the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The importance of plant-water stress for predictions of ground-level ozone in a warm world
Tamara Emmerichs
Yen-Sen Lu
Domenico Taraborrelli
Abstract. Evapotranspiration is important for Earth’s water and energy cycles as it strongly affects air temperature, cloud
cover and precipitation. Leaf stomata are the conduit of transpiration and thus their opening is sensitive to weather and climate
conditions. This feedback can exacerbate heat waves and droughts and can play a role in their spatio-temporal propagation.
Therefore, the plant response to available water is a key element mediating vegetation-atmosphere interactions. Sustained high
temperatures strongly favor high ozone levels with significant negative effects on air quality and thus human health. Our study
assesses the process representation of evapotranspiration in the atmospheric chemistry model ECHAM/MESSy. Diverse water
stress parametrizations are implemented in a stomatal model based on CO2 assimilation. The stress factors depend on either
soil moisture or leaf water potential and act directly on photosynthetic activity, mesophyll and stomatal conductance. Overall,
the new functionalities reduce the initial overestimation of evapotranspiration in the model globally by more than one order
of magnitude which is most important in the Southern Hemisphere. The intensity of simulated warm spells over continents
is significantly enhanced. With respect to ozone, we find that a realistic model representation of plant-water stress depresses
uptake by vegetation and enhances its photochemical production in the troposphere. These effects lead to a general increases
in simulated ground-level ozone which is most pronounced in the Southern Hemisphere over the continents. The uncertainties
for plant dynamics representation due to too shallow roots can be addressed by more sophisticated land surface models with
multi-layer soil schemes. In regions with low evaporative loss, however, the representation of precipitation remains the largest
uncertainty.
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Tamara Emmerichs et al.
Status: open (until 12 Dec 2023)
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RC1: 'Comment on egusphere-2023-2306', Anonymous Referee #1, 25 Nov 2023
reply
In this article, the authors explore representations of evapotranspiration in the chemistry-climate model EMAC, and how this impacts air temperature as well as air pollution, focusing mainly on tropospheric O3. The remarks below need to be addressed before the manuscript can be accepted for publication.
This work presented is within the scope of the journal, and I think the results are robust and quite interesting. However, I have two major concerns:
- I think the manuscript has a lot of build-up but then the implications on air quality are not discussed thoroughly. In other words, Section 3.5 needs to be expanded. I listed some suggestions below.
- I had to re-read some text multiple times to understand what the authors wanted to say. I strongly recommend the re-writing of some parts. It would probably be a good idea to send the manuscript for proofreading.
General comments:
While reading the manuscript I came across words written in US English and British English. Please be consistent.
I think the manuscript would benefit from having an “Experimental design” section where you describe the experiment setup in detail. i.e., not just the relevant parameterisations, but also the submodules responsible for the land surface, vegetation, etc.
It is not clear what vegetation representation is being used. It was only later in L357/358 that I realised that you have no vegetation in your model. This should be made more clear. The suggestion above would help to clarify this.
Figure captions are way too short. Generally one should be able to understand the figure completely from the caption without referring to the main text (i.e. self-describing). E.g., Fig. 2 - “Anual mean maximum assimilation” Assimilation of what?
Fig. 3 was not discussed enough in the text. Please update the figure’s caption (see above), and also provide the geographical extent of the areas considered.
I find some of your colour bars confusing. When using a diverging colour bar, please decide whether you want the min/max to be pointy or not, but be consistent! The min/max of the colour bars is misused in most of the figures presented.
Fig. 4 - Fix the colour bar for subplot (a). Caption - Which panels are from EMAC and which are not?
Fig. 8 - Color bars. Why do you have a change in stomatal conductance over deserted regions, e.g., the Sahara desert? Please explain this, and if the values are negligible consider applying a mask.
Section 3.5: In the supplement, you show a strong decrease in isoprene mixing ratios. Could you also show the absolute difference not just the relative (%) change?
From Fig. 6 I would expect a strong increase in isoprene emissions given the increase in surface temperatures. Would be nice to dig in deeper here and quantify the increase in surface isoprene emissions and see how this increase compares to the decrease in isoprene concentrations from enhanced OH oxidation. Also, provide details on the isoprene emissions and sensitivities. Are you using MEGAN?
Fig. S1: In some places over Africa (e.g. 20-30 deg S), you show a decrease in the OH radicals but this does not correspond to an increase in isoprene. Why is that? This seems to violate your hypothesis that isoprene concentrations go lower because of increased loss by OH. Similarly, could you explain the hotspot (increase in isoprene) over Antarctica where there are no apparent corresponding changes in OH?
Please include some limitations in your study. E.g. No vegetation representation, and the fact that the biosphere and BVOC emissions do not respond to changes in tropospheric chemistry e.g. ozone concentrations.
Specific comments:
L26: When writing “e.g.” for citations, it is generally expected that you mention more than one study. Please correct all other instances in the manuscript.
L29: ”(plant’s pores)” - I don’t think this is needed here.
L36/37: “Currently” should go at the start of the sentence.
L51: “non-stomatal processes in plants” - like what?
L61: “GLEAM” - Is this an acronym? Please define acronyms on their first instance.
L61: More details are needed on the EUMETSAT satellite you are referring to.
L90: Include unit for LAI m2/m2.
L98: Latter not “later”
164: EUMETSAT was already used. Please define in the first instance.
L184/85: Not clear what you mean here. Do you mean the sum of the bare soil, short/tall vegetation evapotranspiration per grid box?
L193: mmday-1 - why italic?
L246/253: Text not clear. Please consider rewriting this part. “Saharian” - do you mean Sahara desert?
L250: The distribution of transpiration inf Fig. 2b follows patterns of vegetation distribution e.g. LAI. Why would the additional soil moisture in the tropics explain the geographical distribution?
L302: “African desert” - Sahara desert
L308: “...in the southern part of South America …..”
L344: “to to”
L352: “... respective changes in tropospheric ozone….”
L392/393: Provide more details on the “bucket model” in EMAC or JSBACH.
L394: JSBACH - define the model acronym.
L405: “...(LSMs) generally agree with…”
L414: Double brackets.
L401/402: Give more details on the ‘ozone-climate penalty’. What is the benefit suggested in Zanis et al. 2022?
Citation: https://doi.org/10.5194/egusphere-2023-2306-RC1
Tamara Emmerichs et al.
Tamara Emmerichs et al.
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