the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Information Content of Brightness Temperature Differences of Spaceborne Imagers with respect to Cloud Phase
Abstract. This study investigates the sensitivity of two brightness temperature differences (BTDs) in the infrared (IR) window of the SEVIRI imager to various cloud parameters in order to better understand their information content, with a focus on cloud thermodynamic phase. To this end, this study presents radiative transfer calculations, providing an overview of the relative importance of all radiatively relevant cloud parameters, including thermodynamic phase, cloud top temperature (CTT), optical thickness (τ), effective radius (Reff) and ice crystal habit. By disentangling the roles of cloud absorption and scattering, we are able to explain the relationships of the BTDs to the cloud parameters on the one hand by spectral differences in the cloud optical properties. In addition, an effect due to the nonlinear transformation from radiances to brightness temperatures contributes to the specific characteristics of the BTDs and their dependence on τ and CTT. We find that the dependence of the BTDs on phase is more complex than sometimes assumed. Although both BTDs are directly sensitive to phase, this sensitivity is comparatively small in contrast to other cloud parameters. Instead, the primary link between phase and the BTDs lies in their sensitivity to CTT, which is associated with phase. One consequence is that distinguishing high ice clouds from low liquid clouds is straightforward, but distinguishing mid-level ice clouds from mid-level liquid clouds is challenging. These findings help to better understand and improve the working principles of phase retrieval algorithms.
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CC1: 'Comment on egusphere-2024-540', Andrew Heidinger, 22 Mar 2024
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This paper is a very thorough sensitivity analysis of SEVIRI IR brightness temperature differences. It will serve as a reference for future studies. Excellent text and high quality figures. I urge its publication and my comments should be considered optional.
Line 30. Many algorithms use these brightness temperature differences along with other measurements (brightness temperature of other channels, reflectances …) to retrieve phase simultaneously with other cloud properties (temperature, reff, tau). Are you saying this is necessary?
Figure 2. SEVIRI offers channels at 9.7 and 13.3um but they occur in absorption bands. These seem to offer different single scattering properties that add to the phase story. Do you see any benefit of their inclusion or is the atmospheric absorption too much in your opinion?
Line 40.0 You mention Parol (1991). One of the main tools of his analysis was to convert BTDs in beta ratios which are directly linked to the single scattering properties and remove many of the temperature, optical depth dependences. Do you have a position on the use of beta ratios in cloud phase determination?
Line 355, I am confused by the two statements “the single scattering properties
are not CTT dependent” and “cloud emissivity is not CTT dependent (as it is a function of the absorption coefficient)”. Is not the absorption coefficient a single scatter property? Emissivity is solely a function of reff and tau unless you mean the “effective emissivity” observed through brightness temperatures.Line 490. One of the big issues in satellite imager cloud phase determination is the discrepancy in cloud phase from IR and VIS/NIR approaches for mid-level clouds. Is this something you see in your analysis?
Line 550: Is this study only to show these dependencies or is targeted for improving or understanding a current or future cloud phase product?
Overall, excellent. It might add to the story to show a compelling SEVIRI scene or two.
Citation: https://doi.org/10.5194/egusphere-2024-540-CC1 -
RC1: 'Comment on egusphere-2024-540', Anonymous Referee #1, 01 Apr 2024
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