Cloud mask algorithm from the EarthCARE multi-spectral imager: the M-CM products

Hünerbein, Anja; Bley, Sebastian; Horn, Stefan; Deneke, Hartwig; Walther, Andi

doi:https://doi.org/10.5194/egusphere-2022-1240

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https://doi.org/10.5194/egusphere-2022-1240

Preprints

25 Nov 2022

| 25 Nov 2022

Cloud mask algorithm from the EarthCARE multi-spectral imager: the M-CM products

Anja Hünerbein, Sebastian Bley, Stefan Horn, Hartwig Deneke, and Andi Walther

Abstract. The EarthCARE satellite mission will provide new insights into aerosol-cloud and radiation interactions by means of synergistic observations of the Earth's atmosphere from a collection of active and passive remote sensing instruments, flying on a single satellite platform. The Multi-Spectral Imager (MSI) will provide visible and infrared images in the cross-track direction with a 150 km swath and a pixel sampling at 500 m. The suite of MSI cloud algorithms will deliver cloud macro- and micro-physical properties complementary to the vertical profiles measured from the ATmospheric LIDar (ATLID) and the Cloud Profiling Radar (CPR) instruments. The MSI cloud mask algorithm (M-CM) provides a cloud flag, cloud phase and cloud type product which are essential parameters for the cloud optical and physical properties (M-COP) as well as for the aerosol optical properties (M-AOT). Synthetic test scenes from the EarthCARE end-to-end simulator provide a first overview of the expected performance of the M-CM products before launch. Validation of the MSI cloud mask algorithm against satellite products from MSG SEVIRI and MODIS demonstrates a good performance of the algorithm. The MSI cloud detection performance is not completely aligned with that from MODIS, which is however not surprising when considering the rather limited number of spectral channels of MSI in comparison to MODIS. The direct comparison to MODIS shows that some threshold tuning should be done especially over desert or sun glint regions once real observations are available. However the MSI bands are less affected by sun glint because of the asymmetric view of the imager.

Received: 17 Nov 2022 – Discussion started: 25 Nov 2022

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Journal article(s) based on this preprint

07 Jun 2023

Cloud mask algorithm from the EarthCARE Multi-Spectral Imager: the M-CM products

Anja Hünerbein, Sebastian Bley, Stefan Horn, Hartwig Deneke, and Andi Walther

Atmos. Meas. Tech., 16, 2821–2836, https://doi.org/10.5194/amt-16-2821-2023,https://doi.org/10.5194/amt-16-2821-2023, 2023

Short summary

Anja Hünerbein et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1240', Anonymous Referee #1, 11 Jan 2023

In the study “Cloud mask algorithm from the EarthCARE multi-spectral imager:

the M-CM products” by Hünerbein et al. the MSI cloud mask algorithm is described and its performance compared to selected test scenes of the EarthCARE simulator. A first validation shows a good agreement of the M-CM algorithm with 45 MODIS scenes as well as 12 MSG SEVIRI cloud mask algorithms of one selected scene.

A description and first tests of the M-CM algorithm is very valuable for the cloud community and on time before the launch of EarthCARE that will provide simultaneous measurements of clouds and aerosol properties as well as radiation fluxes.

The paper is written in a clear and concise way. Some minor deficits are noted in the abstract and conveyed messages of the paper. It is recommended for publication after minor revisions.

Minor comments:

The abstract needs modification as the study’s purpose is not clearly stated. What is the study’s motivation, goal, result and outlook?

From my perspective a description of the M-CM algorithm is provided together with a first validation.

The terms “validation” and “verification” should not be used interchangeably as they describe different things. Verification determines if the algorithm developed is suited to the specified requirements. This can be done by testing selected scenes. However, a profound validation should contain a quantitative analysis of a larger and well suited test data set that will follow after the launch as explained in the conclusion. Please check and correct the appropriate usage of these terms throughout the manuscript.

l.14 Comma is missing after “However”. Please check this throughout the manuscript.

l.34 “a a decision tree”. Delete "a"

l.122 "makes no sense" Please be more specific. Why does it make no sense? Is a probability more useful for the user's applications as it provides a confidence measure or is there another reason? Please rephrase l.121-124 for clarity.

l.130 How are these surface dependent thresholds derived?

l.180 Please rephrase the sentence for clarity. Is the following correct?

The same applies for the tri-spectral brightness temperature difference test. Further investigation is needed to define the base threshold, which is strongly dependent on surface and water vapor.

l.190 “probabilities that the pixel is clear” --> probabilities of a pixel being clear

l.237 "the the" --> that the

l. 269 “synthetic atmospheric test scenes created with” atmospheric test scenes created synthetically with …

Fig. 6 Figure subtitle >0.1 disagrees with caption and text (≥ 0.1)

l. 303 “difference” is not true.The term “confusion matrix” would be more appropriate.

l. 307 "while the true cloud flag". This may be misleading as the 3D simulations are not ground truth and are highly dependent on the assumptions (l. 276). Might be better to use the word test or simulated cloud flag instead.

l. 315 “data form the Copernicus” –> data from the Copernicus

l. 351 "Twelve groups" what is meant by this? Where do the 12 algorithms originate from?

Citation: https://doi.org/10.5194/egusphere-2022-1240-RC1
- AC1: 'Reply on RC1', Anja Hünerbein, 20 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1240/egusphere-2022-1240-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1240-AC1
RC2:
'Comment on egusphere-2022-1240', Anonymous Referee #2, 31 Jan 2023

This paper describes the EarthCARE MSI cloud mask (M-CM) algorithms, showing algorithm descriptions and validations. Validations has been performed by using, synthetic dataset, versus MODIS, versus SEVIRI. This paper is well organized and reliable. However, some minor improvements will be needed before accepting this paper. Reviewer recommends that this paper to be accepted with some minor modifications.
(1) Line 27.

Could you please mention about how large of uncertainties can be lead. Please add some reference papers?
(2)Line 77.

What is "a histgram-based scene analysis"? Please clarify this point.
(3) From Line 111 to 115.

Please unify the symbol that means "phi".
(4) Line 138.

Please define "the sun glint angle" by schematic figure or formulation.
(5) Equation (3) and (4)

Why multiply 100?
(6) Line 253, and 254.

Please write formulations of the NDVI and the NDSI.
(7) Line 299.

That does a sentence "The M-CF algorithm yields a cloud fraction of 50%" mean? Please clarify this point.
(8) Line 329.

I'm confusing. My understanding is that, Fig.8 MODIS cloud flag was generated by using MODIS-Level1b data with use of the M-CLD processor as noted Line around 315. So, spectral channels used in both MODIS cloud flag and MSI cloud flag is the same (is it true?). However, this sentence mentioned that "the MODIS cloud tests are based on much more spectral channels". I imagine that two examinations are mixed in this topic. Please explain more.
That's all of reviewer's comments, questions, and suggestions for improvement.

Citation: https://doi.org/10.5194/egusphere-2022-1240-RC2
- AC2: 'Reply on RC2', Anja Hünerbein, 20 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1240/egusphere-2022-1240-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1240-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1240', Anonymous Referee #1, 11 Jan 2023

In the study “Cloud mask algorithm from the EarthCARE multi-spectral imager:

the M-CM products” by Hünerbein et al. the MSI cloud mask algorithm is described and its performance compared to selected test scenes of the EarthCARE simulator. A first validation shows a good agreement of the M-CM algorithm with 45 MODIS scenes as well as 12 MSG SEVIRI cloud mask algorithms of one selected scene.

A description and first tests of the M-CM algorithm is very valuable for the cloud community and on time before the launch of EarthCARE that will provide simultaneous measurements of clouds and aerosol properties as well as radiation fluxes.

The paper is written in a clear and concise way. Some minor deficits are noted in the abstract and conveyed messages of the paper. It is recommended for publication after minor revisions.

Minor comments:

The abstract needs modification as the study’s purpose is not clearly stated. What is the study’s motivation, goal, result and outlook?

From my perspective a description of the M-CM algorithm is provided together with a first validation.

The terms “validation” and “verification” should not be used interchangeably as they describe different things. Verification determines if the algorithm developed is suited to the specified requirements. This can be done by testing selected scenes. However, a profound validation should contain a quantitative analysis of a larger and well suited test data set that will follow after the launch as explained in the conclusion. Please check and correct the appropriate usage of these terms throughout the manuscript.

l.14 Comma is missing after “However”. Please check this throughout the manuscript.

l.34 “a a decision tree”. Delete "a"

l.122 "makes no sense" Please be more specific. Why does it make no sense? Is a probability more useful for the user's applications as it provides a confidence measure or is there another reason? Please rephrase l.121-124 for clarity.

l.130 How are these surface dependent thresholds derived?

l.180 Please rephrase the sentence for clarity. Is the following correct?

The same applies for the tri-spectral brightness temperature difference test. Further investigation is needed to define the base threshold, which is strongly dependent on surface and water vapor.

l.190 “probabilities that the pixel is clear” --> probabilities of a pixel being clear

l.237 "the the" --> that the

l. 269 “synthetic atmospheric test scenes created with” atmospheric test scenes created synthetically with …

Fig. 6 Figure subtitle >0.1 disagrees with caption and text (≥ 0.1)

l. 303 “difference” is not true.The term “confusion matrix” would be more appropriate.

l. 307 "while the true cloud flag". This may be misleading as the 3D simulations are not ground truth and are highly dependent on the assumptions (l. 276). Might be better to use the word test or simulated cloud flag instead.

l. 315 “data form the Copernicus” –> data from the Copernicus

l. 351 "Twelve groups" what is meant by this? Where do the 12 algorithms originate from?

Citation: https://doi.org/10.5194/egusphere-2022-1240-RC1
- AC1: 'Reply on RC1', Anja Hünerbein, 20 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1240/egusphere-2022-1240-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1240-AC1
RC2:
'Comment on egusphere-2022-1240', Anonymous Referee #2, 31 Jan 2023

This paper describes the EarthCARE MSI cloud mask (M-CM) algorithms, showing algorithm descriptions and validations. Validations has been performed by using, synthetic dataset, versus MODIS, versus SEVIRI. This paper is well organized and reliable. However, some minor improvements will be needed before accepting this paper. Reviewer recommends that this paper to be accepted with some minor modifications.
(1) Line 27.

Could you please mention about how large of uncertainties can be lead. Please add some reference papers?
(2)Line 77.

What is "a histgram-based scene analysis"? Please clarify this point.
(3) From Line 111 to 115.

Please unify the symbol that means "phi".
(4) Line 138.

Please define "the sun glint angle" by schematic figure or formulation.
(5) Equation (3) and (4)

Why multiply 100?
(6) Line 253, and 254.

Please write formulations of the NDVI and the NDSI.
(7) Line 299.

That does a sentence "The M-CF algorithm yields a cloud fraction of 50%" mean? Please clarify this point.
(8) Line 329.

I'm confusing. My understanding is that, Fig.8 MODIS cloud flag was generated by using MODIS-Level1b data with use of the M-CLD processor as noted Line around 315. So, spectral channels used in both MODIS cloud flag and MSI cloud flag is the same (is it true?). However, this sentence mentioned that "the MODIS cloud tests are based on much more spectral channels". I imagine that two examinations are mixed in this topic. Please explain more.
That's all of reviewer's comments, questions, and suggestions for improvement.

Citation: https://doi.org/10.5194/egusphere-2022-1240-RC2
- AC2: 'Reply on RC2', Anja Hünerbein, 20 Mar 2023
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2022/egusphere-2022-1240/egusphere-2022-1240-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2022-1240-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Anja Hünerbein on behalf of the Authors (20 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (23 Mar 2023) by Robin Hogan

Many thanks for the revised paper. This addresses most but not all of the comments raised by the reviewers. Please provide another version of the manuscript in which you make the following corrections, where the line numbers refer to the revised manuscript (except where noted):

1. Abstract: it would be helpful to clarify that M-COP and M-AOT are downstream EarthCARE products, e.g. replace "which are essential parameters for the cloud optical and physical properties (M-COP) as well as for the aerosol optical properties (M-AOT)" to "which are essential inputs to downstream EarthCARE algorithms providing cloud optical and physical properties (M-COP) and aerosol optical properties (M-AOT)".

2. Reviewer #2 requested changes at the original line 27: your reply provided the new text but the new manuscript does not include this text. Please add it, and also include some of the papers that you reference in the reply.

3. Reviewer #2 requested changes at the original line 77: your reply provided the new text but again the new manuscript does not include this text. Please add it.

4. L118: "way how" -> "way that". Also you probably want to replace "adopted" with "adapted".

5. L120: "As the thresholds varying globally only" -> "As the thresholds vary globally, only".

6. L121: "determine the probability how close the observation is to the limits" is not clear. Perhaps you mean "determine the probability that a cloud is really present based on how close the observation is to the limits"?

7. Reviewer #1 asked how the surface-dependent thresholds were defined (L130 in the original manuscript) but your reply does not answer this (you just say they are "defined before") and no change appears to have been made to the paper. Please explain in the manuscript how these thresholds were defined.

8. L158: "The pixels identified as cloudy if" -> "The pixel is identified as cloudy if". Moreover, the two sentences on lines 155-157 are each their own paragraph. Generally speaking a paragraph should be composed of more than one sentence, so I suggest these two sentences are combined into the paragraph beginning on line 158. Likewise line 166 is another one-sentence paragraph.

9. Eqs. 11 and 12: NDVI and NDSI are acronyms so should be rendered in upright roman in equations, just like in the text. In Latex equations this can be done with \mathrm{NDVI}.

10. L304. "confusion matrix" may not be familiar to some readers, so I suggest this is replaced by "hits, misses, false alarms and correct negatives (i.e. occurrences of elements of the confusion matrix)".

11. L316: "(MOD021KM) seven similar" -> "(MOD021KM) of seven similar" (your reply to reviewer #2 included the "of", but not your new text).

12. Many of your references say "to be submitted" but I think all but one (Eisinger et al.) has now been submitted, so you can replace these with the proper references including the DOI.

Hide

AR by Anja Hünerbein on behalf of the Authors (31 Mar 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (28 Apr 2023) by Robin Hogan

AR by Anja Hünerbein on behalf of the Authors (05 May 2023) Manuscript

Journal article(s) based on this preprint

07 Jun 2023

Cloud mask algorithm from the EarthCARE Multi-Spectral Imager: the M-CM products

Anja Hünerbein, Sebastian Bley, Stefan Horn, Hartwig Deneke, and Andi Walther

Atmos. Meas. Tech., 16, 2821–2836, https://doi.org/10.5194/amt-16-2821-2023,https://doi.org/10.5194/amt-16-2821-2023, 2023

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Anja Hünerbein et al.

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The Multi-Spectral Imager (MSI) onboard of the EarthCARE satellite will provide the information needed for describing the cloud and aerosol properties in the across-track direction complementing the measurements from the cloud profiling radar, atmospheric lidar and broadband radiometer. The accurate discrimination between clear and cloudy pixel is an essential first step. Therefore, the cloud mask algorithm provides a cloud flag, cloud phase and cloud type product for the MSI observations.


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