Impacts of Urban Expansion on Relatively Smaller Surrounding Cities during Heat Waves

Round 1

Reviewer 1 Report

The article has a good scope. But, I am afraid it is not well written. It took me a while to understand what the authors want to achieve from this article due to the following reasons. 

The title is misleading. 

The article has a poorly written introduction. Until line 148, I didn't realize that authors are performing a green roof sensitivity study. 

The authors need to review current literature and work extensively on introduction and methods. Few recent references to guide the authors are: 

Sharma, A., Conry, P., Fernando, H.J.S., Hamlet, A.F., Hellmann, J.J. and Chen, F., 2016. Green and cool roofs to mitigate urban heat island effects in the Chicago metropolitan area: Evaluation with a regional climate model. Environmental Research Letters, 11(6), p.064004.

Li, D., Bou-Zeid, E. and Oppenheimer, M., 2014. The effectiveness of cool and green roofs as urban heat island mitigation strategies. Environmental Research Letters, 9(5), p.055002.

Tewari, M., Yang, J., Kusaka, H., Palou, F.S., Watson, C. and Treinish, L., 2018. Interaction of urban heat islands and heat waves under current and future climate conditions and their mitigation using green and cool roofs in New York City and Phoenix, Arizona. Environmental Research Letters.

Yang, J. and Bou-Zeid, E., 2019. Scale dependence of the benefits and efficiency of green and cool roofs. Landscape and Urban Planning, 185, pp.127-140.

If authors used WRF-SLCUM, why not compare with the same model with the green roofs; why use a PUCM model for green roofs. Either perform an inter-comparison model study OR be consistent with one model (it can we PUCM or SLUCM with and without green roofs). The mix of model configurations increases the variability in comparison. 

 In conclusion, the green roof mitigation study seems like a side story. It needs proper integration in the article to improve readability. 

Author Response

We thank the reviewer and the Editor for their thoughtful and constructive comments

 

Reviewers’ comments black

Responses in red.

 

1: The title is misleading.

The focus of the study is to explore the impact of urban expansion on its own and downwind areas, especially when combined with heat waves. Green roof as a mitigation measure is a complementary aspect but not the most important part. Hence we use the title “Impacts of urban expansion on nearby cities under heat waves”. (The previous version is “Impacts of upwind urban expansion on regional climate under heat waves”)

 

2: The article has a poorly written introduction. Until line 148, I didn't realize that authors are performing a green roof sensitivity study.

See previous comment. We are concerned with the urban expansion process and its impact on the downwind areas, especially under the extreme weather conditions such as heat waves. Thus, the key points in this article are:

•      From the four urbanization scenarios simulated to assess the sprawling effect of the megacity Hangzhou: the downwind small cities are far more affected than the upwind cities by the expansion of megacity on regional climate.

•      One mitigation strategy (green roofs) is examined in the upwind megacity. This is found to greatly reduce the air temperature of both megacity and the downwind city.

Therefore, the first part of the introduction is the urban expansion and the negative effects of high temperature in urban area. And the second part is the impact of megacity on downwind city and its research method. To alleviate this impact, green roofs are discussed. The introduction of green roofs is added in second last paragraph (line 54-60).

 

3: The authors need to review current literature and work extensively on introduction and methods.

I have read the recommended articles and cited them in the article, especially in the introduction of green roofs (line 54-60). (Li D’s article has been cited before, and has not been added again.)

 

4: If authors used WRF-SLUCM, why not compare with the same model with the green roofs; why use a PUCM model for green roofs. Either perform an inter-comparison model study OR be consistent with one model (it can we PUCM or SLUCM with and without green roofs). The mix of model configurations increases the variability in comparison.

In this study SLUCM and PUCM have the same urban physics except for the green roof. The other PUCM improvements (e.g. wall) are not used. Hence when GR fraction is set to 0 they are consistent (line 150). 

If GR (GR Fraction≠0) the PROM (Princeton ROof Model) [Sun et al., 2013] considers both heat transfer and hydrologic processes. The roof in PROM is divided into three parts (concrete deck, medium layer and vegetation-soil layer), and Green’s function based solution of the heat equation is used in the multilayer heat transfer scheme. Meanwhile, a Richards equation-based hydrologic module is implemented which considering infiltration, hydraulic diffusion and runoff generation processes.

The formulas and figures can be found in detail in Sun et al. [2013], section 2.1.

 

Sun, T., E. Bou-Zeid, Z.-H. Wang, E. Zerba, and G.-H. Ni (2013), Hydrometeorological determinants of green roof performance via a vertically-resolved model for heat and water transport, Building and Environment, 60, 211-224, doi:10.1016/j.buildenv.2012.10.018.

 

5: In conclusion, the green roof mitigation study seems like a side story. It needs proper integration in the article to improve readability.

The focus of the study is to explore the impact of urban expansion on its own and downwind areas. GR as a mitigation measure is a complementary study of this impact. Thus, the conclusion part mainly mentions the former one.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper was well-written, concise, and informative. Other than a few formatting issues, I would not have an issue with the paper being published in its present form. I am intrigued by the application of green roofs in mitigating the UHI effect. In this paper, the authors state that the addition of green roofs may actually make it more unpleasant Lines 277-278, due to an increase in humidity associated with the vegetation and low wind environment. Additional quantification of this would be intriguing. I suspect that for an arid environment, the overall green roof effect with light winds would be positive to the mitigation of heat in large cities.  

Author Response

We thank the reviewer and the Editor for their thoughtful and constructive comments.

Thank you very much for your affirmation.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thank you for the responses. I would still encourage the authors to change the title. 

"Impacts of urban expansion on nearby cities under heat waves"

Maybe try replacing "nearby" with "surrounding relatively smaller". This is just a suggestion. 

Author Response

We thank the reviewer and the Editor for their thoughtful and constructive comments.

we have replaced "nearby" with "surrounding relatively smaller" in title.