Review for “Scaling Artificial Heat Islands to Enhance Precipitation in Arid Regions”

In their manuscript “Scaling Artificial Heat Islands to Enhance Precipitation in Arid Regions”, the authors present analysis of several numerical simulations exploring the impact of decreased land surface albedo in squares of various sizes over two desert regions of the United Arab Emirates. They find that, when the imposed squares of low albedo land are larger than 20 km per side, there is a statistically significant increase in precipitation in the surrounding region. The simulations are conducted using the Weather Research and Forecasting model, with boundary conditions forced for four distinct weather events in the region. The methodology is appropriate for the aims of the study, and the topic is appropriate for GMD. I have a few addressable (but still important) concerns, outlined below, as well as several minor comments. Following revision, this manuscript would be a valuable addition to the literature.

Major comments:

Line 225: “It is not linear, but more exponential in appearance – which is reasonable, given the areal size increments.” --- I wouldn't make this claim unless you back it up with analysis, and you'd want to do the analysis with area of imposed ABS, not width of imposed ABS square (e.g. the 20km box is 4x larger than the 10km box); IS the relationship non-linear with area of forcing? That isn’t at all obvious to me.

Line 240: It seems necessary at this point in the analysis to discuss the *temperature* and *heat stress* impacts of the ABS  - are these locally confined to the region of the ABS or do they extend regionally?  (I wondered if perhaps this would come up later in the paper, but (a) it didn’t and (b) this is where I felt like I wanted to see it addressed.)



Line 250: “As a result, net-shortwave radiation absorbed at the surface is increased by 25% of the incident

solar radiation, and this extra energy is partitioned into turbulent and ground heat fluxes.” ... presumably also into heating the surface which leads to increased LW up. Maybe this is what you mean by "ground heat flux", but typically we think of "ground heat flux" as energy stored by the ground at each timestep, ie that doesn't have to be removed as LW, SH, or LH. But heating up the surface leads to higher LW out , which isn't a turbulent flux.

Minor comments:

Line 16: what do you mean by "one-day cases over a 24 hour period"?

Line 26: how much direct warming comes from the albedo change?

Line 34: specify with albedo *brightening* - will be clear to some but not all, especially since the bulk of the paper is about albedo *darkening*

Line 40: “. Branch et al., 2014, measured albedos of0.17 and 0.12 for jatropha and jojoba plants, and the surrounding desert ~0.3, leading to temperatures up to 4°C higher than the surrounding desert (see also Saaroni et al., 2004). This heating led to greater simulated cloud development and convection initiation (CI) (Branch & Wulfmeyer, 2019).” --- make clear in 2nd sentence you're no longer talking about measured or observed things, but rather a model simulation

Lines 46/47: be consistent - earlier references to albedo would make this "0.05"

Line 47: PV - define (photovoltaic I assume)

Line 51: not just sensible heating, but also high surface temperatures (increasing LW out of the surface through increased surface T)

Line 51: “net total” - net total what? (I assume "net total SW absorbed that goes directly into the local surface energy budget" or something along those lines)

Line 52: Is this equation necessary / especially as an equation on its own line? It doesn’t get used in the rest of the paper, and you don’t actually simulate these combined albedo/energy uptake surfaces, so it is kind of distracting… You could put it in-line and emphasize that it is for background info only, and NOT what you’re going to try to model here.

Line 54: “delta” - This is the "regular" surface albedo, correct?

Line 55: This comment is kind of long and the actual paper doesn’t simulate solar panels taking energy “out” of the surface energy budget, so perhaps consider altering this text to emphasize that it is background information for the reader, and that you *aren’t* going to be trying to capture this in this study! This discussion definitely sent me down a rabbit hole when you were describing your methods, until I realized in the results that this discussion isn’t actually applicable to the actual simulations you ran.



To simulate this, would have to modify the surface energy budget to have a special "energy out" term that is energy production; this would be the most physically consistent way. Instead, it sounds like the authors have imposed surfaces brighter than actaul PV panels, such that the energy that typically would go to energy production is instead reflected away from the surface as SW radiation. For the most part, this probably isn't going to qualtiatively change their answers given the magnitude of changes imposed here. But the atmosphere *does* absorb SW radiation, including SW reflected from the surface, and also altering SW albedo in these low water vapor, relatively cloud-fre desert regions will alter the top of atmosphere energy balance in a way that could influence circulation.

If you instead had the surfaces as dark as they actually are, with an extra term in the surface energy budget that removes energy used for power generation, you'd have to release that energy *somewhere* in a coupled model to conserve energy. In a regional model, though, you could just assume that the energy is moved out of the region you're simulating. I'm not suggesting you re-do your simulations this way - just that you explain what is and is not representative of the actual physical system (real world) in the way youv'e chosen to simulate this.

And of course, if the surface is made artificially dark without any power production - e.g. just painting surfaces dark, which the authors do discuss as one option of ABS - the power / energy conservation thing isn't a concern!

Line 56: What is reducing the cell efficiency with precip? Being low efficiency makes it lower efficiency with time? Or getting rained on makes it lower efficiency with time?

Line 84: Is "material" really appropriate here? Maybe "Model and Methods"?

Line 87: specify the region (title just says "arid regions", and intro discussed the Middle Eastern Gulf region, but here specify that that is indeed where you're going to simulate, and maybe modify the title to reflect the actual region of study – it would be a leap to extrapolate from this analysis to all arid regions, as the background flow and moisture sources are pretty critical to the results).  

Line 90: in the same domain? Please specify.

Line 92: MYNN - what is this?

Line 98: I know the choice of convection scheme needs to be specific to the model resolution, but am not up-to-speed enough with the WRF convective scheme options to know what the appropriate choice here is. Also, please specify which scheme you used.

Line 105: are you still talking about the outside of domain forcing? Clarify. Sounds like you're talking about the model, but the land surface you use in the model domain is NOAH-MP, right?

Figure 1: what is the 899x699x100? number of x, y, and z grid cells?

Figure 1: Could you please make ocean a different colour than low (0-400m) land?

Line 128-131: this discussion is a great orientation to the base-state of the region! nice!

Line 136: what four selected cases? To help the reader not get confused, it would be helpful if before this point, you say that you're going to do, for each ABS box, 4 different runs of YYYY days each, each forced with weather conditions from <insert time periods here>

Line 137: “Typical CI…” --- Define (I assume "convective initiation")

Figure 2: Adding a map, either as a separate figure or in one of figure 1 or 2, that shows the prevailing near-surface and aloft wind directions (on the map) for these two time slices would be really helpful. (Figure 3 has this for 10am, I assume near the surface? But maybe not near the surface – not specified!)

Figure 3: what are 18.07, 24.07, 25.07, and 27.07? Can you write "July 18", "July 24", etc instead? otherwise this reads like a number and the reader gets confused about what the number means (when in fact it is a date)

Figure 3: figure 3 b, c, d - make the reference vector larger, like in panel a --- otherwise it is too tiny for my poor eyes...

Figure 3: what level are these winds from? surface?

Figure 4: Again, please label with "July 18, July 24" etc. What are the arrows for? Wind vectors on the control would be helpful, but I don't think that is what the arrows are for here?

Figure 4: Inconsistency between text (line 180) and figure caption. Do you calculate the precipitation response in a circle of radius 90 km (ie diameter 180 km), as on line 180, or a circle of diameter 150 km (radius 75 km) as in the caption of figure 4?

Line 200: panel e of what? Sounds like you're talking about figure 5 in this paragraph, but figure 5 only has panels a-d.

Figure 5: clarify "millions of m3" somewhere

Figure 5: again, please write "July 18, July 24..."

Line 207: clarify this is combined accumulated precipitation volume for *both* regions

Line 207: “…UAE per capita supply based on 500-liter capita-1 day-1 (right axis, people yr-1), amongst the

highest in the world” --- I read this as the UAE having the highest per capita supply of water in the world... is that right? That is not intuitive to me! Or is it the volume of supple that is high? Please clarify.

Line 212: “There is a small surplus on 24 July though.” --- Just for the 50 km case, right? No, I'm confused - what do you mean "surplus"? I don't see the demand listed anywhere on thesee plots, so what does a "surplus" mean?

Figure 6: So, Panel C is showing that the 50 km blobs of ABS lead to almost a 50% increase in precipitation, is that right? That seems worth highlighting!!!

Line 258: influence of what from the surrounding what?

Figure 7: it would be helpful to highlight the zero line here, e.g. with a darker horizonal line - at least in the pdf I have, there is no / almost no line visible at 0, 50, and 300. The one at 0 is particularly important to have though, and to make stand out more than the rest!

Text/Grammatical:

Line 30: “are become” - "are becoming" or "have become"

Line 258: “flu” -> “flux”

I have attached a PDF with my review comments. Thanks!