the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2023
Unfiltering of the EarthCARE Broadband Radiometer (BBR) observations: the BM–RAD product
Abstract. The methodology to determine the unfiltered solar and thermal radiances from the measured EarthCARE BBR shortwave (SW) and totalwave (TW) filtered radiances is presented. Within the EarthCARE ground processing, the correction for the effect of the BBR spectral responses, the unfiltering, is performed by the so-called BM-RAD processor which produces the level 2 BM-RAD product. The BM-RAD product refers to unfiltered broadband radiances that are derived from the BBR and the Multi-Spectral Imager (MSI) instruments onboard of the forthcoming EarthCARE satellite. The method is based on theoretical regressions between filtered and unfiltered radiances, as is done for the Clouds and the Earth's Radiant Energy System (CERES) and the Geostationary Earth Radiation Budget (GERB) instruments. The regressions are derived from a large geophysical database of spectral radiance curves simulated using radiative transfer models. Based on the radiative transfer computations, the unfiltering error, i.e., the error introduced by the small spectral variations of the BBR instrument response, is expected to remain well below 0.5 % in the shortwave (SW) and 0.1 % in the longwave (LW), at 1 standard deviation. These excellent performances are permitted by the very simple optics used in the BBR instrument: a telescope with a single paraboloid mirror. End-to-end verification of the unfiltering algorithm has been performed by running the BM-RAD processor on modeled Level 1 BBR radiances obtained for three EarthCARE orbits simulated by an integrated forecasting and data assimilation system. The resulting unfiltered radiances are eventually compared to the solar and thermal radiances derived by radiative transfer simulations over the three EarthCARE orbits. In addition, this end-to-end verification has provided further evidence on the high accuracy of the unfiltered radiance process, with accuracies better than 0.5 % for SW and better than 0.1 % for LW.
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RC1: 'Comment on amt-2023-170', Anonymous Referee #1, 27 Sep 2023
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-170/amt-2023-170-RC1-supplement.pdf
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AC1: 'Reply on RC1', Almudena Velazquez Blazquez, 31 Jan 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-170/amt-2023-170-AC1-supplement.pdf
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AC1: 'Reply on RC1', Almudena Velazquez Blazquez, 31 Jan 2024
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RC2: 'Comment on amt-2023-170', Anonymous Referee #2, 12 Dec 2023
General points
The manuscript describes the algorithm to obtain spectrally unfiltered shortwave (SW) and longwave (LW) radiances at top-of-atmosphere from the observations by the BBR instrument on board of EarthCARE. The spectral unfiltering is needed to correct for the spectral sensitivities of the instrument and for the contamination of SW radiation in the LW channel and vice versa. This paper is part of the special issue describing the algorithms for EarthCARE.
The structure and overall story of the manuscript is quite clear. However, when it comes to the details of the algorithm, the equations, and the steps that are taken, the paper is not clear and sometimes ambiguous, especially in Section 2. Since the BM-RAD algorithm should be perfectly clear to avoid errors later on, corrections in the manuscript are needed. These changes are not very difficult but necessary, and therefore these are major revisions. Furthermore, several technical corrections should be applied throughout the paper.
Specific points
- In the Introduction (Section 1) more high-level information on the aim of the BBR instrument in combination with other EarthCARE instruments is needed.
- The paragraph starting on L. 29 seems too detailed for the Introduction.
- L. 25: what is the function phi? Please mention that it is the spectral response function, and is needed to separate between reflected solar and emitted thermal fluxes.
- L. 29: all three references should be between brackets.
- Section 2: This section contains bugs, is unclear and should be improved and clarified. The symbols are unclear, the different unfiltering steps are not clearly separated.
- Equation 1: please give the integration limits; this holds for all integrations.
- L. 58: SW channel: do you mean SW radiation ?
- L. 58: efficient > an efficient
- L. 62, Eq. 5: subscript SW - in other places you use sw. Please be consistent in the subscripts throughout the paper. SW and LW are clearer subscripts than sw and lw.
- L. 64: observed. > observed:
- Eqs. 7 and 8: I find it confusing that the spectrally integrated radiance has the same symbol L as is used for the spectrally dependent L(\lambda). Please use a clear distinction in symbols.
- L. 76-79: “This conversion …. solar radiation”: Please clearly separate these 3 factors.
- Eqs. 8-9: The terminology in these equations, e.g. the subscripts “sw,sol” and “lw,th”, is not clear.
- L. 85: L_sol,L_th: where is now the subscript unfil, which was used in Equations 7-10 ?
- L. 93-94: please clearly separate these different steps.
- Eqs. 13-14: what happened to the fil and unfil subscripts introduced in earlier equations?
- L. 97: alpha_sw, alpha_lw: these are new variables! Where are alpha_sol and alpha_th introduced in Eqs. 9-10?
- L. 97: the name “unfiltering process” is quite confusing: there is an unfiltering step and a decontamination step. Please clarify the entire Section 2.
- L. 102: double bracket )
- L. 103: physical > geophysical
- L. 104: remove , in such as,
- L. 106: aerosols: how about clouds? what are the ranges in optical thickness and height of aerosols and clouds?
- L. 114: droplets
- L. 119: are the three spectral response functions of the instrument independent of the (3) viewing directions?
- L. 136: MSI-based algorithm
- Figure 2: please use the same font style for the symbols and equations in this figure.
- L. 140: The Fig. > Figure; same comment on L. 156.
- L. 148: It is quite remarkable that alpha does not depend on cloudiness. What is the explanation that the solar contamination in the longwave channel does not depend on cloudiness, since thick clouds in the daytime reflect a lot of radiation.
- L. 150: m/s; note that all units should be written in upright font.
- Fig. 3: the red colored dots in the two lower plots are unrelated, I assume, to the legend of the upper plots. Then please make these dots black, and define the residual in the main text.
- Eq. 16: symbol a is already used in Eq. 15, and has a different meaning there. Please use unique symbols for each quantity. The same remark holds for the next equations. Please be consistent in symbols and terminology.
- Fig. 4: On the basis which points did you determine the precise relationship shown in the 4 plots? The a and b coefficients are exactly the same, but the data points are different. That seems strange.
- Caption Fig. 4: please explain the two typs of points in each subplot.
- Sect. 4.4, first sentence: Please explain where we are in the procedure of the flow diagram. Do you mean spectral unfiltering to obtain the correct SW radiances? Is this the step after the decontamination?
- Eq. 17: apart from the reuse of earlier symbols, this is an unclear formula. Please make a multiplication with the inverse or add brackets.
- Fig. 5: the plots and their legends are poorly readable. Please use a larger font.
- L. 190: stand-alone algorithm
- L. 196: “much smaller than for the CERES and GERB instruments”: what is the reason?
- Fig. 6: what are the fit coefficients? This fit function does not hold for larger radiances.
- Caption Fig. 6: use correct ordering: (a) ..., (b) .....
- Caption Fig. 6: please explain: are these the alpha factors of Eq. 10 or of Eq. 14?
- Table 1 header: what is the reason that you switch between LW and th, and between SW and sol?
- Section 5: Section 5 is very short. How did you do the analysis? Please explain how you arrive at Table 1.
- Title Section 5: do you mean algorithm verification instead of performance verification? The next section is about performance verification.
- L. 217: Table 2
- L. 225: Please summarize the results of Fig. 7: which conclusions do you draw from it?
- Fig. 7: The plots in the right column are somewhat smaller than in the other columns. Please make all subplots the same size. How will you orient this figure to make it readable? Preferably landscape.
- Caption Fig. 7: "RT sim (truth)"
- References: Please correct all references, because the initials should be put after the surname.
Technical corrections throughout the manuscript- level 1 > level-1, level 2 > level-2
- Subscripts which are words, abbreviations or acronyms should be in upright font. For example, fil in L_fil on l. 50 should be upright. This occurs many times in the manuscript.
- Symbols should be in italic font.
- L. 67: Fig. 5 > Figure 5. If it is the starting word of a sentence, Fig. should be written in full.
- Always use a space between number and unit: e.g. L. 91: 5 µm
- Units should be written in upright font (e.g. l. 152, and many other places)
- Fig., Eq., Sect. should be written with capital.
- Tables: please put the table caption above the table.Citation: https://doi.org/10.5194/amt-2023-170-RC2 -
AC2: 'Reply on RC2', Almudena Velazquez Blazquez, 31 Jan 2024
The comment was uploaded in the form of a supplement: https://amt.copernicus.org/preprints/amt-2023-170/amt-2023-170-AC2-supplement.pdf
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AC2: 'Reply on RC2', Almudena Velazquez Blazquez, 31 Jan 2024
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EC1: 'Comment on amt-2023-170', Robin Hogan, 26 Feb 2024
Dear authors and reviewers,
Thank you to the reviewers for their first set of comments and to the authors for their revisions. I am going to send the manuscript out for a second review, and the reviewers/authors should take note of my comments below in the second reviews/revisions.REVIEWER 1
1. Regarding the comment on the flowcharts in Fig. 2, readers of this paper are very likely to have the same questions as the reviewer, so the authors should update the text to add clarity and explanations. The reviewer is not asking why the B-NOM and B-SNG products are the way they are (indeed their design is out of your hands) but to explain the reasoning for BM-RAD to contain data from two different sources (B-NOM and B-SNG), and explain how you envision the two being used - perhaps different applications will require different sources?
2. Further to my comment 1, the caption of Fig. 2 could be clearer, because it implies the flowchart is for the processing carried out in the production of B-NOM and B-SNG, when in fact it is the processing applied to B-NOM and B-SNG data to produce BM-RAD (which contains both outputs?)
3. Figure 3: please label all four panels as (a) to (d), and refer to them by letter in the caption.
4. Regarding the comment on L219-220 of the original manuscript, I agree with the reviewer that the text is inconsistent with both the table and the conclusions. The new text in section 6 reads:
"The error metrics show that the MSI-based shortwave unfiltering does not perform significantly better than the stand-alone unfiltering approach. The gaining of including MSI information in the unfiltering process while improving results is not very relevant."
The conclusions state:
"Scene information from the MSI radiances (from M-RGR product), MSI cloud retrievals (from M-CLD processor), or snow products (from X-MET product) are useful to further reduce the unfiltering error."
These are inconsistent. Why not state in section 6 that a small improvement is detected but that the difference might not be significant in practice because of parallax effects (which you state in the conclusions)? The current use of the word "significant" in section 6 implies you are talking about formal statistical significance, but I don't think you have tested this. The word "relevant" also doesn't seem correct - surely any improvement of accuracy is "relevant"? Perhaps you mean "large" (as in "not very large")?
REVIEWER 25. Regarding the definition of \phi(\lambda), if you use it in the introduction, it must be defined in the introduction not in the section after. I suggest changing the text simply to "measured by a perfect instrument, i.e. one with a flat spectral response $\phi(\lambda)=1 (where $\lambda$ is wavelength), ..."
6. Regarding the comment on the original L148, I think the explanation that alpha does not depend on cloudiness is that clouds are not reflective in the longwave, which could be stated in the manuscript. Indeed, if you plug cloud single-scattering albedo and asymmetry factor into the equation for the reflectance of a cloud in the limit of infinite optical depth (e.g. Eq. 13.45 of Petty's book on atmospheric radiation) then you get a value typically in the range 0.02-0.1 in the thermal infrared. So there is much less contrast with the underlying surface than you get in the shortwave.
OTHER COMMENTS7. Figs. 4 and 5: Yellow is a poor choice of colour as it appears quite faint - can you use a darker shade, e.g. orange?
8. L248 of revised manuscript: the reference to Fig. 7 appears as "Fig. ??"
Citation: https://doi.org/10.5194/amt-2023-170-EC1
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Cited
4 citations as recorded by crossref.
- Broadband radiative quantities for the EarthCARE mission: the ACM-COM and ACM-RT products J. Cole et al. 10.5194/amt-16-4271-2023
- The generation of EarthCARE L1 test data sets using atmospheric model data sets D. Donovan et al. 10.5194/amt-16-5327-2023
- The EarthCARE mission – science and system overview T. Wehr et al. 10.5194/amt-16-3581-2023
- The EarthCARE mission: science data processing chain overview M. Eisinger et al. 10.5194/amt-17-839-2024