the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jun 2024
Water Vapor Content Retrieval Under Cloudy Sky Conditions From SWIR Satellite Measurements in the Context of C3IEL Space Mission Project
Abstract. A retrieval algorithm of integrated water vapor content above cloud, using shortwave infrared observations, is developed and evaluated through idealized and realistic atmospheric profiles. Water vapor plays a crucial role in cloud formation and development, particularly those resulting from convective processes. They influence locally the spatiotemporal variability of atmospheric water vapor content, through exchanges between clouds and their immediate environment. Therefore, a better understanding of water vapor content above and around cloud is necessary to improve our comprehension of interactions between water vapor and cloud to better constrain Large-Eddy Simulation and numerical weather forecasting models. The developed algorithm is part of the Cluster for Cloud evolution, ClImatE and Lightning, C3IEL space mission project. This mission, scheduled for 2027 aims to enhance our knowledge of the 3D convective cloud development velocities, the electrical activity associated with convective systems and the water vapor content above and around cloud. The retrieval algorithm presented in this study was achieved through a Bayesian probabilistic approach, the optimal estimation method. The atmosphere was assumed to be composed of homogeneous plane-parallel layers, and synthetic radiance datasets were generated for testing the developed retrieval algorithm. The feasibility of retrieving integrated water vapor content above cloud and over the ocean from SWIR radiances was shown to have a precision of approximately 1 kg.m-2 for optically thick clouds under idealized cloudy sky conditions. Tests using realistic water vapor and cloud extinction profiles that present non-homogeneous vertical distributions show that integrated water vapor content above low/mid-level clouds could be retrieved with a positive bias related to cloud vertical penetration of approximately 2.6 kg.m-2. The algorithm fails for very low water vapor content encountered in the presence of high deep convective clouds.
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Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2024-1560', Anonymous Referee #1, 22 Jul 2024
This paper describes and assesses a preliminary version of an algorithm to infer water vapor content above cloud top from shortwave infrared observations included in the C3IEL Mission. The paper is generally well written. The topic is of interest for AMT. As discussed in my main comments some crucial information is missing from the paper in my opinion and some more analysis and discussion is needed to complete the paper and make it acceptable for publication.
Main comments:
- I would expect that the method is quite sensitive to the assumed cloud base height. In my view the assumed uncertainty of 100 m in CBH is much too small. Boundary layer heights vary more than that. Furthermore, the targeted cumulonimbus and congestus clouds are often anvil-shaped and/or slanted, so much of the cloud (part) seen at the top at nadir have an (effective) base that is much higher than 0.2 km. Furthermore, I expect that the sensitivity to the assumed cloud base height is far from linear (as the current error estimate approach assumes). I strongly suggest performing simulations with other cloud base heights to assess this influence.
- At the scales of the observations (~125 m), 3D radiative transfer effects are expected to be non-negligible. I realize that their influence on the water vapor product is hard to assess and I am not asking for 3D simulations, but a discussion about the possible influence of 3D radiative transfer effects will be suiting.
- The test in section 5 is rather comprehensive, but some crucial information is missing: How are the cloud top height and cloud top phase defined and determined from the IFS profiles, as this is input for the retrieval? What water vapor profile first guess is used here in the retrieval?
- Not all retrievals are converged, especially for high clouds, but it is not defined when a retrieval is considered to be converged. At least for the case with one band in the water vapor band, I expect you can always find a minimum to match the observations, so I am confused that non-convergence can occur at all. Please discuss the convergence definition and why non-convergence occurs and for which cases.
- I am a bit confused about the mission operations described in section 2 and how it relates to the choices made in this study. The mission will look at several viewing angles at the same cloud (parts). Will all of these observations be taken into account for the water vapor retrievals or just those near Nadir? The simulations seem to be just for nadir observations. If other viewing angles will be considered in the operational retrievals, I would suggest to also assess those. At the C3IEL spatial resolutions, many of the observations at off-nadir views will look at the side of the cloud rather than the top. Are those observations also used? Also, what is the region (swath?) in which the observations are made?
General comment about the text:
The text is changing from past tense to present tense quite a lot. In general, scientific articles refrain from using past tense, so I suggest just using present tense throughout (also in abstract and conclusions).
Specific comments:
Abstract:, Line 3: What is meant with “they”? I suggest to rewrite the sentence.
Intro, Line 22-26: Liquid clouds also absorb infrared radiation, and ice clouds also reflect at all solar wavelengths. This explanation of the balance of interactions between clouds and solar and thermal radiation needs to be more accurate.
Line 69: The acronyms “C3IEL” and “IWV AC” are not defined here yet.
Section 3, Line 116: When setting the a priori errors to such extreme values, doesn’t the optimal estimation scheme essentially act as a common linear least squares fitting approach?
Section 4, Figure 3: What are the assumed solar and viewing geometries here?
Section 5:
Line 185: What is the assumed cloud phase in these simulations?
Line 231: How are low/mid-level (and high) clouds defined?
Small textual edits:
line 5: “clouds” instead of “cloud”Line 6: “simulations” instead of “simulation”
Line 36: “dynamical” instead of “dynamic”
Line 37: “In turn” instead of “in return”
Line 56: I suggest to correct this sentence as: “Moreover, as clouds do not act as a perfect reflector, radiation penetrates the cloud and gets scattered, effectively extending the radiation path through the atmosphere and consequently increasing absorption by water vapor.”
Citation: https://doi.org/10.5194/egusphere-2024-1560-RC1 -
CC1: 'Revision of the manuscript', Raphaël Peroni, 24 Jul 2024
Since the submission of the article, improvements have been made to our study, leading to notably enhanced results. These improvements require modifications to figures and comments within the manuscript. Consequently, we prefer to stop the review process here to revise properly the manuscript accordingly to the new results and resubmit it later in order to ensure that our work is presented in its best form.
We are very grateful to the reviewer 1 for the pertinent comments and will include some discussions regarding the points mentioned, in the future version of the paper.
Citation: https://doi.org/10.5194/egusphere-2024-1560-CC1
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-1560', Anonymous Referee #1, 22 Jul 2024
This paper describes and assesses a preliminary version of an algorithm to infer water vapor content above cloud top from shortwave infrared observations included in the C3IEL Mission. The paper is generally well written. The topic is of interest for AMT. As discussed in my main comments some crucial information is missing from the paper in my opinion and some more analysis and discussion is needed to complete the paper and make it acceptable for publication.
Main comments:
- I would expect that the method is quite sensitive to the assumed cloud base height. In my view the assumed uncertainty of 100 m in CBH is much too small. Boundary layer heights vary more than that. Furthermore, the targeted cumulonimbus and congestus clouds are often anvil-shaped and/or slanted, so much of the cloud (part) seen at the top at nadir have an (effective) base that is much higher than 0.2 km. Furthermore, I expect that the sensitivity to the assumed cloud base height is far from linear (as the current error estimate approach assumes). I strongly suggest performing simulations with other cloud base heights to assess this influence.
- At the scales of the observations (~125 m), 3D radiative transfer effects are expected to be non-negligible. I realize that their influence on the water vapor product is hard to assess and I am not asking for 3D simulations, but a discussion about the possible influence of 3D radiative transfer effects will be suiting.
- The test in section 5 is rather comprehensive, but some crucial information is missing: How are the cloud top height and cloud top phase defined and determined from the IFS profiles, as this is input for the retrieval? What water vapor profile first guess is used here in the retrieval?
- Not all retrievals are converged, especially for high clouds, but it is not defined when a retrieval is considered to be converged. At least for the case with one band in the water vapor band, I expect you can always find a minimum to match the observations, so I am confused that non-convergence can occur at all. Please discuss the convergence definition and why non-convergence occurs and for which cases.
- I am a bit confused about the mission operations described in section 2 and how it relates to the choices made in this study. The mission will look at several viewing angles at the same cloud (parts). Will all of these observations be taken into account for the water vapor retrievals or just those near Nadir? The simulations seem to be just for nadir observations. If other viewing angles will be considered in the operational retrievals, I would suggest to also assess those. At the C3IEL spatial resolutions, many of the observations at off-nadir views will look at the side of the cloud rather than the top. Are those observations also used? Also, what is the region (swath?) in which the observations are made?
General comment about the text:
The text is changing from past tense to present tense quite a lot. In general, scientific articles refrain from using past tense, so I suggest just using present tense throughout (also in abstract and conclusions).
Specific comments:
Abstract:, Line 3: What is meant with “they”? I suggest to rewrite the sentence.
Intro, Line 22-26: Liquid clouds also absorb infrared radiation, and ice clouds also reflect at all solar wavelengths. This explanation of the balance of interactions between clouds and solar and thermal radiation needs to be more accurate.
Line 69: The acronyms “C3IEL” and “IWV AC” are not defined here yet.
Section 3, Line 116: When setting the a priori errors to such extreme values, doesn’t the optimal estimation scheme essentially act as a common linear least squares fitting approach?
Section 4, Figure 3: What are the assumed solar and viewing geometries here?
Section 5:
Line 185: What is the assumed cloud phase in these simulations?
Line 231: How are low/mid-level (and high) clouds defined?
Small textual edits:
line 5: “clouds” instead of “cloud”Line 6: “simulations” instead of “simulation”
Line 36: “dynamical” instead of “dynamic”
Line 37: “In turn” instead of “in return”
Line 56: I suggest to correct this sentence as: “Moreover, as clouds do not act as a perfect reflector, radiation penetrates the cloud and gets scattered, effectively extending the radiation path through the atmosphere and consequently increasing absorption by water vapor.”
Citation: https://doi.org/10.5194/egusphere-2024-1560-RC1 -
CC1: 'Revision of the manuscript', Raphaël Peroni, 24 Jul 2024
Since the submission of the article, improvements have been made to our study, leading to notably enhanced results. These improvements require modifications to figures and comments within the manuscript. Consequently, we prefer to stop the review process here to revise properly the manuscript accordingly to the new results and resubmit it later in order to ensure that our work is presented in its best form.
We are very grateful to the reviewer 1 for the pertinent comments and will include some discussions regarding the points mentioned, in the future version of the paper.
Citation: https://doi.org/10.5194/egusphere-2024-1560-CC1
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