A new WRF-Chem treatment for studying regional-scale impacts of cloud processes on aerosol and trace gases in parameterized cumuli

Berg, L. K.; Shrivastava, M.; Easter, R. C.; Fast, J. D.; Chapman, E. G.; Liu, Y.; Ferrare, R. A.

doi:https://doi.org/10.5194/gmd-8-409-2015

Articles | Volume 8, issue 2

https://doi.org/10.5194/gmd-8-409-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue:

The community version of the Weather Research and Forecasting...

https://doi.org/10.5194/gmd-8-409-2015

© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 8, issue 2

Development and technical paper

|

24 Feb 2015

Development and technical paper |

| 24 Feb 2015

A new WRF-Chem treatment for studying regional-scale impacts of cloud processes on aerosol and trace gases in parameterized cumuli

L. K. Berg, M. Shrivastava, R. C. Easter, J. D. Fast, E. G. Chapman, Y. Liu, and R. A. Ferrare

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Final revised paper (published on 24 Feb 2015)
Preprint (discussion started on 29 Apr 2014)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC C616: 'gmdd-7-2651-2014', Anonymous Referee #1, 23 May 2014
- AC C2042: 'Response to Referee #1', L. K. Berg, 17 Oct 2014
RC C1214: 'Referee comment on "A New WRF-Chem Treatment for Studying Regional Scale Impacts of Cloud-Aerosol Interactions in Parameterized Cumuli", Berg et al., 2014', Anonymous Referee #2, 22 Jul 2014
- AC C2043: 'Response to Referee #2', L. K. Berg, 17 Oct 2014

Peer-review completion

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

AR by L. K. Berg on behalf of the Authors (17 Oct 2014) Manuscript

ED: Referee Nomination & Report Request started (24 Oct 2014) by Tim Butler

RR by Anonymous Referee #1 (03 Nov 2014)

Suggestions for revision or reasons for rejection

Berg et al. have addressed some of the criticisms by the referees. Yet, I find that they have failed on two major issues. Firstly, Berg et al. have not sufficiently addressed my earlier criticisms regarding the model formulation in the manuscript (see details below). Secondly, the revised manuscript does not convince me that the simulations with the modified model yield results that are closer to the observations. In the light of the rather obvious problems with the model formulation in combination with a lack of convincing results, I recommend against final publication in GMD. I recommend to improve the treatment of aerosol-cloud interaction and of trace gas scavenging.

Major comments

In their response, Berg et al. state that "[t]he manuscript has been modified to include 'When air is detrained from sub-grid convective clouds, any detrained cloud-borne aerosol is added to the grid resolved interstitial aerosol in that model grid box where the aerosol can potentially interact with resolved clouds.' While this statement clarifies what is done in the model, I still think that this is inappropriate. Convective updrafts detrain liquid water or ice to the environment. Thus, it does not make sense to automatically release tracers that have previously been taken up. This seems especially important since no new uptake on ice is parameterized. Furthermore, assuming that no moderately soluble tracer reaches the upper troposphere would also be wrong. I think that what is needed here is a change in the model formulation, even if this does not have too big of an impact on the case studied by Berg et al. Typical WRF users are not always aware of the model's limitations when doing their own case studies, so it would be best to avoid such unnecessary problems from the beginning.

Regarding my criticism of the lack of uptake by rain drops, Berg et al. argue:

"We agree that the uptake of gases by rain should ultimately be included in treatment of
cloud effects on aerosol and gases. In this work, however, we have neglected this process
because of its relative importance compared to uptake in cloud. Within clouds, the wet
removal associated with cloud drops is larger than raindrops due to the much larger
surface area of cloud drops compared to the larger, but less plentiful, raindrops. "

I can think of several studies with results that point to a rather important role of trace gas scavenging by rain drops.

Furthermore, in defense of their treatment of trace gas transport and scavenging, Berg et al. state that their "parameterization was designed with climate-relevant cloud-aerosol interactions in mind, and less attention on the wet removal of gases." However, as the other referee has rightly pointed out in the very first comment, the influence of aerosols on precipitation formation is not taken into account in the work by Berg et al.This process is included in many other global and regional models and is fairly straight forward to implement. While one could turn it off for the sake of specific sensitivity studies, in my opinion it should be implemented and tested before including the code in a public release of WRF. All in all, I do not think that the code should be released to the public in its present form.

Finally, the newly added Figures 7 and 8 fail to show how the model results compare to the observations.

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RR by Anonymous Referee #3 (24 Nov 2014)

RR by Anonymous Referee #2 (18 Dec 2014)

ED: Publish subject to minor revisions (Editor review) (14 Jan 2015) by Tim Butler

The authors have clearly put a lot of work into the revised version of this manuscript, resulting in an improved version which addresses many of the concerns expressed by the anonymous referees in the open discussion phase of the manuscript. Referee #2, as well as an additional referee (#3) have both recommended to accept the manuscript subject to a number of minor revisions and technical corrections. Referee #1 is however not satisfied with the revisions, and recommends rejection of the manuscript, mostly on the grounds that the model formulation is not appropriate since it omits some key physical processes.

The authors argue that the manuscript falls within the scope of GMD as a technical paper describing these particular model developments, and that such a detailed description of their new treatment is of value to the community of users who may be considering using it in their simulations. Referee #1 recommends against releasing the model code in its present form due to the many shortcomings they have pointed out. My decision about whether to accept or reject this manuscript for publication in GMD is however not the same as the decision about whether or not the WRF maintainers should release this model code to the public. The code may well be released anyway, and I think it will be valuable to the community to have this manuscript published in GMD along with all of the criticism which has been made of the model formulation by the referees. It is the responsibility of all model users to determine whether or not the model they are using is appropriate for the intended application, and I believe that publication of this manuscript will help to enable this, as long as the criticisms of referee #1 are clearly acknowledged.

As well as responding to the minor and technical comments of referees #2 and #3, I would like to ask the authors to respond to the comments of referee #1 by adding additional text, in appropriate points in the manuscript, clearly pointing out that several important physical processes are missing or not well implemented in the current model version. Tracer release during detrainment, scavenging by raindrops, and the influence of aerosols on precipitation formation are mentioned by referee #1 as key shortcomings of the current model version, and these should be more clearly acknowledged as such by the authors before the manuscript is finally published in GMD.

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AR by L. K. Berg on behalf of the Authors (23 Jan 2015) Author's response Manuscript

ED: Publish as is (28 Jan 2015) by Tim Butler

AR by L. K. Berg on behalf of the Authors (29 Jan 2015)

Short summary

This work presents a new methodology for representing regional-scale impacts of cloud processing on both aerosol and trace gases in sub-grid-scale convective clouds. Using the new methodology, we can better simulate the aerosol lifecycle over large areas. The results presented in this work highlight the potential change in column-integrated amounts of black carbon, organic aerosol, and sulfate aerosol, which were found to range from -50% for black carbon to +40% for sulfate.