|This work presents results of collocated (spatially and temporally) shortwave, spectral cloud radiation (downwelling and upwelling) and concurrent microphysical measurements from instrumentation on an aircraft above a thin, cirrus layer and from a platform (called the AIRTOSS) towed below and behind the aircraft. For the case study presented, the atmospheric layer bounded by the aircraft and the towing platform, which had a vertical distance of ~ 200 m, which also contained the (thin) cloud. Using the combination of radiation measurements from the two platforms observations of spectral (from 300 to 2200 nm) cloud absorptivity, transmissivity, and reflectivity of the layer were provided as well as the cloud top albedo. The measurement results presented are the first for true, collocated sampling of radiation fields above and below a cloud; other studies have applied approaches to perform this sampling with a single aircraft consecutively, or with two aircrafts, but with some temporal lag between. The authors mention how these other, previous, measurement sampling configurations can only be applied to clouds with little static development and that are horizontally homogenous (i.e. no 3-D effect); this discussion, at first, implies that the presented aircraft + AIRTOSS sampling configuration is unaffected by these constraints. It becomes more clear (but could probably be more explicitly stated) that the aircraft + AIRTOSS sampling configuration would still be subject to uncertainties in horizontal radiation flow out/in. |
The authors then present various sensitivity analyses of the cirrus layer properties to changing cloud microphysics, namely particle size and shape, and to changing underlying surface conditions. In this study, the predominant variability in underlying surface conditions was from a lower-level water cloud, as the measurements were obtained over the ocean. Microphysical measurements of number size distribution were used in concert with tables of single scattering properties for varying ice particle shapes and sizes to evaluate assumptions in particle shape and size on the simulated cirrus cloud layer properties. A second sensitivity analysis to assess changes in radiative forcing of the cirrus by an unaccounted for underlying low level water cloud was also done; this impact would be to overestimate the shortwave cooling (reflectance) by cirrus cloud.
The presented work establishes the complexity of clouds (and cirrus clouds) and this case study of the spectral measurements of cirrus cloud layer properties is a valuable contribution to the sparse database of these observations. The sensitivity analysis the authors present also establishes the challenges in simulating cirrus cloud optical properties due to uncertainties and variability in shape of ice crystals and in the atmospheric conditions below the cirrus cloud.
I do have a criticism regarding the discussion of radiative forcing presented in this work for these reasons:
a) it is not discussed that the radiative forcing is typically defined as the net of shortwave and longwave, and that the warming by cirrus comes from absorbing the outgoing energy from the earth (i.e. the LW component). Please add to the intro material.
b) while it is mentioned in intro and conclusion that the radiative forcing presented in this study is shortwave radiative forcing (“solar cooling”), I think this should be emphasized once. Perhaps along with Equation 6. In light of concern a), please refine text following equation 6 to reflect that the LW component is additionally necessary to evaluate whether a cloud has a net warming or cooling effect on the underlying atmosphere/surface.
c) the discussion in regards to Figure 12 (page 11; lines 355-357) refers to “noticeable” results where there is a change in sign from cooling to warming at near-infrared wavelengths for a cirrus cloud in the presence of an underlying water cloud. I agree I can see a tiny blip in the spectrum, but it is practically indistinguishable from the zero-change line. While I don’t argue that systematic uncertainties are very important, I do think this particular aspect of the results has been overly interpreted.
Page 7, line 224 – I think you are mistaken when you attribute the low reflectivity in Fig 7b to the cirrus cloud and a brighter warm cloud underneath. I think this is just a typo as earlier in the manuscript, the clear difference between a layer property and that of cloud layer plus atmosphere was established.
Page 11, line 342 – In the discussion of the cirrus and underlying low-level cloud, the additional contribution from multiple reflections of radiation from the low-level cloud that are reflected upward and then interact with the cirrus cloud have been attributed as a property of the cirrus layer. I think it would be important to make clear that this is only true according to the measurement-based definition of transmissivity defined in Eq (2). In actuality, the true cirrus "layer" properties would be unaffected by the lower level cloud.
Figures 10&11&12: It would seem in Figure 10, that the greatest factor to the differences between the various simulations and the measurements (with diamonds) arises from the contributions by the underlying surface (a water cloud) to the cirrus layer. Is a claim of the paper that there is a potential for the retrieval of particle shape using measurements of cirrus layer absorption– for limited, idealistic cases (dark ocean, no underlying clouds before the cirrus or 3-D effects)? The sensitivity to the changing surface properties (Fig 12) would seem to swamp any of the spectral signature change due to particle shape (within the uncertainty of the measurements).
In Figure 11b, do I understand correctly that these results would reflect the inverse relationship between particle size and scattering? So, while Table 1 does not include a value of effective diameter (or effective radius), I assume that the ice models with the largest optical thickness would correlate to smallest effective diameter. Then, were the simulations for Approach II also to be shown for a constant optical thickness, any resulting differences could be attributed to a size effect.
Could you supply the wavelength for which the optical thickness values are applied in the caption to Table 1?
Page 9, line 275 – ‘The ice crystal shape is assumed do (to) be constant..”
Page 7, line 214 – ‘…according the (to) Eq,’