General comments

Black carbon (BC) aerosols have significant impacts on earth system radiative balance due to its strong light-absorbing properties in the visible wavelength. Once BC deposited in snow, it accelerates snow melting, and further impacts the climate. In this study, a physically based snow radiative model was coupled into Polar-WRF to investigate the SDE of BC in the arctic region. The topic is interesting, however, this manuscript cannot be published in the current state, the authors need to resolved several major issues before it can be reconsidered. Please see my comments in detail below.

1. Title of the manuscript underlines the time period is melting season, however I do not see any significance of snow melting in the manuscript. The biggest problem is that the authors did not take BC amplification effects during melting period into modeling. The authors have to solve this issue before it can be reconsidered by ACP. See Flanner et al., JGR, 112, D11202, doi:10.1029/2006JD008003, 2007 and Doherty et al., JGR, 118, 5553–5569, doi:10.1002/jgrd.50235, 2013.
2. An assumption of 50 ppb was applied in this study, which largely deviates from the real world. The BC mixing ratios in the arctic can vary from <10 to several hundred ppb. See Doherty et al., ACP, 10, 11647–11680, 2010 and Doherty et al., JGR, 120, 11,391–11,400, doi:10.1002/2015JD024018, 2015. Therefore, I think the SDE values reported by the authors are not trusted.

Specific comments

1. Abstract: Title of the manuscript is “Modeling study of the snow darkening effect by black carbon deposition over the Arctic during the melting period.” However, I did not see any discussion related to the effects of melting on SDE due to BC in the abstract.
2. The abstract is unnecessarily long, needs to be shortened substantially.
3. L21: instantaneous RF or daily-averaged RF here? Please clarify.
4. L31: What is Noah-MP?
5. I think the authors need to clarify in the introduction that why did they want to investigate the SDE during melting season instead of snow accumulation season or stable season? And proper citations are required.
6. L98: SSNICAR->SNICAR
7. L99: Please briefly describe surface feedbacks in the modeling experiments.
8. L105: Please delete “and” after “processes through”.
9. L109: What is Noah-LSM? Same for CLASS and BATS in L113-114. I highly suggested the authors generate a list of Abbreviations in the appendix for the readers, too many model names in the manuscript.
10. Please think about remove Sec. 3.2. Similar results have been reported in other studies such as He, C. 2022, https://doi.org/10.1071/EN22013, and nothing interesting of the results.
11. 3.3: I do not agree with the authors that they used a fixed BC mixing ratios in Arctic snow, and the values about BC RF they reported are with very low confidence.
12. 3.4: Where is the BC deposition time series data from? I did not see the data source.

This study investigates the effect of black carbon (BC) on snow surfaces over the Arctic using the Snow, Ice, Aerosol, and Radiation (SNICAR) model online and offline with a polar-optimized version of the Weather Research and Forecasting model (Polar-WRF). The research focuses on an important problem as a better understanding of the impacts of snow conditions and land-atmosphere interactions is critical in more accurately assessing the overall impact of BC in the Arctic. However, there are a few major flaws in the manuscript.

1. The modeling seems detached from the reality. Only validation of meteorological factors is included. However, other variables, such as BC concentration, snow depth, surface albedo, runoff, etc, also need to be compared with observations to ensure the numbers are in line with those in the real Earth system in order to generate meaningful results. For example, it is unclear how 50 ng/g is selected as BC mixing ratio. For the existing validation, please provide explanations of the simulated biases, especially temperature and wind direction. Also, observations from the two locations cannot sufficiently represent the whole Arctic condition.

1. The manuscript states that weather models with high temporal and spatial resolution and conducting ensemble simulations are important in accurately assessing the impacts of the snow-darkening effect (SDE). However, these statements are not well justified. Also, this research uses a resolution of 27 km, which is not high.
2. The manuscript needs a more comprehensive literature review. There are more studies focusing on BC impacts on snow surfaces using SNICAR over the Arctic areas.
3. The Mellor-Yamada-Nakanishi-Niino (MYNN) scheme is used to represent the boundary layer. However, modeling the Arctic boundary layer is very challenging. Please explain why the MYNN is selected and how it behaves over the Arctic. Also, do clouds play a role in the simulations?
4. Please explain the difference between CLM and Noah-MP, and highlight why Noah-MP is selected.
5. A five-day spin up is used. However, it is unclear whether this is sufficient. For example, less variabilities are simulated during the first few days, as shown in the temporal evolution plots. Should these first few days also be considered spin-up?
6. Please provide more detailed description about experimental design. For example, which experiments are offline and which ones are online. Also, what’s the meaning of “except for snow depth” etc.
7. Please clarify how the model accounts for snow-ice transition. For example, after melting and refreezing, can we still treat the surface as snow? As the study does not account for ice, clarification like this is necessary.
8. Section 3.4 is hard to follow. The time series plots show dense information, but the explanations are brief. It is unclear how shallow, moderate, and deep snow depths are defined. There are brief explanations on Line 444, yet information like this should be placed in the beginning of the section and also be expanded on to give a clear background to facilitate a better understanding of the temporal evolutions.

Minor:

• Inconsistent titles in the manuscript and the supplementary materials.
• In the result section, please clarify which results are coming from which experiments.
• Many typos, such as Line 105 “through and”, Line 318 “f-j”, Line 342 “each maximum levels”; Line 481 “shows the how,” etc.
• Please add more descriptions regarding the advantage of Polar-WRF. Simply refereeing to other documents is not good enough to provide a background for this study.
• Please provide brief description of the data downloaded from the Arctic Data Center.
• Line 325, how is the “absolute mass of BC represented by the snow depth”?
• For Section 2.3, please clarify whether the calculations are included in WRF as one of the diagnostics, or are these offline calculations based on modeled results.
• Please clarify “The high RF caused by BC deposition reached 1-4 W m^-2”