The Vertical City Weather Generator (VCWG v1.3.2)

Moradi, Mohsen; Dyer, Benjamin; Nazem, Amir; Nambiar, Manoj K.; Nahian, M. Rafsan; Bueno, Bruno; Mackey, Chris; Vasanthakumar, Saeran; Nazarian, Negin; Krayenhoff, E. Scott; Norford, Leslie K.; Aliabadi, Amir A.

doi:https://doi.org/10.5194/gmd-14-961-2021

Articles | Volume 14, issue 2

https://doi.org/10.5194/gmd-14-961-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/gmd-14-961-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 2

Model description paper

|

18 Feb 2021

Model description paper |

| 18 Feb 2021

The Vertical City Weather Generator (VCWG v1.3.2)

Mohsen Moradi, Benjamin Dyer, Amir Nazem, Manoj K. Nambiar, M. Rafsan Nahian, Bruno Bueno, Chris Mackey, Saeran Vasanthakumar, Negin Nazarian, E. Scott Krayenhoff, Leslie K. Norford, and Amir A. Aliabadi

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Final revised paper (published on 18 Feb 2021)
Supplement to the final revised paper
Preprint (discussion started on 27 Aug 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Comments', Anonymous Referee #1, 22 Oct 2019
RC2: 'review', Anonymous Referee #2, 19 Dec 2019
AC1: 'Author Comments in Response to All Reviewers', Amir A. Aliabadi, 06 Mar 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Amir A. Aliabadi on behalf of the Authors (06 Mar 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Mar 2020) by Wolfgang Kurtz

RR by Anonymous Referee #1 (01 May 2020)

Suggestions for revision or reasons for rejection

The effort of the authors in revising the manuscript is appreciable. The clarity of the manuscript and the scope of the results are improved through the revisions. Considering the major changes implemented in the model and manuscript, I reviewed the paper as a new submission, while the authors’ response was helpful in clarifying some parts.

My comments address some major and minor issues:
-The two main equations 8 and 9 are showing that the assumption of horizontal homogeneity in the mean flow is used for their derivations. This assumption is heavy for complex terrains of urban areas, when the focus lays within the UCL. The authors’ response to this concern is not convincing and the dispersion term does not account for the horizontal mean velocity gradients. I still believe that Santiago and Martilli (2010) cannot be referenced for this assumption as their work has a different focus and refers to a mesoscale model. While respecting the work of other researchers, referring to other works on this matter does not help changing the fact, either.

-In Fig. 7, right panel, I expect to see reduced wind speed within the UCL (< Havg) for larger Lamda_p. On page 20, line 2 the authors mention that they expect to see this behavior, too. However, Fig. 7 does not show this behavior. The red line shows larger velocities within the UCL.

-The model is also not showing the expected behavior for the wind profile in Fig. 9, where it is expected to see lower velocities within the UCL in case of bigger trees in the simulation.

-While it is mentioned that the vertical profile of TKE is also modeled, there is no evidence of its results in the paper. Given the extremely long length of the paper, I am not sure whether I should propose to include new results in the paper, but at the same time, there is no way to evaluate the model performance over this aspect.

-I noticed that while the model contains four sub-models, only three are mentioned in the abstract (excluding the radiation model)

-On page 4, line 17: I suggest removing the general statement of “(the direction in which turbulent transport is significant)” from the sentence, as this is not accurate. Turbulent transport, in general, could be important in all three directions.

-Equation 2: the power -1/4 for the unstable condition is incorrect. It should be -1/2.

-Add the units for all terms that are mentioned within the text

-Figure 2: There is no information about the tree figure in the caption
Page 12, section 2.1.5: Tree shape parameterization is widely proposed in other work. Please provide a reference for this part, or mention the major changes implemented in this work.

-It is not clear to me what the right panel of Figs. 12 and 13 are showing. The caption does not help either. If simulations were performed for one day, then it is expected to see similar downward shortwave radiation in the top and bottom figures of Fig. 12 and 13. Then I wonder whether S_downward is the incoming solar radiation?

-Figures, in general, are very far from where they are mentioned for the first time
-Page 6, line 15: …within ‘an’ above… --> …within ‘and’ above…
-Page 6, line 18: ‘surfaces’ --> ‘surface’
-Page 9, line 15: ‘based’ --> ‘based on’

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RR by Anonymous Referee #3 (22 May 2020)

Suggestions for revision or reasons for rejection

This a timely attempt at overhauling the UWG model. The topic is highly relevant, in particular for practitioners (architects, urban planners) wishing to incorporate urban microclimatic effects in their building energy models. Therefore, the effort of the authors is to be commended.

Generally speaking, the manuscript is not yet ready for publication. Several of the points raised in the previous review round have not been addressed convincingly in the current version, and doing so may require major rework. More fundamentally, the validity/superiority of the proposed VCWG model over the original UWG model is not clearly established, despite the improvements proposed in this revised version—most notably, the incorporation of the Monin-Obukhov parameterization in the rural model.

1. New rural model: Why do the authors think that the Monin-Obukhov rural model is superior to the one in your original manuscript? This change seems to be mainly triggered by the first-round review comments highlighting “unjustified parameters” incorporated in the rural model of the previous manuscript. However, this is a major change and the transition from one to the other merits detailed discussion and justification in the final manuscript. Both parameterizations are imperfect, and the choice of one versus the other should be based on ultimate model accuracy, rather than the satisfaction of a review comment.

2. Assumption of constant specific humidity in the vertical direction in the rural model
* The authors’ replies to both reviewers’ comments on this topic are not convincing. The answer to the first reviewer is that the assumption is valid as long as vapour pressure is below its saturation value; and they proceed to show that this condition is indeed verified, at least over the limited two-week period of analysis. While the answer to the second reviewer seems to accept the reviewer’s viewpoint that even if this condition is verified, it does not constitute sufficient basis for the validity of said assumption. The authors’ final rational seems to be that there is no other feasible way to approach this matter: “This assumption is made, for lack of a better assumption”
* Why not use the Monin-Obukhov parameterization also for humidity? The authors mention the lack of surface latent flux measurement in the EPW file, but a basic soil water diffusion model similar to the one implemented in ENVI-met could overcome this problem. Of course, precipitation measurements would be required. The authors mention that this measurement is often missing in EPW files, but I don’t think that the authors should limit their methodology on the basis of such considerations. Even if precipitation happens to be missing, daily or monthly values are generally not difficult to obtain even for the most remote locations, and are probably sufficient to feed said soil model. Such a model would also help with the necessary incorporation of the evapotranspiration phenomenon (see my comment below).

3. Lack of proper evapotranspiration model to account for evaporative cooling provided by the vegetation: this was a major shortcoming of the original UWG and does not seem to have been addressed in this updated version (or at least, it is not mentioned in the manuscript)

4. Validation procedure and model accuracy
* Why not use 1 year instead of two weeks? The main advantage of UWG-like models is their ability to conduct annual analysis. The validation therefore should also be applied on an annual time scale. The validation period of two weeks is more appropriate for mesoscale or CFD models.
* Why not use the highly reliable and comprehensive Basel (BUBBLE) or Toulouse (CAPITOUL) observations, which would also make comparison to the original UWG more straightforward? The authors prefer to undertake their own measurement campaign in Guelph which is limited in duration to only two weeks.
* The average bias of the reference variables is actually quite high (−1.43 K, 1.06 m/s, 5 g/kg). The temperature bias in particular is of the same magnitude as the UHI intensity. The temperature RMSE is also quite high at 1.56 K. When it comes to UHI intensity, what is lacking is the RMSE between measured and modelled values—i.e., a measure of the goodness of fit. Calculating the standard deviation of UHI intensity with respect to its own average is not informative. Furthermore, using the proximity of the standard deviations of UHI measurements and UHI model predictions (respectively 1.23 K and 1.53 K) as an indication of the accuracy of the model is questionable.
* After the cursory and unconvincing model validation, the study attempts a sensitivity analysis (“model exploration”, section 3.2) which is now conducted using Vancouver rural weather data. The most important sensitivity analyses, those pertaining to plan area index, frontal area index, leaf area density, building energy configuration and radiation configuration are, again, conducted over a clearly insufficient period of 2 weeks. This is all the more surprising given that, for the sensitivity analysis, no urban measurements are required and computation time is not a major issue. In sections 3.2.5. and 3.2.6 the authors consider model variability for different seasons and different locations. This time the model is simulated for a full year—which shows that computation time is not an issue. Given the weakness of the model validation (my comments above), and of the methodology underlying the sensitivity analysis, I shall not discuss the outcome of the sensitivity study in any detail.

5. Miscellaneous
* Please explain the values of C_k (=2 for unstable, =1 for stable). They do not seem to be taken from the cited paper by Nazarian et al. (2019). In that paper, the product C_k.l_k is parametrized, not C_k separately.
* Why is the waste heat fraction set to 0.3? Please provide a justification for this value or conduct a sensitivity analysis. This parameter can have an important impact on UHI.
* The urban and rural measurement stations (both within the University of Guelph campus) are quite close to each other, separated by about 2 km. Please explain why the rural station is not more distant. Also the rural station is northeast of the urban station, i.e., downstream of the urban station given that the predominant wind direction is from west/southwest. Usually, an upstream rural station is preferred.
* Why do you combine the Guelph rural station data with that of London, Ontario? What do you mean by “combine”? There is also mention of “assembled EPW dataset” which is even more puzzling.
* HMP60 is a Vaisala sensor not Campbell.
* Please explain why an average building height of 20 m was selected. This seems quite high for that urban location.
* Similarly a plan area index of 0.55 (page 17) seems high. That urban area contains many empty (green) spaces. Additionally, this plan area index value largely exceeds the maximum value considered in the CFD-based parameterization of Nazarian (2019) which this paper seems to be using extensively. So you probably had to extrapolate Nazarian’s parameterization. This deserves some discussion. By the way, the plan area index value in Table 1 is given as 0.44 (exactly the maximum value consider by Nazarian) while the frontal area index becomes 055. Which is correct?

In conclusion, the present manuscript does not succeed is unambiguously establishing the superiority of the proposed model over the original UWG. A direct comparison of the performance between the two models is not provided. Without a doubt, the original UWG methodology presents shortcomings that need to be addressed in a demonstrably superior way. There is a key sentence in the abstract: “The results obtained from the explorations are reasonably consistent with previous studies in the literature, justifying the reliability and computational efficiency of VCWG for operational urban development projects”. Rather than being “reasonably consistent with previous studies”, the authors should demonstrate that their approach is superior. This is something that still remains to be established.

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ED: Reconsider after major revisions (07 Jun 2020) by Wolfgang Kurtz

AR by Amir A. Aliabadi on behalf of the Authors (19 Jul 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (05 Aug 2020) by Wolfgang Kurtz

RR by Anonymous Referee #3 (15 Aug 2020)

Suggestions for revision or reasons for rejection

Thanks for the comprehensive response to my review comments. Please see below the remaining issues in the current manuscript.

GENERAL: The discussion of parameter sensitivity is too long. It is more important, for a paper introducing a new model, to solidly establish the validity of said model under strictly controlled conditions (and the choice of the BUBBLE database is excellent in that regard), than to test—in excessive length—its sensitivity to input parameters. If I was involved in the initial review round, I would have suggested that the sensitivity study should be moved to a separate paper. In any case, I propose to reduce/compress these expansive discussions and the associated figures by restricting the sensitivity analysis to Basel (same city that you used for validation), adding waste heat fraction to the analysis, and foregoing other cities/climates. This is consistent with my strong recommendation—also mentioned in the previous review round—to run the validation for a full year instead of two weeks. See one of my comments below.

Eqn3: How do you calculate the Monin-Obukhov length L if the the sensible heat flux is not available? Basel was an exception in that this value could be derived from deep soil temperature (Eqn 7), but how would one proceed if this measurement is not available—as is likely to be the case for the quasi-totality of rural weather stations.

P914: “Friction velocity can be determined by numerically integrating Eq. 4 from the elevation of the rural aerodynamic roughness 15 length z0rur [m] to 10 m in an iterative process”. Do you assume that the zero displacement height is zero?

P9L16: “The potential temperature profiles are also obtained by integration of Eq. 1”. I guess you start the integration from z0H. What is the value of z0H? Do you assume that it is the same as z0 (aerodynamic)? If yes, why? Same question arises for z0Q, although setting z0Q=z0H would be a more acceptable assumption than setting z0H=z0.

P9L17: “Given the similarity of heat and mass transfer”. Do you mean, “given the similarity of sensible and latent heat transfer”?

P12L23: I agree that in Basel, air-conditioning is either non-existent or rejects waste heat at roof-level. But what value of waste heat fraction have you assumed for other cities? In your response to reviewers, you mention that the waste heat rejection has a significant impact on UHI (+1 K), yet you choose not to do a sensitivity analysis on it. Why?

Eqn17: I note that although your urban canopy model is multi-layer, the building energy model seems to be single-zone. To be consistent with the UCM, it would have been preferable to assume a multi-zone BEM (the height of each building zone corresponding to the outdoor UCM layering), especially in view of the varying insolation according to height. Please briefly discuss this choice in the manuscript.

P15L26: the baseline values of lambda_P=0.54 and lambda_F=0.37 that you report for Basel, are not consistent with the values of “horizontal building density” and “vertical-to-horizontal urban area ratio” reported by Bueno (2012) for the same city (resp. 0.4 and 0.8). Please clarify.

P17L20: I am not convinced by your response to my review comments stating that you retain the 2-week validation period in order to avoid a more lengthy manuscript. First of all, since you are now using the BUBBLE data, the size of the manuscript will be reduced, perhaps by a few pages. Secondly, reporting, BIAS/RMSE/R2 for one year instead of 2 weeks does not require more space and the figures can also be easily adapted to be brief. My point is that it is important to assess the dynamic behaviour of the model for a full year, especially since the main goal of this paper is to establish the validity of a completely new model. If you do not have a full year of rural Basel weather data corresponding to the urban weather data, why not use rural data from a standard weather station operated by the national meteorological organization. Of course that raises the question of how to determine the surface heat flux. But this question must be addressed anyway in your next version of the manuscript, since making the algorithm reliant on the exceptional availability of said data in the BUBBLE dataset is not an acceptable solution.

P18: Are all the values in the table taken from already cited papers or are some of them estimated by you—in which case some additional explanation would be in order. For example: z0, beta (is it really constant), etc.

Fig6: Any specific comment about the model/observation discrepancy late at night?

Table2: I cannot read the adjusted values.

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RR by Anonymous Referee #4 (21 Sep 2020)

Suggestions for revision or reasons for rejection

Review of the revised manuscript "The Vertical City Weather Generator
(VCWG v1.2.0)" in discussion for publication in Geoscientific Model Development.

The reviewed manuscript is a revised version of a manuscript
describing the improvement of the "Urban Weather Generator" into the
"Vertical City Weather Generator". Such weather generators occupy a
niche to simulate the meteorological conditions in the urban roughness
sublayer with very low computational cost compared to an urban canopy
model coupled to a mesoscale atmospheric model or an obstacle
resolving model. Given the availability of meteorological observations
in the vicinity of the city, the weather generator can simulate the
meteorological conditions in the urban environment with very low
computational effort and provide input data for building energy models
representative for the urban environment.
Considering the previous comments of the reviewers, the manuscript
has been considerably improved, but there is still room for more improvement.

Major remarks

- At the end of Section 1.1, the urban canopy models are discussed and
it is mentioned that there is "no connection to rural meteorological conditions"
This is a problematic statement, since the objective the UCM is
to solve the urban energy balance at a given point as a function of
the meteorological forcing data. Thus it is trivial that they do not
consider the connection to the adjacent rural area. But if the UCM
is coupled to a mesoscale atmospheric model, this connection is very
well taken into account by the modelling of the process of advection
between rural and urban area, and even a potential urban breeze.
On the contrary, the difficulty in accurately representing advection
or an urban breeze seems rather to be a weakness of the Weather
Generator than of the UCM coupled to a mesoscale atmospheric model.

- Section 1.2 contains many details on the functioning of the VCWG.
It could be shortened considerably by introducing just the main ideas.

- Section 1.3 is also too detailed. It could become just a very short
paragraph at the end of Section 1.2.

- In the description of the VCWG, the rural and the urban model
are described, but how do they interact? The advection of cooler
air from the rural areas to the urban areas with the mean wind speed
and a potential urban breeze is a very important process for the
UHI. Is this considered and how? More details on the urban-rural
interaction and the underlying assumptions of the model are required.

- The entire Section 3.2 (model exploration) could be removed since
it is not very rigorous. This would make the entire paper much
shorter and a subsequent study could be made with a more detailed exploration of parameters.

Minor remarks

- At many occasions, the word "run" is used for "simulation".
In my opinion, this is a bit too technical.

- You might consider replacing "elevation" by height above ground level (a.g.l.).

- The degree symbol "°" should be directly after the value, without space.

- I dont understand why "Winter" and "Summer" are written with capital letters.

- The formulation "not reported here for brevity" is used way too often,
which makes the article tiring to read. It could actually just be omitted at most instances.

- Page 4, L28 "The VCWG is designed to cycle through...". I do
not understand what this means. It sounds as if a series of
arbitrarily chosen meteorological conditons would be tested one
after the other. Please rewrite this to be more clear.

- Page 6, L8, what is "building energy system"?

- Page 7, L10: "Energy Plus"

- Page 10, L28: why are you using the formulation "temperature
(energy)" here and at other instances? Although the equation
for temperature comes from energy conservation, it is not usual to
have such a double writing.

- Page 11, L14: layout problems.

- Page 12, L20: There could also be a waste flux at the wall levels,
e.g. for wall split type air conditioners.

- Page 12, L22-24: these details about BUBBLE should not be
in the general model description section.

- Page 13, L1-12. There are many repetitions in this part of the text,
which might be reduced by reorganisation.

- Page 13, L25: "in Bueno"?

- Page 14, L9: "Trees cannot be higher than the buildings".

- Page 15, L19: "in a typical"?

- Page 17, L16 delete "now".

- Page 18
- Average building height
- Width of street canyon
- Rural Bowen ratio

- Page 23, L22: Usually the drag force is proportional to lamba_f,
and not lambda_p. Only the drag due to the friction on the roofs
depends directly on lambda_p, but it should be small compared
to the drag force due to the vertical walls.

- Page 27: layout problems in Table 2.

- Page 27, L15: "Radiation configuration" sounds as if different
options of the radiation model would be tested. But this seems
not to be the case. The radiation model is the same, but the input
data like the orientation of the street canyon are changed.

- Page 27, L17: "to investigate"

- Page 29, L4: "This is expected".

- Page 29, L9: "loose".

- Page 32, L4-5. I dont understand this way of reporting the results.
If it is BIAS, RMSE and R2 for minimum and maximum UHI, why are
there only 3 numbers reported?

- Figure 14: would it be possible to display the same figure for
the observed values (e.g. in a two panel figure)?

- Page 33, L7: The phrase "Appropriate input parameters are used"
is not very specific.

- Section 3.2.6. Given the strong daily cycle of the UHI it does not
seem very appropriate to compare diurnally-averaged UHI values.

- Page 33, L12: "predicts".

- Page 35, L4: "shows".

- Page 36, L30: This "BIAS +- RMSE" way of reporting the results is a
bit misleading since it looks as if some kind of statistical uncertainty
of the bias would be reported, although it is a different metric
(the RMSE) that is reported.

- Page 36, L31: and what about the RMSE?

- Page 37, L7: "wind speed".

- Page 37, L17: "reveals".

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ED: Reconsider after major revisions (08 Oct 2020) by Wolfgang Kurtz

AR by Amir A. Aliabadi on behalf of the Authors (09 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Nov 2020) by Wolfgang Kurtz

RR by Anonymous Referee #4 (01 Dec 2020)

Suggestions for revision or reasons for rejection

Review of the revised manuscript "The Vertical CityWeather Generator
(VCWG v1.3.0)" by Moradi et al.

The authors have well taken into account the comments by the two
reviewers of this review round. The following minor issues remain.

- For the roughness length for temperature and humidity, you state
that "a typical value" (it is rather a formulation than a value) "is
often used" / "often assumed". But what do you use in the present manuscript?
Probably the formulation you state, but it is not clear enough.

- I do not like the approach to calculate a combined BIAS, RMSE, R2
for minimum and maximum UHI, since the performance of the model
might be very different for minimum and maximum UHI.

- I am a bit surprised about the daily average UHI observations.
Are you sure that it is not just the nocturnal UHI that is observed.
Given that the UHI has a strong daily variation, it should not
be the daily average that is reported, but rather the nocturnal UHI.
Maybe it would be better to focus on a subset of observations for
which it is actually possible to evaluate the daily cylcle of the UHI.

- Page 2, Line 18: I find this formulation a bit strange : "some
efforts have begun by investigators...".

- Page 10, Line 3: "this expression should be integrated". This is not very clear.

- Page 15, Line 30. It seems as if you did one simulation
for each month. But why is this necessary, given that
the simulations are very quick? Only a spin-up
of about 3 days at the beginning would be required.
In addition, I find that the spin-up should be rather 2-3 days
than 1 day.

- Page 16, Lines 12-22. The writing style is very repetitive
and could be synthetised, perhaps even using a small table.
The values for R2 will also be very good if the evaluation is
made for an entire year, since the large annual cycle of temperature
is obviously well reproduced by the model.

- Page 16, Line 26: 'similar level of success' -> "similar skill".

- Page 16, Line 28: "well-behaved" -> "well captured"

- Figure 3: Scatterplots for the entire year are not very useful,
since due to the large annual temperature amplitude, it is clear
that the agreement between the model and the observations will be good.

- Page 20, Line 5. Not "overpredicts the observations", but
"overpredicts the values"

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RR by Anonymous Referee #3 (04 Dec 2020)

ED: Reconsider after major revisions (19 Dec 2020) by Wolfgang Kurtz

AR by Amir A. Aliabadi on behalf of the Authors (23 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (10 Jan 2021) by Wolfgang Kurtz

AR by Amir A. Aliabadi on behalf of the Authors (13 Jan 2021) Manuscript

Short summary

The Vertical City Weather Generator (VCWG) is an urban microclimate model developed to predict temporal and vertical variation of potential temperature, wind speed, and specific humidity. VCWG is forced by climate variables at a nearby rural site and coupled to radiation and building energy models. VCWG is evaluated against field observations of the BUBBLE campaign. It is run under exploration mode to assess its performance given urban characteristics, seasonal variations, and climate zones.