the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Jan 2025
The subtleties of three-dimensional radiative effects in contrails and cirrus clouds
Abstract. The radiative effect of cirrus, contrails, and contrail cirrus affects the energy budget of the Earth and climate change. Those clouds, and especially contrails, are heterogeneous and their holes and sides exert three-dimensional radiative effects. This study uses the htrdr Monte Carlo radiative transfer code to investigate the sensitivity of the cloud radiative effect (CRE) to the geometrical dimensions and optical depth of optically thin ice clouds (cloud optical depth < 4), with particular emphasis on three-dimensional radiative effects. When the Sun is at zenith, an increase in cloud optical depth causes a linear increase in shortwave (SW) CRE but a saturation of longwave (LW) CRE, causing the net CRE to change sign from positive to negative. The optical depth at which this change in sign occurs depends on the cloud geometry. 3D effects make the one-dimensional SW and LW CREs more positive for a Sun at zenith, reaching the same order of magnitude as the 1D CRE itself for clouds with high aspect ratios. The angular dependence of ice crystal scattering strongly increases shortwave CRE when solar zenith angle increases. 3D effects change sign from positive at zenith to negative at large zenith angles as the Sun’s rays interact more with the cloud sides. Integrating instantaneous CRE and 3D effects over selected days of the year indicates compensation of SW with LW 3D effects for some cloud orientations, but 3D effects remain important in some cases. These results suggest that the 3D structure of cirrus and contrails needs to be considered to finely quantify their CRE and radiative forcing.
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RC1: 'Comment on egusphere-2024-3642', Anonymous Referee #3, 05 Feb 2025
The manuscript investigates cloud radiative effect (CRE) (both overcast and all sky) and 3D radiative effect using a Monte Carlo ray tracer run in a plane parallel (1D) and 3D configuration. It investigates the CRE (both shortwave and longwave) of contrails/cirrus considering a) different idealized geometries, b) for 1D vs. 3D calculations, and c) varying solar angles (including time-integrated solar angles). It also considers the 3D radiative effects of contrails for a) the sun at zenith with varying geometries and b) varying solar angles (including time integrated angles) with set geometry. These many analyses are accompanied by thoughtful discussion.
The manuscript contains several neat results:
- Overcast 3D radiative effects for the sun at zenith are positive (warming) for both the SW and LW due to side leakage and side cooling, respectively.
- When varying SZA, 3D radiative effect in the SW switches in sign from positive to negative at high solar zenith angle with a dependence on cloud optical thickness.
- In looking at time integrated solar position and effect on 3D radiative effect, there is a strong dependence on the orientation of the cloud relative to the sun and overall the strongest 3D effects are seen for the winter solstice.
This thorough manuscript would most benefit from better framing. What questions motivated the study? What have the authors learned that was not previously known or anticipated? How might the results be used to generalize understanding of how contrails affect radiation?
The manuscript would also benefit from greater emphasis on novel insights. The manuscript reports on both 1D and 3D calculations of CRE. The former are less novel than the latter, and the new results might come out more clearly if the 1D results were reported more selectively. In particular, the linear relationship between ice water path and CRE for the low cloud optical thickness regime is well established in the literature (and summarised in the authors’ table C2); the manuscript could instead rely upon citations to allow for more attention on 3D radiative effects.
Some more specific suggestions:
- Figures 9 and 10 were the most interesting to me but would benefit from a diagram of the N-S, W-E clouds and path of the sun at equinox and solstice to further illustrate the author’s insights.
- It would be easier to compare between sections if there were consistency in language about cloud orientation; the authors use parallel, perpendicular in figure 8 and N-S, E-W in figures 9 and 10.
- Figure 7 would benefit from a second panel showing the 3D radiative effects so the reader does not have to do the calculation with their eye.
- While the scope of the study does not include the interplay between geometry and solar angles on 3D radiative effects, synthesis and discussion on this topic would be very interesting.
Citation: https://doi.org/10.5194/egusphere-2024-3642-RC1 -
RC2: 'Comment on egusphere-2024-3642', Anonymous Referee #2, 13 Feb 2025
General comments
The study by Carles et al. is a sensitivity analysis of optically thin ice clouds radiative effects (CRE) to their geometrical dimensions and optical thickness investigating also the importance of the 3D effects on these estimations. By using Monte Carlo radiative transfer simulations for both 1D and 3D configurations, the authors derived useful insights for CREs (SW, LW and net) in terms of clouds geometry, their optical thickness and the relative position of the sun, highlighting when the 3D effects are important. To put into perspective their results in terms of the importance of including the 3D effects in estimating cirrus and contrails radiative effects and forcing, CRE estimates integrated for selected days at different latitudes were also calculated and discussed. The objectives of the study are quite straightforward and are addressed by thorough analysis. I consider the topic and results of this manuscript to fit the scope of ACP.
I have some general and minor comments which should be addressed prior to publication.
- It would increase the value of manuscript to elaborate in the introduction which was the gaps identified in previous studies which motivated the objectives of the current study.
- There is an extensive part dedicated to CRE 1D results and to simple analytical models of CRE in relation to cloud optical depth. Apart from explaining the results in a more intuitive way, please provide the added values of this analysis.
- There is a very short discussion in Lines 428-431 comparing the results of the present study with previous studies. I suggest placing at the relevant sections the differences and similarities that are briefly mentioned in the conclusions section.
Specific comments
Table 1. L hasn’t been introduced yet.
Line 154: I cannot see the vertical line mentioned here in Fig. 2b
Line 168: Please, be more specific instead of “cf”
Figure 4: Fig 4b should have the colored light shadowing? I was expecting something more like Fig. 4c. In addition, “Blue” are black lines right?
Technical corrections
Figure 3: (a) in the SW, (b) in the LW instead of “(a) in the LW, (b) in the SW”
Citation: https://doi.org/10.5194/egusphere-2024-3642-RC2
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