This study investigates two factorial effects on net shortwave radiation and albedo. While the manuscript contains important information, it would benefit from (1) providing quantitative reports in terms of both the magnitude and interannual variations, (2) focusing on the main research questions, (3) emphasizing results that take advantage of the long-term dataset, and (4) trying to organize figures concisely and reducing information that is not directly related to research questions.

  Regarding the first point, please notice that there is no quantitative information in the abstract. The authors have reported in the abstract there was a difference in absorbed shortwave radiation between peatland and forest, and their north-south difference. However, it is not clear whether these differences matter in the annual energy budget without quantitative information (e.g., what percentage). The authors also have reported that the interannual variation of the SW difference was explained by snowmelt date (Fig. 10). However, it is not clear to what extent snowmelt date was important in comparison with other attributions, such as summertime albedo, snow depth, and diffuse fraction. In addition, it might be more appropriate to use snow-covered duration rather than snowmelt date in the context of this manuscript.

  Regarding the second point, please consider if all research questions are sufficiently answered in the Abstract. Particularly, the second research question - what determines temporal variations of albedo? - is not quite summarized in the abstract. The authors found albedo depended on snow depth, LAI, zenith angle, diffuse fraction, and the “seasonal cycle” during the snow-free periods. Which factor was more important than others and to what extent? It is also important to focus on the research questions upon analyzing data. Upon finding environmental controls, I suggest giving a full focus on albedo rather than reflected radiation or SW difference. It is not surprising the latter group depends on incoming shortwave radiation, and reporting it does not add any scientific value.

  Regarding the third point, I believe that one key to improving the originality of this work is to take advantage of the long-term data set. Please notice that most of the results other than the last three lines in the abstract can be drawn from a single-year dataset. I believe further attribution analysis as I have mentioned in the first major point would be beneficial in this aspect.

  Finally, I suggest trying to minimize the number of graphs. For example, information in Fig. 3 can be easily found in Fig. 2 and Fig .4. Fig. 4 and Fig. 6 share quite the same information. Important information in Fig. 7 can be incorporated in Fig. 2 or Fig. 4 if they are presented for different years. Please try assigning a unique role to each figure.

Abstract

As stated in the first major point, give a full focus on addressing the results that directly answer the three research questions. Ancillary discussion based on Appendix figures is not appropriate to be included in the abstract. As stated in the third major point, a more in-depth analysis on the last three lines would improve the significance of this study.

L9 The fact of a higher albedo in peatland particularly in the south compared with the forest is more important for annual net shortwave radiation and is directly related to the main topic rather than the seasonal cycle in the snow-free period.

Introduction

It is good to have clear research questions in the end. There are some parts that appear to be irrelevant to this study (e.g., L24, L36).

L43 Effects on what? When using “effect”, the subject on which an effect is imposed must be clearly stated.

L70 Related to my second major comment, to take advantage of utilizing a long-term data set, I suggest revising it to “How do these differences affect the magnitude and variations of annual energy inputs from shortwave radiation?” Here, quantitatively reporting the variation is a key to successfully addressing this aspect.

Method

116, 121 the net shortwave radiation. Net radiation means a different variable.

L122 Please define 1 and 2 subscriptions.

Results and discussion

Related to my second major comment, I feel this section would improve by giving a clearer focus on the research questions and refrain spending too much on other results that are not directly related to the main story.

L147 radically high reflected shortwave radiation?

L163 more variation - related to my first comment, be quantitative.

L166 Again, this could be understood from Fig. 2 without the need of Fig. 3.

L265 If such a statistical model is to be used for data analysis, I suggest taking albedo as a target variable, and snow depth (maybe min{snow depth, 30cm}), diffusion fraction, zenith angle, LAI as explanatory variables. Once albedo is modeled by this way, it may be possible to extract of the significance of each explanatory variable, snow-covered duration, and strength of global radiation upon determining the annual net shortwave radiation.

Figures

Fig. 2, 3 It seems none of the discussion in this article deals with processes at sub-diurnal scales. Therefore, I think it would be better to consistently use daily averaged data instead of 30 mins data.

Fig. 2 What about showing albedo instead of upward shortwave radiation? Doing so eliminates the need for Fig. 4.

Fig. 4 What if the graph is drawn for each year then it would eliminate the need for Fig. 7?

Fig. 6 It is interesting how the impact of diffuse radiation affects differently between the snow-covered and snow-free periods. However, this figure quite resembles Fig. 4, and there would be better ways to graphically present the effects of diffuse radiation on albedo. My suggestion is to take the diffuse fraction on x-axis and albedo on y-axis and draw graphs for snow-covered and free periods, separately.

Fig. 8 This is an interesting graph but it would be interesting to show Panel b for other years too, because doing so provides how this relationship differs from year to year. And again, I think the originality of this article would improve by giving a clearer focus on interannual variations.

Fig. 10 I consider this is a very important part of this study - analyzing interannual variations. Related to my first major comment, it would be more informative to indicate to what extent snowmelt DOY was more important than other factors.

Appendix figures

Please consider relocating these figures to a separate file as supplementary resources rather than in the Appendix. While I do not negate the potential importance of the information presented in these figures, some of these figures, such as Fig. A18, 19, 20, do not have a publication quality in terms of their relevance to the main topic and graphical presentations.

Fig. A1 missing unit and label on the y-axis.

Fig. A4 Why not show albedo vs diffuse radiation or zenith angle? Reflected SW radiation is surely related to global radiation, which gives a misleading impression.

Fig. A5 1:1 comparisons of albedo between different locations do not make sense to me. Peatland albedo greater than forest albedo can be seen in the main figure (Fig. 4).

Fig. A6 Why not incorporate in the main figure 5?

Fig. A10 I feel there is no use in comparing global radiation and albedo.

Fig. A11 If the authors want to emphasize the importance of the seasonal cycle in the snow-free period, it might be worth considering this or Fig. A8 as a main graph.

Fig. A14, 15, 17, 21 Variables on the y-axis is a clear function of global radiation, which does not add any scientific value. As stated in my second major point, please focus on albedo as the research question has also stated so. For example, Fig. A21 should be comparing observed and predicted albedo.

General comments

The manuscript evaluates the differences in shortwave albedos between forested and peatland sites. The topic is important and relevant to the BG journal. The study is based on long time series (up to 12 years) of in situ measurements on four study sites (two site pairs), and the measurements and processing of data have been transparently explained. The results indicate that open peatlands have high albedos compared to forested sites, especially in snow-covered periods, and that the duration of the snow-covered period is a strong predictor of differences in annual net shortwave radiation between the forested and peatland sites. The paper creates valuable fundamental knowledge by carefully explaining the factors influencing the albedo of peatland and forest vegetation. 

I would have expected more quantitative evaluation of the results, especially when the differences are small and difficult to visually observe from the figures. This is the case, for example, when evaluating the effects of forest thinning on albedo.

In addition, I feel that the conclusions could be stronger and more clearly indicate what kind of implications the findings have.

Specific comments

L16-17: The conclusions in the abstract are rather vague. I’d expect some explanation of what kind of implications you are expecting.

L27: High reflectance in which wavelengths? I guess the reflectance of snow depends strongly on wavelength as snow contains water and water is absorbing strongly in the shortwave-infrared region.

L41: Replace “sunlight” with “solar radiation”.

L54-55: “have opposite trends due to warming”. I’d say they respond differently to warming.

L59: Maybe specify which kind of prior information is required?

L59-60: This is true, but the same applies to in situ measurements also: they have lower quality in winter because the amount of incoming solar radiation is low.

L69: Is it “temporal behaviour” or “temporal variation”? Probably both are ok to use. Consider also adding some information on the time scale in the research question, i.e., are you talking about seasonal or interannual variation, or both.

L70: Does “these differences” here refer to the first research question?

L79-80: Consider mentioning the LAI values here in the text also, so that the reader immediately gets an idea of what is meant by “substantially higher”.

L94: “zenith angle” -> “solar zenith angle”. Check also elsewhere in the manuscript.

Table 1 caption: The caption now only says that more details can be found in Table A1. To help the reader evaluate whether they should look at Table A1, you could specify a bit more what kind of details.

Table 1, footnote d: You mention that small trees are underrepresented in the LAI measurements. Is this because of sampling (avoidance of dense bushes of spruce in the field work), or for some other reason?

Fig. A1: The measured and gap filled snow depth data have different symbol types (line vs. dot), which makes one think that the time step of the data are different. But if I understood correctly, this is not the case? Can you use the same symbol for both data?

L129: I think it is a good idea to explain the gapfilling in the appendix in order to keep the manuscript concise. However, I would mention the amount of gaps (as percent of total number of days) in the main text. Now it is mentioned in the appendix, but I think mentioning it in the main text would help the reader to quickly evaluate how important role the gapfilled data have in the analyses.

L159: “diffuse light fractions” -> ”diffuse fractions of radiation”

L166-167: “The distinct bimodal distribution of the reflected solar radiation, now as a function of the global radiation, was clearly seen in the scatter plot as well.”. This sentence is difficult to understand.

L169: “the peatland albedo was higher and more variable in the snow-free period”. Do you mean that in the snow-free period the peatland albedo was higher and more variable than the forest albedo?

L172-174: It is unclear why you compare only snow-free albedos to previous values from literature. Surely some literature values for snow-covered albedos can be found from literature as well, especially for forests.

L182-183: “On all of the sites, there was more absolute variation in the wintertime albedo”. In comparison to summertime albedo?

Figure A5: Why are there empty circles present in the scatterplots?

Figure A5: In the Hyytiälä site there are sometimes albedo values larger than 0.4 when the Siikaneva is snow-free. Is it because the snow cover differs between Hyytiälä and Siikaneva?

L202: “The albedo for both sites showed a slight increase over the summer.”. This is very difficult to see from Figure 4. Consider making the y-axis scale logarithmic, or showing summer months as separate figures (with smaller y-range).

L203-205: “The very similar values for the snow-free albedo for the forest sites are somewhat unexpected, given their substantially different LAI (Table 1), and that albedo has been found to vary with changing LAI (Lukeš et al., 2013).”. Actually, in Figure 3 you have reported the values 0.1 for Hyytiälä and 0.11 for the Halssikangas site. Comparing this to Fig. 6 of Lukes et al. (2013) where much larger range of LAI values has been reported (from almost 0 to over 4 for the pine forests), it seems that the albedo difference of 0.01 is quite expected given the LAI difference that you have reported (LAI = 2.1 for the Hyytiälä, LAI = 1.37 for the Halssikangas site).

L205-206: “However, other studies have found little change in albedo for large variations in LAI (Bright et al., 2018).”. Please specify what is meant by large variations in LAI here.

L209-215: If you want more in situ evidence on the seasonal cycles of albedo in different types of vegetation, Fig. 4 of Betts and Ball (1997) (https://doi.org/10.1029/96JD03876) reports seasonal cycles of broadleaved and coniferous forest and grasslands.

Figure A7: Please explain why you used different measurements (PAR or shortwave) for defining the white-sky and clear sky conditions in the northern and southern sites. I guess this is because of the availability of the measurements, but it would be good to state it explicitly. 

Figure A8: “The Halssikangas reflected PAR sensor appears to be mounted at a slight angle, registering also part of incoming radiation.”. This sentence needs more explanation. How is this visible in the figure?

L230-232: The explanation of vegetation phenology seems somewhat unrelated to the PAR albedo. Perhaps it would fit better earlier where you discuss Figure 4.

Figure 7 caption: The meaning of the unfilled circles is explained twice in the caption.

L239: “light” -> ”radiation”

L237-241: The results on the effect of thinning of forest albedo are solely based on visual examination of the figures. Quantitative evaluation would be preferred, as the differences are small and not possible to clearly from the figures. Especially the validity of the conclusion that in snow-free period thinning does not affect albedo is a bit difficult to evaluate because the authors do not provide any numbers to back up their conclusion.

Figure A13 caption: “The product of these two parameters…”. This sentence is difficult to understand. Which two parameters are you referring to?

L267-268: Please define quantitatively what is meant by model explaining the net SW differences “satisfactorily”.

L278-280: Some quantitative measures of the strength of the correlation and/or linearity would be informative.

L281: “sunlight” -> “solar radiation”

Figure 10: It is interesting that the average differences in net SW radiation between forest and peatland sites is smaller in spring than over the entire year. I may be wrong but based on Fig. 8 I would have expected the opposite.

Figure A16 caption: I’d say the values are predicted rather than fitted. Parameters are fitted to the data, and then the model is used for making predictions.

L287: Do you mean higher in winter compared to summer?

L319: “Wintertime points are generally better described by this model.”. I’d say the model produces generally more accurate predictions in winter than in summertime.

Figure A19 caption: This is a minor detail as these data are not used in any analyses, but it would be good to shortly explain how the water table depth and soil moisture measurements were obtained.

L330: The information on the percent of gapfilled values should be mentioned already in the main manuscript text.

Technical corrections

L31: Should it read “difference between winter- and summertime albedo”?

L63: “above the Arctic Circle”. Above to me refers to elevation. I’d write “north of the Arctic Circle” or “on the northern side of the Arctic Circle”.

L98: “With zenith angles over 87, …”. I’d write “When the solar zenith angle was larger than 87, …”.

L107: “radiations” -> “radiation values”

L165: “some 20 cm” -> ”approximately 20 cm”

L201: You could replace “interannual or within-year” with “inter- or intra-annual”.

L234: “some” -> ”approximately”

Figure A13, Figure A14 & A15 captions: I think “mean global radiation between the sites” should be e.g. “mean global radiation over the sites” or “mean global radiation of both sites”.

L328: “used for the gapfilling”. I’d say “used as an explanatory variable in the model”.