Review of: Afforestation impact on soil temperature in regional climate model simulations over Europe

Sofiadis et al.

Overall Evaluation:

The study examines the results of the LUCAS regional climate experiments, which explored the climate impacts of afforestation and deforestation in Europe. The study focuses on the effects of forest cover on soil temperatures. The study finds that the different regional climate models that participated in LUCAS give contrasting projections of changes in the annual amplitude of soil temperatures. The underlying experiments and motivations of the study are sound, however the presentation and analysis of the results are confusing and incomplete. I recommend major revisions. 

General Comments:

(1) The paper lacks a clear causal explanation of why the models vary so much in the change in the amplitude of soil temperature. The attempt to explain the variation relies of two factors: the annual amplitude of ground heat flux, and soil moisture. Soil moistures is a perfectly valid explanatory variable but ground heat flux is not. Temperature and ground heat flux are both thermodynamic quantities and thus are very closely linked. Without an internal heat source (such are waste heat from soil carbon decay) subsurface temperature is surface heat flux modified changes in thermal diffusivity and heat capacity. In models thermal diffusivity is likely only being changed by soil moisture and maybe soil carbon content. Thus,  it is no surprise that temperature and heat flux correlate well, but also this does not constitute an explanation.

Instead the focus should be on the differences in surface energy balance components (which are briefly examined) and the differences in model structure that may cause these differences. Key features to examine are: how snow is treated, how litter is treated (it is a good insulator), how forest canopies are treated and how root-depth is treated.

(2) Despite being mentioned in the introduction soil profiles are never examined. Instead annual amplitudes of temperature at just one depth are examined. It would be useful to examine how temperature changed with depth in grassland and afforested conditions. Examining these profiles may also be helpful in finding a causal explanation for inter-model variance.

(3) How the models are being forced is unclear. The text implied that RCMs are being used with interactive atmospheres  but the methods section seems to imply the reanalysis data is being used to force the models. The methods may be trying to say the reanalysis is being used at the RCM boundaries but this is not at all clear.

(4) The manuscript has far to many abbreviations. As a rule of thumb, only define an abbreviation if you are going to use it 5 times or more.

(5) Citation parenthesis are used incorrectly. Citations are not placed in parenthesis if they need to be pronounced as part of a sentence. For example "(Davin and de Noblet-Ducoudre, 2010) analysed a GCM’s sensitivity to idealized global deforestation ..." should be: "Davin and de Noblet-Ducoudre 2010, analysed a GCM’s sensitivity to idealized global deforestation ..."

(6) Using Celsius instead of Kelvin would make the manuscript more readable.

(7) The paper is not self-contained and relies on Davin et al. 2020. Elements critical for understanding the experiments should be reproduced here.

Specific Comments:

Abstract: Make it clearer you are examining soils.

Introduction: Briefly introduce the biogeochemical effects of deforestation and make clear that you are only examining the biophysical effects. Also need to explain what RCMs are and how they improve on global studies.

Line 44: Many of the models that you are referring to are Earth system models.

Line 48: Cloud feedbacks?

Line 48: "On contrary" is not grammatically correct. "However" would work.

Line 50: Citation needed.

Line 54: "Inter-comparison" should be "Intercomparison"

Line 106: This table should be reproduced for this paper.

Line 110, 112: Forest and Grass are not acronyms and thus do not need to be in all caps.

Line 113: Show the maps.

Line 117, 118: These abbreviations are barely used. They can easily be eliminated.

Figure 1: The map needs a North arrow, a scale, and inset showing the study domain, and higher resolution territorial boundaries. Using a different line style for national boarders and coastlines would also be helpful.

Line 165-166: Rewrite sentence for clarity.

178: 'assumed' is a poor choice of words. Models suggest.

Line 207: Change 'involve' to 'include'

Line 213, 215: 'Obviously' and 'totally different' are informal constructions 'Clearly' and 'largely different' would be more consistent with formal English.

Figure 3,4: Use Celsius, also in caption give depth of temperature.

Figure 5: In caption explains which direction of heat flow is considered positive.

Figure 8: Net radiation and turbulent fluxes should have opposite signs, one is opposing the other. Is melt energy included in the latent heat flux?

Figure 11: Write out the region names.

Line 371: At what depth is the cooling?

Line 389: This section is the Discussion and Conclusions.

Line 439: 'Nowadays' is English slang, very informal.

The study under eview by Sofiades et al., complements the previous work by Davin et al. (2020) on the biogeophysical impacts of forestation in Europe, both undertaken within the important CORDEX Flagship Pilot Study LUCAS (Land Use and Climate Across Scales). The current work further exploits the multi-RCM output of afforestation experiments (forest minus grass) and reveals (to name a few key results) the crucial role of the employed land surface models, and a dampening of the annual ground heat flux cycle which explains the inter-model variance in the annual amplitude of soil temperature. Also a response to afforestation was indicated of the summer land surface cooling in forested areas compared to open land. This work is nicely written and presented, with adequate statistical and graphical aspects and sound science. It is a valuable contribution towards the understanding of land use change role (or impact) on regional climate (and change).

Science Comments

lines 79-84: a brief justification may be needed here on why this study focuses on the "soil temperature profile" (by looking at the 1 m below ground in section 3.1) and not , also, the uppermost soil layer (= surface) temperture which is ultimately connected to the radiative and heat fluxes that drive the overlying air temperature, the surface climate parameter of main interest

line 98: as opposed to which PBL scheme in WRFb-NoahMP?

lines 125-128: is thermal diffusivity κ (see below) parameterised in the RCMs land surface schemes (and therefore derives from moisture affecting heat capacity, as you mention) or it is taken as a constant from look-up tables?  Could this property be shown for each model (especially if the authors feel it would assist interpretation of results)?

From textbooks (pages 397-398 of McIlveen (2010) or section VIII.B. Conduction of Heat in Soil in Hillel (2003)) the thermal diffusivity is defined as:

κ = k/(ρC) where

k = thermal conductivity

ρ = density

C = specific heat capacity

The authors may consider the information in the Chen and Kling (1996) for better introducing and perhaps diagnosing in future studies, the thermal diffusivity κ.

lines 128-129:  the fact that "GHF is calculated as the residual of surface energy balance because the actual GHF outputs were not available in most models" assumes that model surface energy budgets are balanced, something that it may not be the case for, e.g., WRF (section 3.3 in Constantinidou et al., 2020a)

lines 169-170: Would it be useful to also show (in the Supplementary), not only the forest minus grass effect on the "annual amplitude of soil temperature (AAST) at 1 meter below the ground surface" (as done here), but the absolute value of annual land surface (skin) temperature as well?

line 230: Refarding the afforestation response of GHF, "Scandinavia appears to be the most sensitive among the regions". Any reasons?

lines 425-427: Can you also connect the results with the overarching ambition expressed in line 65 to "better constrain and represent the LUC biophysical forcing in regional climate simulations over Europe"?

lines 431-432: these proposed evaluations should critically include the land surface temperature, as in Constantinidou et al. (2020b)

Minor/Technical Comments

The English need to be checked again as there are a few grammatical erros or suboptimal expressions (some of them are listed below).

line 48: more correctly "On the contrary"

line 85: "second heat conduction law" can be written, more neatly, as "Fourier's second law of heat conduction". Same in lines  120, 226, 289

line 122: in equation (1), strictly, the derivative symbols should be replaced with partial differentials

line 127: "is the only variable which influence" should be "is the only variable which influences"

line 291: "since affecting" should be replaced with "since it affects"

line 402: replace "conducted an approach of" with "employed"

REFERENCES

- Chen, D., and Kling J. (1996) Apparent thermal diffusivity in soil: Estimation from thermal records and suggestions for numerical modeling. Phys. Geogr., 17 , 419–430. https://doi.org/10.1080/02723646.1996.10642593

- Constantinidou, K., Hadjinicolaou, P., Zittis, G., & Lelieveld, J. (2020a). Sensitivity of simulated climate over the MENA region related to different land surface schemes in the WRF model. Theoretical and Applied Climatology, 141(3–4), 1431–1449. https://doi.org/10.1007/s00704-020-03258-5

- Constantinidou, K., Hadjinicolaou, P., Zittis, G., & Lelieveld, J. (2020b). Performance of Land Surface Schemes in the WRF Model for Climate Simulations over the MENA ‑ CORDEX Domain. Earth Systems and Environment, 2014. https://doi.org/10.1007/s41748-020-00187-1

- Hillel D. Soil Physics (2003), Reference Module in Earth Systems and Environmental Sciences Encyclopedia of Physical Science and Technology (Third Edition), Pages 77-97 https://doi.org/10.1016/B0-12-227410-5/00936-4

- McIlveen R. (2010), Fundamentals of Weather and Climate, Second Edition, ISBN: 9780199215423