General comments: 

This paper calculates the radiative forcing of aerosol and water vapor volcanic cloud generated by the Hunga volcano eruption in January 2022. The calculations were performed for the specific location of Reunion Island. The perturbations of stratospheric aerosol were calculated, neglecting the effect of background aerosols, which can cause a 20-25% error; for water vapor, it was assumed that the unperturbed value is 4.5 ppmV for all altitudes, which was not precisely correct. The authors used a Line-by-Line radiative transfer model with the highest resolution of 20 cm-1 for radiative transfer calculations. That might be the course for resolving the effects of stratospheric water vapor, but it worked well. The authors extrapolated the imaginary part of the sulfate aerosol refractive index from near IR to visible and UV. As a result, they overestimated aerosol short wave (SW) absorption. This is especially well seen in stratospheric radiative heating, as the paper reports warming of the stratosphere during, e.g., the first four months after the eruption, while observations show significant cooling. The radiative forcing at the top of the atmosphere is reasonably correct, but SW aerosol radiative forcing at the bottom of the atmosphere (BOA) is exaggerated. These drawbacks have to be rectified before the paper can be published.

Specific comments:

L38: The mass of water retained in the stratosphere was unprecedented, not the amount of emitted water.

L42: Do you mean at the location of Reunion Island or globally? I do not think it is right globally.

L77: In this context, the reference should be "Jenkins et al. (2023)." Please correct the text in many other similar cases.

L108: Legrand et al. (2022) reported that the aerosol spatial distribution was patchy due to dynamic instabilities for more than six months.

L190: Extrapolating the imaginary refractive index could cause spurious absorption in the UV and visible wave bands. It is well known that sulfate aerosols do not absorb in those wave bands.

Figure 1: Please show your aerosol LW SAOD for 10 um.

L236: You should use the word dispersion instead of dilution. SAOD is also defined by the rate of SO2 to SO4 conversion. OMPS-LP misses the initial stage of the SAOD generation, so it is not surprising that you see a discrepancy with OMPS observations at the initial stage.

L257: "zonal scale" - please clarify the sentence.

L263: Please be more specific.

L307: "probably correlated" > "caused"

L309: These results from (Zhu et al., 2022) cannot be used for comparison with your calculations, as without water vapor, volcanic clouds have different evolution and cannot be correctly interpreted.

Figure 7 shows Hunga's aerosol heating rate reaching 0.8 k/day in the first four months after the eruption, while after Pinatubo eruption the aerosol stratospheric heating rates were below 0.3 K/day. This cannot be right.

L433-436: This conclusion about stratospheric warming contradicts observations that reported significant stratospheric cooling.

This paper focusses on the characterization of stratospheric aerosols and water vapor over Reunion Islands in the southern tropical Indian Ocean. The manuscript associated variations in these two atmospheric parameters with the eruption of the Hunga Tonga-Hunga Ha’ apai volcano on January 2022. The methodology used are based on backscattering lidar measurements emitting at 355 nm and on the Ozone Mapper and Profiler Suite Limb satellite. The Microwave Limb Sounder is also used for monthly mean water vapor. In general, the use of these instruments can serve sattisfactory for the purpuses of the study. Authors also claim the use MERRA-2 but it is not clear in the manuscript why they use this data. In generall, the authors presents very interesting measurements that are suitable for publications in Atmospheric Chemistry and Physics due to the possible impacts on the climate.

The manuscript also uses the measurements over Reunion Island in the GAME radiative transfer model for computing direct radiative transfer. I agree with the comments made by the previous referee. For aerosols, the input parameters have large uncertainties. For example, with the limited number of lidar measurements it is not possible the retrieval of aerosol microphysical properties that ultimately may affect GAME computations. I understand the proxies made by the authors, but it must be translated in error bars. This is a weak point that must be addressed before the final publication of the manuscript.

The authors in the last line of the conclusions claim that from this volcano eruption there is a clear impact on the regional climate of the Earh-Atmosphere system in the southern tropical Indian Ocean region’. To me this can not be deduced from the measurements and analyses performed in the manuscript. The tittle is confused as it suggests this impact on climate. I think that the title is incorrect and should be modified to reflect the purpursoes of the manuscript.

Generally the manuscript is well-written, although there are many naive mistakes that must be improved to make the manuscript better legible:

Introduction Section: In general is very well, but I miss many references. For example:

• Lines 33-34: Reference needed after “Several figures are evidences of a record-breaking atmosphere event”. What are you referreing by ‘Figures’
• Lines 59-60: Reference needed after “Because ozone is not emitted primarly during volcanic eruptions, its loss or production by post-eruption reactions are more tedious to estimate”
• Lines 70-71: Reference needed after “… as volcanic sulfates are concerned, these aerosols usually scatter sunlight back to space, cooling the Earth´s atmosphere, and absorb outgoing thermal radiation”

Materials and Methods:

• A brief overview is needed for this lidar – e.g. number of wavelengths, laser power, type of detection.
• Authors mention 87 nights of measurements. How frequently are acquired the measurements.
• Authors use 30 sr as lidar ratio. This is a potential source of errors because i) lidar ratio affect for the computation of the entire profile and ii) it might not be the real values. How do you accounts this possible source of uncertainties in GAME computations ?
• Ozone Mapper and Profiler Suite Limb: Authors just use public data (that must be correctly referenced). But they are introducing additional errors in sAOD by forcing lidar to 745 nm for comparisons. Why not using 510 nm that is the closest wavelength to lidar measurements. If I am right, authors use AE_355/745 of -0.14 that might not be the real value for each specific measurement. That could add errors in direct radiative forcing computations.

The GAME radiative transfer model

• I see that size distribution, single scattering albedo and assimetry parameters must be inputs and assumptions are made. This ok. But what is the final error in the computations? This could be computed assuming other aerosol optical and microphysical properties in the literature. Have you made these computations ?

Results

• Generally, I would like to point out a naive mistake: Many Figures are not introduced in the text and they just show up in the discussions. For a mature paper, every Figure must be appropiately introduced. The same happens for Tables. For example, in 282 says ‘ 4 runs of GAME are performed and summarized in Table’, and when going to the Table I only find the configurations used in GAME.
• Line 217: Background sAOD of 0.00259. How this value is computed ? I guess that the error associated with the measurements is larger than your standard deviations and might not have sense to give three significative values.
• Line 237: The volcano also injected particles in the troposphere.
• Line 437-438: The study does not show the impact of HTHH on the regional climate in the southern tropical Indian Ocean region. To me, it deals with the aerosol and water vapor characterization plus radiative forcing computations. It might the impact claimed by the authors, but it can not be deduced from the results and discussions presented.
• And I would like to add that I agree with the comments made by the other referee

The paper presents a good radiative characterization of the Hunga Tonga Hunga Ha'apai eruption. The authors present measurements and observations obtained at Reunion Island with Lidar and satellite measurements.

in the work an analysis of the results is presented in an analytical but very clear way making the paper clear and sequential in reading. Regarding the methodological part I think that some more details without having to resort to the references indicated would have been useful to make the reader easily informed on the observational capabilities. Specifically a more exhaustive description of the lidar system would give the reader the possibility to understand the characteristics and observational potential of the Reunion observatory.  Even a few brief additions on why certain assumptions were chosen in the data analysis would have provided the reader who is not an expert in Lidar with a more comprehensive explanation of the work (e.g. Line 91 LR=30).

From my point of view therefore the work is important to be published also given the low frequency of these events which as illustrated by the authors see in literature still relevant presentations of the eruption of Pinatubo and El Chichon underlining to the scientific community the importance of these ground and satellite observation systems for the study and characterization of these events.