angle? Please specify.

Figure 3 and Line 202:

Is there a shading for the 5th to 95th percentile missing?

Line 242:

Does this mean that the retrieved Hc, that is used as input to calculate the GHI, is on average over-estimated? (since Hc has to be reduced to get the smallest RSME?)

Line 248:

Several cloud retrieval algorithms use a simplified Lambertian cloud model which is more representative of a "cloud mean height" than a "cloud top height". Have the authors considered to apply such cloud models instead of using Hc as input in order to mitigate this effect of ignoring the cloud's vertical extent?

Figure 5:

If I understand correctly, a negative difference in panel c) indicates that the HR retrievals have a lower RMSE than the SR retrievals and that the HR-based daily or time-step corrections are better for the regimes CR2 and CR5-CR9. However, for CR1, CR3 and CR4 the SR-based corrections seem to be better. Do I understand that correctly? If so, then the term HR RMSE "improvement" in the Figure caption might be misleading since in some cases the improvement is with the SR and not the HR. Please clarify.

Lines 340-341:

The phrases "mainly optimal" and "less optimal" sound strange as "optimal" is an absolute term. Maybe the following sentence sounds more reasonable: "In other words, the optimal shift is mainly suitable for the more variable regimes with lower retrieved clouds and, therefore, slightly less suitable for the more homogeneous regimes with higher retrieved clouds." Please consider to re-phrase this part.

Line 362:

While this is true for the two extreme cases (uncorrected and fully corrected), it seems that the SR RSME is smaller than HR RMSE at relative cloud top heights above 60% and larger than HR RMSE at lower relative cloud top heights. Although not being statistically significant it might still be worth mentioning if the authors consider this relevant.

Lines 365-366:

I don't understand how the statement of this sentence can also be deduced from Figure 5b? Can this be backed up by giving the exact RMSE numbers of the 40% Hc from Fig 7 versus the daily shift RMSE from Figure 5b?

Line 387:

I don't understand how this can be seen from Figure 3a), where, to me, the shifts seem even larger in the morning. Did you mean to refer here to Figure 8a) instead of 3a)? Please clarify.

Lines 457-460:

Maybe it could be useful for the reader if a Table (or text) is added in Section 2 that specifies the wavelegth ranges of the pyranometers and those of the SEVIRI bands that have been used. Or even show a Figure comparing the spectra for one collocated example case?

Lines 461-463:

Could a rough estimation of the temporal mismatch error in a worst-case scenario be added here? E.g. by moving a high cloud for 5 minutes with a high wind speed into a fixed direction and then estimating the induced mismatch? 

Lines 464-472:

Same as above for the temporal mismatch errors, could a rough estimation for the order of magnitude of the spatial mismatch errors be added here? 

Lines 478-484:

Some interesting studies on 3D cloud effects have been done here (although with an application to trace gas retrievals instead of GHI, but still it might be interesting for the authors): https://amt.copernicus.org/articles/15/1587/2022/, https://amt.copernicus.org/articles/15/5743/2022/, https://amt.copernicus.org/articles/15/3481/2022/

Technical corrections:

Line 356:

Remove the comma after Figure 7: "findings, Figure 7, shows"

Line 358:

Remove the dot after Hc: "The median Hc. of the cirrostratus".

Line 391:

Add a blank between "the" and "asymmetric".

Line 425:

For instance, subtropical land regions --> For instance, in subtropical land regions