the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 Aug 2024
Mechanisms of surface solar irradiance variability under broken clouds
Abstract. Surface solar irradiance variability is present under all broken clouds, but the patterns, magnitude of variability, and mechanisms behind it vary greatly with cloud type. In this study, we performed numerical experiments to understand which main mechanisms drive surface solar irradiance extremes across a diverse set of cloud conditions based on observations. The results show that we can capture the essence in four mechanisms. We find that for optically thin (τ < 6) and clouds, scattering in the forward direction (forward escape) is the dominant mechanism. In cloud types such as altocumulus, it is able to produce irradiance enhancements of up to 50 % of clear-sky values due to small gaps in the cloud field. For flat, optically thick clouds (τ > 6) like stratus, downward escape becomes the dominant mechanism, and the irradiance extremes are found underneath the cloud edge or gaps. Albedo has a significant effect under optically thick cloud cover, contributing 10 to 60 % of the total irradiance enhancement for low (0.2) to high (0.8) albedo. For deep convective clouds, side escape is the dominant mechanism enhancing domain-averaged diffuse irradiance. This effect has a large area of influence, extending over 20 km from the sunlit side of the cloud. Extreme irradiance enhancement, however, comes from downward escape and forward escape just underneath the cloud edge on the sunlit side, not from side escape. These results provide a framework for understanding the vast diversity and complexity found in surface solar irradiance and cloudiness. A next step is to apply this analysis to multi-layered cloud fields and non-isolated deep convective clouds.
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RC1: 'Comments and recommendations about manuscript by Mol and Heerwaarden', Anonymous Referee #1, 24 Sep 2024
In my opinion, this paper presents an interesting analysis and provides new insights that can help us better understand cloud-related enhancements in surface irradiance. The proposed theoretical classification makes sense, the methodology is suitable for the task, and the analysis is thorough. (I believe this even though, as mentioned near the end of the manuscript, the study does not use the most powerful analysis approach of examining photon paths and scattering directions.) The presentation is of a generally high quality, but important improvements are still needed, most critically in the introduction section. My specific comments are listed below.
Major issue:
The introduction section needs a thorough revamping, for several reasons.
First, the introduction should provide context and historical perspective to the presented study. For example, it should address the following questions. Did other researchers previously examine (using observations and/or theoretical calculations) cloud-related surface irradiance enhancements, and what were their main findings about the frequency, magnitude, sources, and consequences of these enhancements? What is the underlying motivation for us to care about these enhancements: Is it perhaps something about solar energy production or the health risks of UV radiation, etc.? Did the earlier results leave major gaps that we still need to fill, perhaps in observing, understanding, simulating, or predicting the enhancements? Which of these gaps does the current paper help us fill?
Second, Section 1.2 does not seem introductory, as it proposes a new theoretical framework that is a key element of this study. Therefore, I recommend moving Section 1.2 into a new section of its own.
Third, the introduction (just before the start of Section 1.1) presents a brief preview of what we can expect in each section of the paper, but this preview stops at Section 3 and does not include sections 4 and 5 (which present the results and conclusions, respectively).
Finally, the very first sentence starts the paper off by telling about the introduction section rather than about the paper as a whole, and by referring to a “chapter” and a “thesis” (which suggests that the text was simply copied from an academic thesis).
Minor issues:Line 3: The wording should make it clear that the paper covers only extreme highs and does not discuss extreme lows.
Lines 8-9: The wording should be changed because as is, it discusses an albedo effect but not discussed a mechanism. Perhaps trapping or multiple reflection between surface and clouds could help in phrasing the albedo-related process as a mechanism.
Figure 1: The caption or a newly added legend should explain what the lines of various colors represent.
Line 88: Just a typo: The correct start to the sentence should be “In all cases…”
Line 139: It would help to clarify the main between between Monte Carlo ray tracers and radiative transfer models.
Line 147: It should be clarified what RTE-RRTMGP stands for (I imagine RTE is for Radiative Transfer Equation) and, if possible, a reference should be given.
Line 150: Are longwave simulations used in this study? If yes, it should be mentioned what they are used for; if not, they should not be mentioned.
Line 158: What are the wavelength limits of the used visible spectral band?
Figure 8: In the caption, it would help to clarify what exactly is meant by “relative to clear-sky values”; I guess it’s clear-sky values of diffuse irradiance rather than clear-sky total irradiance. Also, it could help to explain why, in the right-side plot, the area under the cloud is white. (Alternatively, could the location of the cloud be marked by a circle and allow us to see the enhancement inside the circle?) Finally, it might help to clarify in the caption or around Line 245 that the yellow dashed line is not visible in the left side plot simply because it coincides with the solid line.
Line 270: The word “under” could be changed to something like “in cases of”, as most values under clouds (i.e., shaded areas) are blocked out by grey in the key, lowest row in Figure 9.
Figure 11: It should be explained what the “diffuse peak probability” (shown in dashed lines in the right-side column) is.
Figure 12b: It should be clarified whether optical depth increases when cloud depth increases, or the optical depth remains unchanged, and the cloud gets less dense as it gets deeper. This could be clarified either around here or around Line 170.
Line 354: The wording “scattered direct irradiance” seem self-contradicting, as direct irradiance is, by definition, non-scattered.
Figures 12 and 13 (and perhaps others) should be placed after they are described in the text.
Line 370: It might be worth adding “and its immediate surroundings” after “itself”, given the finding that, for 16.5 km cloud depth, some very high values occur outside the updraft.
Lines 396-397: It seems worth mentioning that parts of the scene are shown in the right-side plot of Figure 7.
Lines 420-421: I suggest either deleting the word “zone” or replacing the word “between” by “around”.
Citation: https://doi.org/10.5194/egusphere-2024-2396-RC1 - RC2: 'Comment on egusphere-2024-2396', Philipp Gregor, 15 Oct 2024
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Supplementary data for "Mechanism of surface solar irradiance variability under broken cloud cover" Wouter Mol and Chiel van Heerwaarden https://doi.org/10.5281/zenodo.11503610
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