Review report on “Diurnal variation of amplified canopy urban heat island in Beijing megacity during heat wave periods: Roles of mountain-valley circulation and urban morphology”

This case study work investigated the canopy UHI of Beijing City in the summers of 2016-2020. Based on data from various sources, the authors conducted a systematic analysis,  aiming to get a better understanding of the synergy between heat waves and the canopy UHI, as well as the influences of local circulation and urban morphology on this synergy. It is an interesting work with the support of a dense observation network and other fine-resolution data sets.

The authors try to consider many different aspects, which in turn requires the combination of many different data sources and analysis methods. This, however, makes the paper just as complex as the climate topic. Without improvement on the structure of this paper, especially the clarity of the data and methods, and the fragmented results presentation, it is difficult to follow the flow of the paper and to grasp the key findings.

Here are my major comments:

1. The language needs improvement.
2. Some details in the data and method section need to be clarified, some decisions need to be justified. I will list the issues later.
3. Please rephrase the last paragraph of the introduction section to emphasize your key research questions.
4. Please present your results more concisely while sticking to your key research questions. Try not to overwhelm readers with less important details in numbers, instead, try to summarize and convey the key results, to better connect different fragments of result presentations so that the storyline flows better.

In summary, I think this paper needs to be better structured, some details in the data and method sections need to be clarified, and the results need to be presented more concisely.

Here are some minor comments:

P1Line21: “insight to understand the driving mechanisms…” maybe “insight into understanding the driving mechanisms..” or “insight into the driving mechanisms”

P1Line16: “On a street scale” , maybe “on the street level“

P1Line25: “hasbecome” space in between missing

P2Line43: “Few studies…” Do you mean “A few studies…“ or do you want to say this has not been studied.

P2Line36: “The rate of contribution of urbanization to the excessive mortality caused by high temperatures can reach more than 45% in the high-density urban areas”  The contribution rate of urbanization to excessive mortality caused by high temperatures can exceed 45% in high-density urban areas.

P2Line47: “Overall, the current understanding of the mechanisms through which local circulations modulated the amplified CUHII during HW periods is still in the exploratory stage.”  Overall, the current understanding of how local circulations modulate the amplified CUHII during heatwave periods is still in the exploratory stage.

P2Line49: “impact” not necessary

P2Line59: “However, LCZs are a comprehensive indicator of urban morphology, and the aforementioned studies have not quantified the contribution of different urban morphological parameters to the local thermal environment, nor have they taken into account the nonlinear driving effects of 60 urban morphology on the local thermal environment (Alonso & Renard, 2020; Chen et al., 2022).” However, while LCZs are a comprehensive indicator of urban morphology, the aforementioned studies have neither quantified the contributions of different urban morphological parameters to the local thermal environment nor considered the nonlinear driving effects of urban morphology on the local thermal environment.

P2Line63: “Currently, it is still matter of debate the roles of local circulations and urban morphology in amplifying CUHI in megacities during HW periods.” Currently, the roles of local circulations and urban morphology in amplifying CUHI in megacities during heatwave periods are still a matter of debate

P2Line64: “The main objective of this study is considering as case study the megacity of Beijing, using high-density automatic weather stations (AWS) observations.”  The main objective of this study is to use high-density automatic weather station (AWS) observations to analyze the megacity of Beijing as a case study.

P3Line70: “The terrain of Beijing is exceptionally complex, northly bounded by Yan Mountains by Taihang Mountains in the west.”  The terrain of Beijing is exceptionally complex, bounded to the north by the Yan Mountains and to the west by the Taihang Mountains.

P4: FIG1c, not buit-up -> Non-built-up

P5Line85: “released by Professor Yang and Professor Huang of Wuhan University” This is not necessary, you already cited the corresponding publication.

P5Line85: “made” maybe “produced“

P5Line89: “The building skyline and floor data of the electronic map were extracted using Python language.” Electronic map from which provider?

P5Line90: “was estimated to be 3 m” was set to be 3 m

P5Line112: “This study identified stations that were less influenced by the urban effect” Selecting reference stations is always a challenge and the criteria are subject to the decision of different choices that might be biased. From the map on Fig1, it seems these reference stations are rather close to built-up areas.

P6Line124: “Calculation of mountain-valley breeze”, I suggest adding more technical details on this in the supplementary information.

P6: Table1, “Building cover ratio, which represents the proportion of the roof of the building to that of the entire study area.” This is rather confusing, what do you mean by “the entire study area“? What do you mean by “study area“, do you mean the entire Beijing urban are or only the area within the buffer zone?

P6Line131: T“to measure the morphological characteristics around AWS”, up to which distance surrounding the AWSs?

P6: Table1,“Number of patches.” What patch?

P6: “Table 1: Summary of the spatial morphological parameters.” I know it is rather complicated and might require too much effort, but it would be much better to give the equations for calculating these factors, especially since you failed to describe them clearly in the text.

P7: Table1, “in the buffer zone”, you should really mention the buffer zone at the beginning of section 2.3.3, and as per my comment above, what is the size of the buffer zone? How do you define these buffer zones? This needs to be mentioned in the text.

P7Line151: “The impact of urban spatial morphology on urbanization bias” not sure what you mean by “urbanization bias“

P8Line165: “relatively weaker urban excess warming,” Not sure if the HW should be the basis for defining urban excess warming. My intuition would be that CUHII is a measure of urban excess heat/warming.

P8: Fig2, it seems there is no correlation between CUHI and HWs. How is the CUHII calculated? By averaging all the hourly CUHII within one month?

P12Line205: “the mountain-valley breeze strongly impacts the thermal dynamic field near-surface of Beijing megacity” -> the mountain-valley breeze strongly impacts the near-surface thermal dynamic field of the Beijing megacity

P14: Fig6d, the wind speed in UN is still larger than in US, what are the possible reasons and implications?

P10Line185: “It should be noted that the ΔCUHII remained positive throughout the daytime and nighttime,” It is not always true, for example in 2018 in Fig3c.

P15Line255: “Therefore, on an urban scale, the turning mountain255 valley breeze caused horizontal transport of heat inner city, resulting in the north-south asymmetrical pattern of urban excess warming during HW periods.” do you mean “horizontal heat transport of heat of the inner city”, or just “horizontal heat transport in the city”?

P16: Fig7c, “Difference value (D-value) in CUHII across different urban configuration structures.”  It reads a bit weird,

P18Line296: “The linear model has shown considerable strength” I suppose a new section should start from here. This is also why I have the impression that the results parts a rather fragmented. You need some connection between them to let the contents flow smoothly so that they form a whole and readers can better follow what you want to convey.

P20Line330: “As the previous text demonstrated” -> As previously demonstrated