the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Nov 2024
Assessment of ACE-MAESTRO v3.13 multi-wavelength stratospheric aerosol extinction measurements
Abstract. The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite provides aerosol extinction measurements in multiple solar wavelength bands. In this study, we evaluate the quality and utility of MAESTRO version 3.13 stratospheric aerosol extinction retrievals, from February 2004–February 2021, through comparison with measurements from other satellite instruments. Despite significant scatter in the MAESTRO data, we find that gridded median MAESTRO aerosol extinctions and stratospheric aerosol optical depth (SAOD) values are generally in good agreement with those from other instruments during volcanically quiescent periods. After volcanic eruptions and wildfire injections, gridded median MAESTRO extinction and SAOD are well-correlated with other measurement sets, but generally biased low by 40–80 %. The Ångström exponent (AE), which can provide information on aerosol particle size, is derived from the MAESTRO spectral extinction measurements in the lowermost stratosphere, showing perturbations after volcanic eruptions qualitatively similar to SAGE III for the eruptions of Ambae (2018) and Uluwan (2019). Differences in AE anomalies after the 2019 extratropical Raikoke eruption may be due to the different spatiotemporal sampling of the two instruments. Furthermore, we introduce a method to adjust MAESTRO extinction data based on comparison with extinction measurements from the Stratospheric Aerosol and Gas Experiment on the International Space Station (SAGE III/ISS) during the period from June 2017 to February 2021, resulting in improved comparison during volcanically active periods. Our work suggests that empirical bias-correction may enhance the utility of MAESTRO aerosol extinction data, which can make it a useful complement to existing satellite records, especially when multi-wavelength solar occultation data from other instruments are unavailable.
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CC1: 'Comment on egusphere-2024-3286', Travis N. Knepp, 02 Dec 2024
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3286/egusphere-2024-3286-CC1-supplement.pdf
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RC1: 'Comment on egusphere-2024-3286', Anonymous Referee #1, 20 Dec 2024
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The manuscript presents stratospheric aerosol observation from the MAESTRO, with comparison to SAGE and OSIRIS. Such comparison for MAESTRO have, to my knowledge, not been published before. The instrument provides data at high latitudes - regions where SAGE III-ISS does not cover. The manuscript is well-written and it is easy to follow the presentation and discussions. Comparing stratospheric aerosol observations among different instruments is important for providing more reliable data to the research community and to compile stratospheric aerosol climatologies for simulations. I would like to see more discussion about reasons behind the discrepancy between the datasets.
Regarding the comparison of AOD and aerosol extinction coefficients, it is not unexpected to see large differences in data from different satellite instruments. Do you have any estimate on the impact of patchiness in aerosol concentrations on the MAESTRO observations? To retrieve aerosol data one must assume homogeneous conditions throughout the entire line-of-sight, as far as I understand it.
The instrument’s line of sight (semi-horizontal) requires a transformation algorithm that computes the conditions at the tangent point. In patchy conditions, the tangent altitude of the retrieved aerosol ext coef. will be lower than the real altitude of aerosol layers. Do you have an estimate on how large this displacement may be? If so, how does this relate to the same phenomenon for the SAGE instruments? It should lead to spread in the data of your comparisons.
Solar occultation instruments cannot quantify aerosol extinction coefficients when the AOD in the line-of-sight is high, leading to a low bias in the data. Is this limit for MAESTRO the same as SAGE’s, and if not, does that have an impact on discrepancies in the AOD comparisons of the two instruments? Do they have similar challenges with missing data, leading to similar underestimates of the AOD when sampling optically dense aerosol? Is this why there is a factor of 2 difference between the instruments after wildfire or volcanic events, and why it has a low bias in general?:
• L196 “…MAESTRO underestimates peak extinction values after major volcanic eruptions and wildfires by a factor of 2 or more…”
• L213: “…MAESTRO extinction at shorter wavelengths has a low bias of 40-80% compared to SAGE III nearly everywhere in the lower stratosphere except right above the tropical tropopause region…”
• L248 “…corresponds to a relative underestimation of 32% by MAESTRO…”• L406: "…The difference in spectral response of MAESTRO measurements, especially following major events may be a limiting factor in accurately characterizing stratospheric aerosol particle size information with MAESTRO….”.
This is important to know for a user, it is important for the reader to understand during which conditions the data may be useful, and it is important to know this when compiling an aerosol climatology like CREST or GloSSAC.How was bias from ice-clouds treated? Data within 2-3 km of the tropopause may be affected by ice clouds to a large degree, suggesting the need to screen for signals from ice-crystals within at least 2-3 km above the tropopause + some kilometer extra (depending on instruments vertical resolution).
Figure 7. Which latitudes and times are shown here? Is it latitude binned monthly mean data? Part of the spread in data may come from periods with patchy conditions. Have you tried excluding the first 2-4 months after volcanic eruptions or wildfires when fitting the data to see if these effects may cause large spread in your’ comparison?
L302: I understand why you do not fit linearly, but why use a power law? Is it the best possible fit to use in this case?
Citation: https://doi.org/10.5194/egusphere-2024-3286-RC1
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