the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 May 2025
Case study on the impact of moving broken clouds on the spectral radiance
Abstract. This case study shows the possibility of using the Advanced MUltiDIrectional Spectralradiometer (AMUDIS) to simultaneously measure the temporal impact of moving clouds on the spectral radiance in different directions and also the first results. For these aims measurements from January 2024 with the AMUDIS in Hannover (Germany 52.39° N and 9.7° E) for cloudless sky and two moving broken clouds situation with different cloud cover are analysed.
From the measuring range of 380–890 nm and the 140 directions, three wavelengths (480 nm, 530 nm, 585 nm) and three directions were selected. These three directions are in the area between north and east and were measured simultaneously.
From these datasets, the temporal variation was calculated for each case with reference to a cloudless moment in the situation (no clouds were observed in the field of view of the input optics). The variations during a broken cloud situation on 18 January were compared with the variations on 10 January under cloudless conditions. It was determined that the spectral radiance was higher by a factor of up to 3 under cloudy conditions. The results show also that the impact of the clouds is dependent on the wavelength, direction and cloud type. The AMUDIS offers another option for validating radiation transfer models, for example.
- Preprint
(1878 KB) - Metadata XML
- BibTeX
- EndNote
Status: final response (author comments only)
-
RC1: 'A review of manuscript by Duffert et al.', Anonymous Referee #1, 02 Jul 2025
In my opinion, the development of the AMUDIS instrument would be of interest to the community and is worth publishing. However, I believe that the current manuscript needs substantial changes before it could be considered suitable for publication. The necessary changes include explaining the novelty and potential scientific benefits of the instrument and clearly explaining some important instrument characteristics and data interpretation issues. Please find my specific comments below.
Major comments
1.
The paper should clearly explain the novelty and benefits of the new instrument. The manuscript should discuss any scientific, engineering, practical, or financial benefits over existing instruments such as all-sky imagers and spectroradiometers. This includes discussing how exactly AMUDIS differs from MUDIS.2.
The paper should discuss what specific cloud retrievals AMUDIS seeks to enable: optical thickness, particle size, or perhaps something else? If the targeted retrieval approach has been used in past studies, they should be referenced; otherwise, the concept of the envisioned retrieval approach should be outlined. The discussion should include explaining whether absolute calibration is needed for the envisioned retrievals, and if it is needed, the manuscript should outline plans or expected opportunities for absolute calibration.3.
Since the last sentence of the abstract says “The AMUDIS offers another option for validating radiation transfer models”, the paper should explain how one could validate radiative transfer models using AMUDIS data that, at least for the moment, lacks absolute calibration.4.
Section 2.1: Several important aspects of the AMUDIS instrument should be added to this section. Specifically:
a) What is the spectral resolution and the number of wavelengths within the three spectral ranges?
b) Why do the field-of-views have different sizes for different fibers (as shown in Fig. 1 and Table 1)?
c) Why does it take a few seconds to take all the measurements, as mentioned in Line 70? Is there some sort of scanning or perhaps a rotating spectral filter involved?
d) What is the instrument structure like, and how does it enable a single camera to take measurements for 140 fibers? I recommend expanding the instrument description and adding a schematic drawing to show the key components of the system.5.
Line 120: The words “was calibrated” should be replaced, as Section 2.1 only discusses characterizing the stability of the instrument, whereas calibration would mean that we determine what instrument output corresponds to what radiative quantity (such as radiance in units of W/m**2/sr/µm).6.
Table 2: It should be clarified what it means that the shutter was “on”.7.
Section 3.3 should discuss how clouds influencing nearby cloudless locations impact the combination/comparison of cloudy and cloudless data. This is important because cloudless areas are known to be affected by clouds through several mechanisms (ranging from increased light scattering by aerosols that swell in the extra-humid air near clouds to increased light scattering in cloudless areas due to the extra illumination coming from nearby bright cloud sides)—whereas if the cloudless reference case is from a completely cloud free time period, this is not an issue (but then temporal variations in atmospheric properties can cause complications).8.
Line 101: In contrast to the text and Figure 3, Figure 1 shows Fiber 28 in the northwest segment (and not in the northeast segment); the discrepancy should be resolved. Also, the text should discuss somewhere why these three fibers were selected for analysis. It should also explain why the number of fibers is set to 140. Also, it should be explained what the overlapping circles represent near the bottom right and bottom left horizon lines in the right panel of Fig. 1, and why having some overlapping fibers is useful.9.
Section 3 should discuss the results not only in terms of fibers, but also in terms of relative azimuth angles and provide more thorough physical interpretation/explanations. Even basic intuitive explanations would help, for example pointing out that the high ratios occur because sunlit cloud sides are brighter than blue sky in between clouds. Following up on this, it would be helpful to also consider data from a fiber that views the shadowy (and not the sunlit) side of clouds: How do the observations behave for those fibers? The discussion of physics should also consider the physics behind the statement in Line 400: “the greater the wavelength, the higher the ratio” by pointing out that this is largely (or at least partly) due to the fact that scattering by air molecules and small aerosol particles is weaker at longer wavelengths. (This makes the sky appear blue and be darker at longer wavelengths.)Minor comments
Lines 23-24: It is unclear what “source” refers to; this should be clarified, or the word should be deleted.
Line 24: I recommend adding “Arguably,” between “influence” and “Spectral”.
Line 24: The abbreviation "cf." stands for the Latin word "confer", meaning "compare", which is not needed here as there is no comparison in the sentence.
Lines 24 and 435: This reference format is unfamiliar to me and should be clarified. Is “DIN” the last name of the author? If so, it should be changed to “Din”, and the first name should also be provided.
Line 27: I guess “radiation properties for” should be changed to “radiative properties when using”, because the current wording is unclear.
Line 34: The word “like” should be replaced by “such as”.
Line 38: Clouds are not parameters of the atmosphere.
Line 56: I recommend adding “such as”, “e.g.,”, or “for example” right after the first opening parentheses.
Line 60: As the meaning of “DFG approval: GZ: INST 187 / 555-1 FUGG” is unclear (this is not a reference format), this text should be deleted or clarified.
Line 60: It is unclear what is meant by “large-scale device”; this should be explained, or the wording should be changed.
Lines 61-62: The words “the temporal uncertainties in measurements in different direction of the atmospheric variability” should be changed as this is confusing. For example, what is meant by “temporal uncertainties”: The instrument does not keep track of when exactly it takes certain measurements? Also, what is meant by “different direction of the atmospheric variability”: What is the direction of atmospheric variability?
Lines 106-110: The caption should have three parts: Top left, bottom left, and right. The current description of three images using four sections (Left, top, bottom, right) is confusing. Also, the acronym IMUK should be explained.
Line 108: The word “objective” should be replaced as it has a different meaning than intended.
Figures 1 and 3: I recommend flipping the sky images to put east to the right side and west to the left side, which is the usual orientation.
Line 126: The text “In total, 21 measurements were carried out” should be modified to indicate that there were 21 measurements for each of the 140 views and each of the spectral ranges (if this is correct).
Equation (4): Over what time period were the 10 measurements spread out? (In other words, how far apart in time were the 10 measurements?) This becomes clear in later section, but as is, readers could get discouraged by not understanding this here. If preferred, even just a sentence saying that this will be discussed later could help.
Line 398: The text in the parentheses “(2 of them close to each other from each other)” should be reworded.
Citation: https://doi.org/10.5194/egusphere-2025-1118-RC1 -
AC1: 'Reply on RC1', Jens Duffert, 19 Aug 2025
Dear Editorial Team,
Dear Reviewer,
Thank you very much for your efforts and for taking the time to read our paper “Case study on the impact of moving broken clouds on spectral radiance”. Your comments and suggestions were helpful and will further improve the quality of the paper. We hope we were able to answer your comments satisfactorily. The answers are in the pdf-document.
Kind regards
Jens Duffert (lead author)
-
AC1: 'Reply on RC1', Jens Duffert, 19 Aug 2025
-
RC2: 'Comment on egusphere-2025-1118', Anonymous Referee #2, 20 Jul 2025
General comments:
The manuscript presents a case study in which the Advanced MUltiDIrectional Spectralradiometer (AMUDIS) was used to measure spectral radiance in different directions under cloudy and cloud-free conditions on two different days. It was observed that the spectral radiance was by a factor up to 3 higher under cloudy conditions.
The availability of ground-based spectral radiance is – despite of their relevance for cloud-radiation interaction studies – very limited and thus this study is in my opinion highly important to the community and suited for publication in AMT.
The manuscript is in general quite well structured and - with some exceptions - clearly written. The literature has been carefully selected and cited. Graphics and tables are in general clear and the captions self-explanatory (but not always, see minor comments). The physical interpretations of the results could partly be improved. In addition, an outlook on further planned activities and capabilities of AMUDIS should be included in the conclusions. When the substance of the content is improved by giving more thorough physical explanations of the results, and the focus is sharpened towards a more original, better structured and formulated conclusion including an outlook, this study will be an interesting and valuable contribution to the atmospheric science community and is in my opinion suited for publication in AMT.
Specific comments:
Section 2. Devices Materials and Methods.
- state the spectral resolution of AMUDIS (e.g. in line 70)
- what about stray light and potential correction? Can you comment on that?
- Why is the FOV different for different fibers? Production and alignment of the fibers?
- calibration: I am confused: the method of Niedzwiedz et al. (2021) was used to calibrate AMUDIS (line 120). This is an absolute calibration method. However, you did not point out the absolute nature of the calibration but highlighted its stability only. Later you used counts for the individual measurements instead of W/m2/sr/µm what is in principle appropriate because you calculated ratios. Nevertheless, you should clearly state if you have calibrated AMUDIS absolutely, especially since you highlighted in the abstract that AMUDIS can be used for the validation of RTM, which requires an absolute calibration.
Section 3: Results
- why have you chosen the three fibers 4, 28 and 90 for analysis? Is it for these particular cloud scenes? Would different directions may be more suitable for these specific cloudy scenes?
- you pointed out a potential impact of aerosols in the cloud-free scene (line 224). Is there any aerosol information available (AOD, single scattering albedo…), e.g. from a nearby AERONET site?
- the physical interpretations of the observed features are sometimes limited, e.g. what is the physical cause of individual readings to be lower than the uncertainty limit of the cloud-free reference (line 350)? Or what is the physical explanation for higher ratios for longer wavelengths (line 400)? Try to give a more concise and physically-based interpretation for such cases
Section 4: Conclusion and Discussion results
I propose to include an outlook: what are your plans for AMUDIS in terms of technical activities and analysis, for instance:
- implementation of a stray-light correction (if necessary)?
- absolute calibration (if not yet conducted, see my comment above)
- are there any plans/possibilities to retrieve some macro-/microphysical aerosol and/or cloud properties from spectral radiance observations of AMUDIS? If yes, which ones?
- quantification of 3D radiative effects?
- validation of RTM
Minor comments
Line 15: may insert “…the temporal variations of the spectral radiance was calculated…”
Line 19: Validation of RTM: I would rather put this in the outlook. In addition, an absolute calibration of AMUDIS is required (see my comments above)
Line 23: delete “source”
Line 27: cloud and radiative properties
Lines 33/34: May rephrase this sentence, e.g.: “Solar radiance at the surface can be observed using various measurement systems, such as:”
Line 38: May rather use “constituents” than “parameters”
Line 39: “…such as clouds, aerosols may not be detected…”
Lines 46/47: may rephrase this sentence, e.g.: “…(HIS) systems, although they are mainly used to detect clouds, calculate the total cloud cover, classify clouds and study their radiative effects”
Line 51: Due to their automated…
Line 56: May rephrase, e.g.: “Non-scanning multidirectional spectroradiometers such as the multidirectional spectral radiometer (MUDIS) and the advanced multidirectional spectral radiometer (AMUDIS)…”
Line 60: What does “large-scale device” mean?
Line 60: I would delete the DFG approval
Line 62: “temporal uncertainties in measurements in different direction of the atmospheric variability” sounds strange to me. You may mean that with AMUDIS you can observe the spatial and temporal variability of the atmospheric constituents and properties or similar, I guess.
Line 71: “…as described in Seckmeyer et al. (2018 and Tobar Foster et al. (2021), it is based on…”
Line 87: may use “…the light is detected by three CCD image sensors”
Lines 106-110: Caption of Figure 1: Use “upper left, bottom left and right” for the description of the three images. Define IMUK
Line 150: May add “Thus, an instrumental uncertainty of 3.5 % is assumed “ or similar
Table 2: Shutter on: Replace “on” by “open”
Line 234: May replace “at” by “for”
Line 253: Isn’t it section 3.1 instead of section 3.2?
Line 269 and Line 345: Table 4 and Table 5: The minimum of the ratio is not indicated.
Citation: https://doi.org/10.5194/egusphere-2025-1118-RC2 -
AC2: 'Reply on RC2', Jens Duffert, 19 Aug 2025
Dear Editorial Team,
Dear Reviewer,
Thank you very much for your efforts and for taking the time to read our paper “Case study on the impact of moving broken clouds on spectral radiance”. Your comments and suggestions were helpful and will further improve the quality of the paper. We hope we were able to answer your comments satisfactorily. The answers are in the pdf-document.
Kind regards
Jens Duffert (lead author)
-
AC2: 'Reply on RC2', Jens Duffert, 19 Aug 2025
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 318 | 57 | 19 | 394 | 10 | 29 |
- HTML: 318
- PDF: 57
- XML: 19
- Total: 394
- BibTeX: 10
- EndNote: 29
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1