My initial comments were in large part addressed by the authors, but some questions were not fully addressed in my opinion. Please find details below.
Specific comment #3
- Initial comment
The notion of effective column extinction coefficient is introduced line 188, but is not defined until lines 307-309 in Section 4.1. Can you explain why you need to introduce this effective extinction as a value representative of the entire cirrus? Is there a need to have one single set of parameters over a smaller geometric thickness for the radiative transfer calculations? My understanding is that COD is unchanged and that the effective extinction is twice the mean extinction. Can you clarify? How does this change of visible extinction coefficient affect IWC, other extinction coefficients, SSA, and asymmetry parameter? In Section 4.1 (Fig. 3), you indicate that your average IWC of 5 mg m-3 is close to a value of 3 mg m-3, which is in agreement with other studies. Does this suggest that mean extinction would be better suited than effective extinction?
- Authors’ response
After applying the two-way transmittance method, we have vertical profiles of extinction and temperature for each cirrus cloud, with a vertical resolution of 75 m. Before applying the self-consistent scattering model for cirrus clouds, we degrade the vertical resolution of these profiles to constrain them to the model vertical resolution, through vertical averaging. This vertical resampling of the cloud is necessary to be able to distribute it in different model layers and not to consider the cirrus cloud in a single 1 km thick layer.
Figure 3 shows the mean values for each cloud. To avoid confusion, the term effective column extinction coefficient has been changed to extinction coefficient in each vertical layer of the model and in Figure 3 it is specified that these are the mean values of each cirrus cloud. That the mean IWC is 5 mg m-3, being a close value of 3 mg m-3, which is in agreement with other studies means that 1) the new methodology for calculating the optical scattering properties of cirrus clouds adequately estimates the IWC, 2) the new approach of the two-way transmittance method inverts well the lidar signal and 3) the identification of cirrus clouds made with measurements from the MPL at the Barcelona lidar station is correctly.
The vertical resampling of the cirrus cloud in the self-consistent scattering model could sometimes lead to a cloud layer having a low ice water content and consequently low optical scattering values, as mentioned in lines 298-305.
- My response
Thank you for adding lines 173-174: “To align these calculations with the model vertical resolution, we previously degrade the vertical resolution of the cloud extinction and temperature profiles through vertical averaging”.
Because the model has not been presented at this stage, it might be useful to clarify that the model resolution is 1 km at cirrus level.
Specific comment # 4
- Initial comment
Can you please give T and IWC for the cloud used to create Fig. 2?
- Authors’ response
Figure 2 has been achieved by averaging the optical scattering properties in all the layers of a cirrus cloud measured on 8th December 2018 at 12 UTC in Barcelona lidar station. The figure with the vertical distribution of the optical scattering properties is shown below.
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The vertical profiles of the extinction, temperature and ice water content of this cirrus cloud are shown below.
My response
The figures provided by the authors in their response are very informative and I suggest including them in the manuscript. Properties at 10, 11, and 12 km could be added to the current Fig. 2, and I suggest showing also the corresponding vertical profiles of extinction, IWC, and temperature.
Specific comment #5
- Initial comment
Line 240: where in the paper is the “with gases only” configuration used?
- Authors’ response
This configuration is used to calculate the direct radiative effect of the cirrus cloud as the difference between the radiative fluxes of the simulations in which cirrus clouds and gases and only gases are considered. This direct radiative effect has been defined as DREGC-G. I recommend reviewing Section 4.3, where these direct radiative effects are analyzed.
My response
Thank you for this explanation. Note that I found the sentence at the beginning of Sect. 4.3 a little confusing. Perhaps refer to Eqs. (2) and (3) for more clarity regarding “the difference between the radiative fluxes”.
Specific comment #9
- Initial comment
Section 4.2, lines 340-342: I do not understand the reasoning: the authors first derive IWC from the lidar visible extinction coefficient and then derive all the other properties. Can you explain why the issue is related to small IWCs? I am wondering how the asymmetry parameter could play a role. Simple sensitivity studies could strengthen the discussion.
- Authors’ response
We derive the IWC from the extinction coefficient obtained with an elastic lidar signal from a MPL because there is no ground-based instrument available at the Barcelona lidar station to directly measure IWC.
The issue of small values of IWC lies in the degradation of the cirrus vertical distribution when is constrained to the model vertical resolution. Initially, the vertical extinction profile has a resolution of 75 m and is converted into a vertical profile of 1 km resolution by averaging. Cirrus clouds are not uniformly distributed over the entire 1 km layer. As a result, in some vertical layers of the model a low extinction value is obtained and after applying Eq. 3, these layers have a low IWC estimation.
Additionally, as you suggested, a sensitivity study was done, but due to the length of the manuscript, it has not been included. Below, I present the optical scattering properties derived from the self-consistent scattering model for cirrus clouds at 0.55 μm, corresponding to initial extinction values between 0 and 2 Mm⁻¹ at a temperature of -37ºC. By initial extinction values I mean the extinction values entered in Eq. 3 to obtain the IWC. The results are shown below.
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My response
As noted by the authors, SSA in the visible should be close to 1. The authors explained that the instances of SSA < 0.6 at 0.55 µm are due to lidar extinctions at 532 nm (initial extinction in the figure they provide in their response) smaller than 1 Mm-1. These tiny initial extinction values could be obtained because the initial vertical resolution of 75 m was degraded to 1-km to match the vertical resolution required by ARTDECO. The resulting SSA values in Fig. 3 smaller than 0.9 are not physical and seem to indicate that Eq. 1 is not valid for very small initial extinctions, at least for SSA. Furthermore, it is my understanding that σext (4th panel from the top) at 0.55 µm should be very close to “Initial extinction” (X-axis) at 0.532 µm, which is clearly not always the case. I see for instance σext = 12 Mm-1 where “Initial extinction” = 0.1 Mm-1.
I do not understand why “the self-consistent scattering model would associate low extinction values to super-cooled liquid water clouds”, as stated by the authors. Another explanation could be simply that the fitting coefficients yield valid results only for initial extinction larger than a certain value. This question should be addressed in the manuscript.
Technical comments:
- Please make sure that the asymmetry factor is now always noted “g”. For instance, I see “asyF” in Fig. 1, but there might be other instances. |