the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 28 May 2025
Altitude-Dependent Formation of Polar Mesospheric Clouds: Charged Nucleation and In Situ Ice Growth on Zonal and Daily Scales
Abstract. Polar mesospheric clouds (PMCs), composed of ice particles, play a crucial role in mesospheric H2O redistribution, yet their microphysical formation mechanism – particularly ice nucleation – remains incompletely understood. Using AIM satellite observations, we reveal a previously unreported hemispheric asymmetry: southern hemisphere (SH) PMCs show a significant latitudinal decrease in column ice particle concentration, while their northern hemisphere (NH) counterparts exhibit zero trend. Our further analysis demonstrates that the column-averaged ice particle concentration (Nc) and radius (rc) are primarily governed by PMC height (h), rather than environment temperature (Tenv). To explain these observations, we propose the charged meteoric smoke particle (MSP) nucleation (CMN) scheme, an altitude-dependent framework based on two key postulates: (1) charged-MSPs serve as ubiquitous ice nuclei throughout the PMC layer, and (2) ice particles grow predominantly in situ with negligible sedimentation. The CMN scheme naturally accounts for the observed vertical gradients in ice particle concentration (increasing with altitude due to charged-MSPs distribution) and size (decreasing with altitude due to H2O competition among ice particles). By eliminating sedimentation, the CMN scheme introduces a novel bottom-up H2O redistribution mechanism we term the cold-trap effect. This mechanism is driven by summer polar upwelling dynamics: upward H2O transport induces hydration, while simultaneous ice particle formation (facilitated by upwelling-induced cooling) blocks further H2O transport, ultimately causing dehydration above PMCs. While the traditional growth-sedimentation (GS) scheme and freeze-drying effect are well-validated, our CMN scheme and cold-trap effect provide an alternative paradigm particularly for understanding zonal and daily-scale PMC variability and associated H2O redistribution processes.
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RC1: 'Comment on egusphere-2025-2330', Anonymous Referee #1, 04 Jul 2025
Dear authors,
thank you very much for presenting your work on "Altitude-Dependent Formation of Polar Mesospheric Clouds: Charged Nucleation and In Situ Ice Growth on Zonal and Daily Scales". The proposed new mechanism of a charged meteoric smoke particle nucleation scheme to explain the characteristics of noctilucent clouds is very interesting and complements standard microphysical models. I only have minor suggestions for the text that hopefully increase the readability for the reader and makes your argument easier to follow.
A lot of acronyms such as GS or PMC are introduced in the abstract only. Please consider to reintroduce them in the body of the text again, i.e. when they are first mentioned somewhere else than the abstract. Moreover, you introduce the CMN scheme in this manuscript, that is in contrast to the conventional GS (growth-sedimentation) scheme. Later in the discussion, you mention the freeze drying effect that, if I understand it correctly, is used nearly synonymous to the GS scheme. Additionally, the cold trap effect is used as a synonym for the GMN scheme. This might be confusing to the reader so please consider to stick to one name per mechanism if it is possible.
Here are some more technical remarks:
l. 9: please provide the full name of AIM
l.73: the "r" is missing in "wate content"
Figure 7 - 9: you are using a significance criteria of +-0.25 for your correlation coefficients. Could you shortly mention why you chose this threshold? Could you mention your significance criteria in the text?
Thank you very much for preparing the manuscript and presenting as well as discussing your findings so clearly.
Citation: https://doi.org/10.5194/egusphere-2025-2330-RC1 -
RC2: 'Comment on egusphere-2025-2330', Anonymous Referee #2, 11 Aug 2025
GENERAL COMMENTS
This paper proposes an approach to characterizing polar mesospheric cloud (PMC) growth and evolution that differs from the conventional approach. The authors suggest that the behavior of key microphysical parameters such as column-averaged ice particle concentration and particle radius are governed by PMC height, rather than by the background temperature. They analyze a portion of the data record from the SOFIE and CIPS instruments, flown on the AIM satellite, to develop the basis for their approach. This approach also relies on the presence of small charged meteoritic smoke particles to initiate nucleation of ice particles.
A significant concern with the approach presented in this paper is the assumption that the latitude of SOFIE occultation measurements remains constant throughout a PMC season. This is not correct, and since various microphysical parameters utilized by the authors do have a latitude dependence, there may be biases or errors in season-long calculated averages of SOFIE data that do not consider this variation.
SPECIFIC COMMENTS
Page 2, line 37: The term “inconclusive” is an overstatement regarding long-term trends. The Kirkwood et al. (2008) study only addresses ground-based noctilucent cloud observations from selected Northern Hemisphere stations. The DeLand and Thomas (2019) study uses satellite data to show statistically significant increasing trends at multiple latitude bands in both NH and SH ice water content during 1978-1997, and significant increasing trends in the NH for the period 1998-2018 as well.
Page 2, lines 57-59: Vellalassery et al. (2023) presents recent 3-D model results that also support the freeze-drying approach.
Page 3, lines 66-68: Note that the AIM satellite re-entered the atmosphere in August 2024.
Page 3, line 71: The SOFIE observation latitude is not constant during the PMC season. Figure 1(b) of Hervig et al. (2009a) shows that for the NH 2007 season, the sampling location varies from ~68°N at DFS (days from solstice) = -10 down to ~66°N at DFS = 0, then up to ~72°N by DFS = +50. This variation means that the latitude dependence in key PMC parameters should not be ignored when seasonal averages are created.
Page 3, line 72: Why are no SOFIE data after 2014 considered? While orbit drift of the AIM satellite does have a more significant impact on SOFIE sampling in later years, extending coverage to 2016-2017 would provide continuity with the choice of CIPS data record coverage.
Page 3, line 79: The PMC height H (calculated by averaging Z_bottom and Z_top) may be approximately equal to the Z_max value reported by SOFIE, but the latter term should be a more accurate representation of the largest portion of cloud particles.
Page 3, lines 80-81: Simple averaging of ice concentration at all altitudes between Z_bot and Z_top is not necessarily appropriate. Figure 3(e) of Hervig et al (2009a) shows that the altitude dependence of the concentration throughout the NH 2007 season is closer to exponential, with values of ~20 cm^-3 near Z_bot, increasing to ~500-1000 cm^-3 near Z_max, with no useful data for 1-2 km below Z_top.
Page 3, lines 81-83: It would be helpful to see plots of the inter-season variation in Z_max or H. The SOFIE sampling latitude drifts Equatorward in both hemispheres by 2014, particularly in the SH (Hervig et al. (2016), Figures 5(b) and 10(a)). This will impact the sampling of latitude-dependent quantities.
Page 4, lines 92-93: It is difficult to believe that the average of 10 season-long zonal averages of PMC properties such as IWC and radius can frequently have a standard deviation that is less than 2% of the original quantity. It would be helpful to show the yearly values of IWC for a few latitude bands for comparison with other published papers that show such time series.
Page 4, lines 101-103: Decreasing PMC altitudes in the SH during the core of the season have also been shown by Bailey et al. (2005) using SNOE data, and by DeLand and Gorkavyi (2020) using OMPS LP data.
Page 4, lines 110-118: Why do you disregard the effect of significant cooling between December and January in the SH as a mechanism for changes? You say that profiles are stable, but show a decrease in concentration and an increase in radius. The latter effect (at the bottom of the profile) is consistent with larger particles sedimenting and sublimating (consistent with H2O changes at 83 km and below).
Page 5, line 129: Why is the duration of the PMC season for the CIPS averages different than the PMC season defined for the SOFIE analysis?
Page 9, lines 146-148: The correlation analysis used here uses “anomaly” data from which a 35-day running mean has been subtracted for each season. This step incorrectly removes true variations in Z_max during a season (see Hervig et al. (2009a), Bailey et al. (2005)). NOTE: This information is only presented in the caption for Figures 5-6. This is an important feature of the data analysis that should be stated (and justified) in the text as well.
Page 9, lines 148-149: Positive correlations < 0.3 do not seem to be very strong.
Page 9, lines 156-161: Extending the region for averaging T_env down to 78 km includes a significant altitude region that does not impact PMC microphysical properties, because PMCs are not observed at such warm temperatures (see average Z_bot in Figures 3-4). Why not limit the lowest altitude of the T_env calculation to 81 or 82 km to be more representative of only the PMC region?
Page 9, lines 161-163: It is easy to understand that reducing T_env will lower Z_bot, since that level is defined by the existence of PMCs. It is not as obvious that Z_top will be raised by a corresponding amount to maintain a constant value of H. You have already shown in Figure 2 that H has a clear decrease during the PMC season in the SH, and temperatures at PMC formation altitudes are also decreasing (Figure 3). It seems simpler to assume that the lower temperature enables PMC formation at lower altitudes.
Page 9, line 165: Larger particles and higher concentration (more nucleation) do not necessarily occur simultaneously in the same altitude region.
Page 16, lines 211-213: It is not obvious how there can be “negligible sedimentation” without requiring increasing vertical winds to maintain a constant particle altitude. As the ice particles grow, gravitational effects become more important. If the PMC reaches local equilibrium, what mechanism then dissipates a stable PMC?
Page 17, lines 247-248: SH PMC altitudes do not show a latitude dependence in multiple satellite data sets: Bailey et al. (2005) using SNOE data (Figure 7), DeLand and Gorkavyi (2020) using OMPS LP data (Figure 11).
Page 17, lines 255-259: The CMN scheme does not seem to be required to explain this case. The large influx of H2O from altitudes above the PMC formation zone enables initial formation and rapid growth at these higher levels. Are charged MSPs really necessary as well?
ADDITIONAL REFERENCES
Bailey, S. M., Merkel, A. W., Thomas, G. E., and Carstens, J. N. (2005). Observations of polar mesospheric clouds by the Student Nitric Oxide Explorer. J. Geophys. Res., 110, D13203, https://doi.org/10.1029/2004JD005422
DeLand, M. T., and Gorkavyi, N. (2020). PMC observations from the OMPS Limb Profiler. J. Atmos. Solar-Terr. Phys., 213, 105505, https://doi.org/10.1016/j.jastp.2020.105505
Hervig, M. E., Gerding, M., Stevens, M. H., Stockwell, R., Bailey, S. M., Russell III, J. M., and Stober, G. (2016). Mid-latitude mesospheric clouds and their environment from SOFIE observations. J. Atmos. Solar-Terr. Phys., 149, 1-14, https://doi.org/10.1016/j.jastp.2016.09.004
Vellalassery, A., Baumgarten, G., Grygalashvyly, M., and Lubken, F.-J. (2023). Greenhouse gas effects on the solar cycle response of water vapor and noctilucent clouds. Ann. Geophys., 41, 289-300, https://doi.org/10.5194/angeo-41-289-2023
TYPOGRAPHICAL ERRORS
Page 3, line 73: “wate” should be “water”.
Page 6, line 133: “could” should be “cloud”.
Page 7, Figure 3: “Temperautre” should be “Temperature”.
Page 16, line 203: “statistic” should be “statistical”.
Page 18, line 262: “grower lager” should be “grow larger”.
Citation: https://doi.org/10.5194/egusphere-2025-2330-RC2 - AC1: 'Comment on egusphere-2025-2330', liang zhang, 26 Aug 2025
Status: closed
-
RC1: 'Comment on egusphere-2025-2330', Anonymous Referee #1, 04 Jul 2025
Dear authors,
thank you very much for presenting your work on "Altitude-Dependent Formation of Polar Mesospheric Clouds: Charged Nucleation and In Situ Ice Growth on Zonal and Daily Scales". The proposed new mechanism of a charged meteoric smoke particle nucleation scheme to explain the characteristics of noctilucent clouds is very interesting and complements standard microphysical models. I only have minor suggestions for the text that hopefully increase the readability for the reader and makes your argument easier to follow.
A lot of acronyms such as GS or PMC are introduced in the abstract only. Please consider to reintroduce them in the body of the text again, i.e. when they are first mentioned somewhere else than the abstract. Moreover, you introduce the CMN scheme in this manuscript, that is in contrast to the conventional GS (growth-sedimentation) scheme. Later in the discussion, you mention the freeze drying effect that, if I understand it correctly, is used nearly synonymous to the GS scheme. Additionally, the cold trap effect is used as a synonym for the GMN scheme. This might be confusing to the reader so please consider to stick to one name per mechanism if it is possible.
Here are some more technical remarks:
l. 9: please provide the full name of AIM
l.73: the "r" is missing in "wate content"
Figure 7 - 9: you are using a significance criteria of +-0.25 for your correlation coefficients. Could you shortly mention why you chose this threshold? Could you mention your significance criteria in the text?
Thank you very much for preparing the manuscript and presenting as well as discussing your findings so clearly.
Citation: https://doi.org/10.5194/egusphere-2025-2330-RC1 -
RC2: 'Comment on egusphere-2025-2330', Anonymous Referee #2, 11 Aug 2025
GENERAL COMMENTS
This paper proposes an approach to characterizing polar mesospheric cloud (PMC) growth and evolution that differs from the conventional approach. The authors suggest that the behavior of key microphysical parameters such as column-averaged ice particle concentration and particle radius are governed by PMC height, rather than by the background temperature. They analyze a portion of the data record from the SOFIE and CIPS instruments, flown on the AIM satellite, to develop the basis for their approach. This approach also relies on the presence of small charged meteoritic smoke particles to initiate nucleation of ice particles.
A significant concern with the approach presented in this paper is the assumption that the latitude of SOFIE occultation measurements remains constant throughout a PMC season. This is not correct, and since various microphysical parameters utilized by the authors do have a latitude dependence, there may be biases or errors in season-long calculated averages of SOFIE data that do not consider this variation.
SPECIFIC COMMENTS
Page 2, line 37: The term “inconclusive” is an overstatement regarding long-term trends. The Kirkwood et al. (2008) study only addresses ground-based noctilucent cloud observations from selected Northern Hemisphere stations. The DeLand and Thomas (2019) study uses satellite data to show statistically significant increasing trends at multiple latitude bands in both NH and SH ice water content during 1978-1997, and significant increasing trends in the NH for the period 1998-2018 as well.
Page 2, lines 57-59: Vellalassery et al. (2023) presents recent 3-D model results that also support the freeze-drying approach.
Page 3, lines 66-68: Note that the AIM satellite re-entered the atmosphere in August 2024.
Page 3, line 71: The SOFIE observation latitude is not constant during the PMC season. Figure 1(b) of Hervig et al. (2009a) shows that for the NH 2007 season, the sampling location varies from ~68°N at DFS (days from solstice) = -10 down to ~66°N at DFS = 0, then up to ~72°N by DFS = +50. This variation means that the latitude dependence in key PMC parameters should not be ignored when seasonal averages are created.
Page 3, line 72: Why are no SOFIE data after 2014 considered? While orbit drift of the AIM satellite does have a more significant impact on SOFIE sampling in later years, extending coverage to 2016-2017 would provide continuity with the choice of CIPS data record coverage.
Page 3, line 79: The PMC height H (calculated by averaging Z_bottom and Z_top) may be approximately equal to the Z_max value reported by SOFIE, but the latter term should be a more accurate representation of the largest portion of cloud particles.
Page 3, lines 80-81: Simple averaging of ice concentration at all altitudes between Z_bot and Z_top is not necessarily appropriate. Figure 3(e) of Hervig et al (2009a) shows that the altitude dependence of the concentration throughout the NH 2007 season is closer to exponential, with values of ~20 cm^-3 near Z_bot, increasing to ~500-1000 cm^-3 near Z_max, with no useful data for 1-2 km below Z_top.
Page 3, lines 81-83: It would be helpful to see plots of the inter-season variation in Z_max or H. The SOFIE sampling latitude drifts Equatorward in both hemispheres by 2014, particularly in the SH (Hervig et al. (2016), Figures 5(b) and 10(a)). This will impact the sampling of latitude-dependent quantities.
Page 4, lines 92-93: It is difficult to believe that the average of 10 season-long zonal averages of PMC properties such as IWC and radius can frequently have a standard deviation that is less than 2% of the original quantity. It would be helpful to show the yearly values of IWC for a few latitude bands for comparison with other published papers that show such time series.
Page 4, lines 101-103: Decreasing PMC altitudes in the SH during the core of the season have also been shown by Bailey et al. (2005) using SNOE data, and by DeLand and Gorkavyi (2020) using OMPS LP data.
Page 4, lines 110-118: Why do you disregard the effect of significant cooling between December and January in the SH as a mechanism for changes? You say that profiles are stable, but show a decrease in concentration and an increase in radius. The latter effect (at the bottom of the profile) is consistent with larger particles sedimenting and sublimating (consistent with H2O changes at 83 km and below).
Page 5, line 129: Why is the duration of the PMC season for the CIPS averages different than the PMC season defined for the SOFIE analysis?
Page 9, lines 146-148: The correlation analysis used here uses “anomaly” data from which a 35-day running mean has been subtracted for each season. This step incorrectly removes true variations in Z_max during a season (see Hervig et al. (2009a), Bailey et al. (2005)). NOTE: This information is only presented in the caption for Figures 5-6. This is an important feature of the data analysis that should be stated (and justified) in the text as well.
Page 9, lines 148-149: Positive correlations < 0.3 do not seem to be very strong.
Page 9, lines 156-161: Extending the region for averaging T_env down to 78 km includes a significant altitude region that does not impact PMC microphysical properties, because PMCs are not observed at such warm temperatures (see average Z_bot in Figures 3-4). Why not limit the lowest altitude of the T_env calculation to 81 or 82 km to be more representative of only the PMC region?
Page 9, lines 161-163: It is easy to understand that reducing T_env will lower Z_bot, since that level is defined by the existence of PMCs. It is not as obvious that Z_top will be raised by a corresponding amount to maintain a constant value of H. You have already shown in Figure 2 that H has a clear decrease during the PMC season in the SH, and temperatures at PMC formation altitudes are also decreasing (Figure 3). It seems simpler to assume that the lower temperature enables PMC formation at lower altitudes.
Page 9, line 165: Larger particles and higher concentration (more nucleation) do not necessarily occur simultaneously in the same altitude region.
Page 16, lines 211-213: It is not obvious how there can be “negligible sedimentation” without requiring increasing vertical winds to maintain a constant particle altitude. As the ice particles grow, gravitational effects become more important. If the PMC reaches local equilibrium, what mechanism then dissipates a stable PMC?
Page 17, lines 247-248: SH PMC altitudes do not show a latitude dependence in multiple satellite data sets: Bailey et al. (2005) using SNOE data (Figure 7), DeLand and Gorkavyi (2020) using OMPS LP data (Figure 11).
Page 17, lines 255-259: The CMN scheme does not seem to be required to explain this case. The large influx of H2O from altitudes above the PMC formation zone enables initial formation and rapid growth at these higher levels. Are charged MSPs really necessary as well?
ADDITIONAL REFERENCES
Bailey, S. M., Merkel, A. W., Thomas, G. E., and Carstens, J. N. (2005). Observations of polar mesospheric clouds by the Student Nitric Oxide Explorer. J. Geophys. Res., 110, D13203, https://doi.org/10.1029/2004JD005422
DeLand, M. T., and Gorkavyi, N. (2020). PMC observations from the OMPS Limb Profiler. J. Atmos. Solar-Terr. Phys., 213, 105505, https://doi.org/10.1016/j.jastp.2020.105505
Hervig, M. E., Gerding, M., Stevens, M. H., Stockwell, R., Bailey, S. M., Russell III, J. M., and Stober, G. (2016). Mid-latitude mesospheric clouds and their environment from SOFIE observations. J. Atmos. Solar-Terr. Phys., 149, 1-14, https://doi.org/10.1016/j.jastp.2016.09.004
Vellalassery, A., Baumgarten, G., Grygalashvyly, M., and Lubken, F.-J. (2023). Greenhouse gas effects on the solar cycle response of water vapor and noctilucent clouds. Ann. Geophys., 41, 289-300, https://doi.org/10.5194/angeo-41-289-2023
TYPOGRAPHICAL ERRORS
Page 3, line 73: “wate” should be “water”.
Page 6, line 133: “could” should be “cloud”.
Page 7, Figure 3: “Temperautre” should be “Temperature”.
Page 16, line 203: “statistic” should be “statistical”.
Page 18, line 262: “grower lager” should be “grow larger”.
Citation: https://doi.org/10.5194/egusphere-2025-2330-RC2 - AC1: 'Comment on egusphere-2025-2330', liang zhang, 26 Aug 2025
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