the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 May 2023
Towards a more reliable forecast of ice supersaturation: Concept of a one-moment ice cloud scheme that avoids saturation adjustment
Dario Sperber
Abstract. A significant share of aviation's climate impact is due to persistent contrails. Avoiding the creation of contrails that exert a warming impact is thus a crucial step in approaching the goal of sustainable air transportation. For this purpose, a reliable forecast of when and where persistent contrails are expected to form is needed, that is, a reliable prediction of ice supersaturation. With such a forecast at hand it would be possible to plan aircraft routes on which the formation of persistent contrails can be avoided. One problem on the way to these forecasts is the current systematic underestimation of the frequency and degree of ice supersaturation on cruise altitudes in numerical weather prediction due to the practice of "saturation adjustment". In this common parameterisation, the air inside cirrus clouds is assumed to be exactly at ice saturation, while measurement studies have found cirrus clouds to be quite often in an ice supersaturated state.
In this study, we propose a new ice cloud scheme that overcomes saturation adjustment by explicitly modelling the decay of the in-cloud humidity after nucleation, thereby allowing for both in-cloud super- and subsaturation. To achieve this, we introduce the in-cloud humidity as a new prognostic variable and derive the humidity distribution in newly generated cloud parts from a stochastic box model that divides a model grid box into a large number of air parcels and treats them individually.
The new scheme is then tested against a parameterisation that uses saturation adjustment, where the stochastic box model serves as a benchmark. It is shown that saturation adjustment underestimates humidity both shortly after nucleation, when the actual cloud is still highly supersaturated, and also in aged cirrus if temperature keeps decreasing, as the actual cloud remains in a slightly supersaturated state in this case. The new parameterisation on the other hand closely follows the behaviour of the stochastic box model in any considered case. We conclude that our parameterisation is promising but needs further testing in more realistic frameworks.
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Dario Sperber and Klaus Gierens
Status: closed
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RC1: 'Comment on egusphere-2023-914', Blaž Gasparini, 26 Jun 2023
The manuscript by Sperber and Gierens, 2023 introduces a new cirrus parameterization that avoids saturation adjustment and can be implemented in models using one-moment microphysical schemes. The scheme adds an additional prognostic variable describing the in-cloud humidity to the reference model using the saturation adjustment assumption. The authors describe and compare the results of their scheme with the benchmark stochastic model and the reference saturation-adjusted cloud model, demonstrating the advantage of their parameterization.
This is a valuable study that could potentially in the long-term lead to improvements in coarse-resolution models using simple, 1-moment microphysical schemes and the saturation adjustment assumption. Such simplified representations of cirrus are common in current weather prediction models. Overall, it is a very nice parameterization development study that has some potential for improving ice supersaturation forecasts. However, I have a number of questions that should be addressed before the study can be published in its final form.
General comments:
- The conditions where the new parameterization improves the modeled moisture fields the most (slow, constant updraft) are most likely to be associated with heterogeneous freezing at lower supersaturations. If this is the case, and nucleation occurs at lower supersaturations, the advantage of your scheme would probably be smaller compared to the cases presented. Also, even if your scheme removes one of the biases, bias due to the lack of heterogeneous ice nucleation may persist.
I am wondering which of the two biases could have larger implications on the simulated supersaturation fields, particularly in the context of a weather model. Please, discuss!
Also, I think it might be worthwhile to add an additional sensitivity test assuming an idealized type of heterogeneous ice nucleation. This test might give some more clues about the importance of the new parameterization.
Indeed, I know that weather models typically don't have either interactive or prescribed aerosol fields that could modify cirrus formation. However, prescribing an aerosol or ice nucleating particle climatology would be a fairly straightforward task.
- How realistic is the assumption of a constant updraft for periods longer than 12 hours? Does this really occur in the real atmosphere? How well are such processes simulated by weather models?
- A number of studies point to the importance of rapid temperature/updraft variations in the formation of cirrus, particularly in the context of homogeneous freezing. The manuscript mentions the inclusion of such variability in the scheme. However, I didn't understand how such fluctuations are included in the new scheme. Could you explain this better? What if the variability were larger/smaller?
- I believe the authors hate to provide a data availability statement, as per https://www.atmospheric-chemistry-and-physics.net/policies/data_policy.html
Specific comments:
Page 1, abstract: I’m missing a sentence on the conditions in which the new scheme is improving humidity most compared to saturation adjustment
Page 2, first paragraph: I like the sharp focus on contrails. However, from the climatic point of view, the scheme would have likely a much larger impact on natural cirrus coverage and their radiative effects.
Page 5, equation 10: Why is the assumption of a hyperbolic supersaturation? Where does this come from?
Page 6: Why can’t growing ice crystals decrease the relative humidity to 100%?
Section 2.2: please note that I did not go through the equations, but I just tried to qualitatively understand the concept of the new scheme. An illustration/schematic could help to effectively convey the key concept of the new scheme.
Section 3: Visualizing the updrafts considered in the considered cases would improve the clarity of the results, particularly for the cosine-like cases.
Figure 4 and 8: based on the caption I expect an increase in updraft from 2 to 5 cm/s. Please, just plot the updraft.
Page 18, lines 364-370: Would be great to see that in a separate sensitivity test and a new figure.
Page 19, lines 404-405: language, please rewrite the sentence. Saturation adjustment => parameterization using saturation adjustment (or similar)
Page 19, line 408: “keeps getting restored” => is that true? I guess it’s simply continuous?
Page 19, line 426-427: suggestion “…a significant improvement compared with a saturation adjustment scheme…”
Citation: https://doi.org/10.5194/egusphere-2023-914-RC1 -
AC3: 'Reply on RC1', Klaus Gierens, 10 Oct 2023
The replies to the comments are given in the final author comment.
Citation: https://doi.org/10.5194/egusphere-2023-914-AC3
- The conditions where the new parameterization improves the modeled moisture fields the most (slow, constant updraft) are most likely to be associated with heterogeneous freezing at lower supersaturations. If this is the case, and nucleation occurs at lower supersaturations, the advantage of your scheme would probably be smaller compared to the cases presented. Also, even if your scheme removes one of the biases, bias due to the lack of heterogeneous ice nucleation may persist.
-
RC2: 'Comment on egusphere-2023-914', Anonymous Referee #2, 02 Aug 2023
-
AC2: 'Reply on RC2', Klaus Gierens, 10 Oct 2023
The replies to the comments are given in the final author comment.
Citation: https://doi.org/10.5194/egusphere-2023-914-AC2
-
AC2: 'Reply on RC2', Klaus Gierens, 10 Oct 2023
- AC1: 'Comment on egusphere-2023-914', Klaus Gierens, 10 Oct 2023
Status: closed
-
RC1: 'Comment on egusphere-2023-914', Blaž Gasparini, 26 Jun 2023
The manuscript by Sperber and Gierens, 2023 introduces a new cirrus parameterization that avoids saturation adjustment and can be implemented in models using one-moment microphysical schemes. The scheme adds an additional prognostic variable describing the in-cloud humidity to the reference model using the saturation adjustment assumption. The authors describe and compare the results of their scheme with the benchmark stochastic model and the reference saturation-adjusted cloud model, demonstrating the advantage of their parameterization.
This is a valuable study that could potentially in the long-term lead to improvements in coarse-resolution models using simple, 1-moment microphysical schemes and the saturation adjustment assumption. Such simplified representations of cirrus are common in current weather prediction models. Overall, it is a very nice parameterization development study that has some potential for improving ice supersaturation forecasts. However, I have a number of questions that should be addressed before the study can be published in its final form.
General comments:
- The conditions where the new parameterization improves the modeled moisture fields the most (slow, constant updraft) are most likely to be associated with heterogeneous freezing at lower supersaturations. If this is the case, and nucleation occurs at lower supersaturations, the advantage of your scheme would probably be smaller compared to the cases presented. Also, even if your scheme removes one of the biases, bias due to the lack of heterogeneous ice nucleation may persist.
I am wondering which of the two biases could have larger implications on the simulated supersaturation fields, particularly in the context of a weather model. Please, discuss!
Also, I think it might be worthwhile to add an additional sensitivity test assuming an idealized type of heterogeneous ice nucleation. This test might give some more clues about the importance of the new parameterization.
Indeed, I know that weather models typically don't have either interactive or prescribed aerosol fields that could modify cirrus formation. However, prescribing an aerosol or ice nucleating particle climatology would be a fairly straightforward task.
- How realistic is the assumption of a constant updraft for periods longer than 12 hours? Does this really occur in the real atmosphere? How well are such processes simulated by weather models?
- A number of studies point to the importance of rapid temperature/updraft variations in the formation of cirrus, particularly in the context of homogeneous freezing. The manuscript mentions the inclusion of such variability in the scheme. However, I didn't understand how such fluctuations are included in the new scheme. Could you explain this better? What if the variability were larger/smaller?
- I believe the authors hate to provide a data availability statement, as per https://www.atmospheric-chemistry-and-physics.net/policies/data_policy.html
Specific comments:
Page 1, abstract: I’m missing a sentence on the conditions in which the new scheme is improving humidity most compared to saturation adjustment
Page 2, first paragraph: I like the sharp focus on contrails. However, from the climatic point of view, the scheme would have likely a much larger impact on natural cirrus coverage and their radiative effects.
Page 5, equation 10: Why is the assumption of a hyperbolic supersaturation? Where does this come from?
Page 6: Why can’t growing ice crystals decrease the relative humidity to 100%?
Section 2.2: please note that I did not go through the equations, but I just tried to qualitatively understand the concept of the new scheme. An illustration/schematic could help to effectively convey the key concept of the new scheme.
Section 3: Visualizing the updrafts considered in the considered cases would improve the clarity of the results, particularly for the cosine-like cases.
Figure 4 and 8: based on the caption I expect an increase in updraft from 2 to 5 cm/s. Please, just plot the updraft.
Page 18, lines 364-370: Would be great to see that in a separate sensitivity test and a new figure.
Page 19, lines 404-405: language, please rewrite the sentence. Saturation adjustment => parameterization using saturation adjustment (or similar)
Page 19, line 408: “keeps getting restored” => is that true? I guess it’s simply continuous?
Page 19, line 426-427: suggestion “…a significant improvement compared with a saturation adjustment scheme…”
Citation: https://doi.org/10.5194/egusphere-2023-914-RC1 -
AC3: 'Reply on RC1', Klaus Gierens, 10 Oct 2023
The replies to the comments are given in the final author comment.
Citation: https://doi.org/10.5194/egusphere-2023-914-AC3
- The conditions where the new parameterization improves the modeled moisture fields the most (slow, constant updraft) are most likely to be associated with heterogeneous freezing at lower supersaturations. If this is the case, and nucleation occurs at lower supersaturations, the advantage of your scheme would probably be smaller compared to the cases presented. Also, even if your scheme removes one of the biases, bias due to the lack of heterogeneous ice nucleation may persist.
-
RC2: 'Comment on egusphere-2023-914', Anonymous Referee #2, 02 Aug 2023
-
AC2: 'Reply on RC2', Klaus Gierens, 10 Oct 2023
The replies to the comments are given in the final author comment.
Citation: https://doi.org/10.5194/egusphere-2023-914-AC2
-
AC2: 'Reply on RC2', Klaus Gierens, 10 Oct 2023
- AC1: 'Comment on egusphere-2023-914', Klaus Gierens, 10 Oct 2023
Dario Sperber and Klaus Gierens
Dario Sperber and Klaus Gierens
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