Review of GMD-2020-441

“WRF-GC (v2.0): online two-way coupling of WRF (v3.9.1.1) and GEOS-Chem (v12.7.2) for modeling regional atmospheric chemistry-meteorology interactions” By Feng et al. (2021)

General Comments:

The authors developed and presented the WRF-GC model v2.0, an online two-way coupling of the Weather Research and Forecasting (WRF) meteorological model (v.3.9.1.1) and the GEOS-Chem chemical model (v12.7.2). The main feature of the WRF-GC v2.0 includes aerosol-radiation Interactions (ARI) and aerosol-cloud interactions (ACI). The authors evaluated the sensitivity simulations with different combinations of ARI and ACI over East Asia during January 2015 and July 2016 using the WRF-GC v2.0. The manuscript is well structured. The objective and methodology described in the manuscript are clear. However, some revisions on the results (chapter 4 and 5) are required prior to the recommendation. Please refer to the major comments below.

Major comments:

1. Table 1. Why did the authors conduct the sensitivity experiments with ARI and ACI only on Case Summer? If possible, please add the additional sensitivity experiments on Case Winter or one-year test.

1. Chapter 4.3, 4.4, 4.5, 4.6 (Validation of the simulated AOD, LCOD, surface downward shortwave radiation, and air temperature). Among the sensitivity simulations with different combinations of ARI and ACI, the Case ACR (ARI and ACI are both turned on) had the best consistency with the observed values? Author should show the model performance evaluation (MPE) in depth prior to the sensitivity experiments. I believe model sensitivity itself does not tell many things.

1. Overall, this manuscript is missing interpretations of key physical/chemical processes related to ARI and ACI in the WRF-GC model v2.0. In chapter 5, there is no explanation on the impacts of ARI and ACI on the meteorological factors and air pollutants.

1. Considerations of ARI and ACI in the WRF-GC v2.0 can alter a variety of meteorological factors in addition to the surface temperature and shortwave radiation as mentioned in the manuscript. Authors should add the analysis for the impacts of ARI and ACI on other meteorological factors, such as planetary boundary layer, relative humidity, and so on.

1. The results and discussion are too general. What would be benefits of the on-line model applications? If the purpose of the model update or development is to simulate the regional-scale interaction between meteorology and air pollutants, the modeling capability should be thoroughly validated as if other regional photochemical models conventionally do. Model performance evaluations against the surface measurements of the criteria air pollutants including PM2.5 and O3 are recommended. After that, model sensitivities with or without the ARI and ACI can show the results of the interactions.

Specific comments and typos:

Line 407 : Add a comma before the “2015” à during January 8 to 28, 2015.

Line 414 : at 550 nm in July

Line 474 : Please revise the subscripts. (for example, PM^2.5 à PM_2.5)

This is a well-organized and written manuscript. Built on their previous work published on GMD, the authors further developed WRF-GC to enable feedbacks of aerosol-cloud and aerosol-radiation interactions.  The latest model edition also supports the nest-domain capability. Both GEOS-Chem and WRF models are actively developed and embrace a large user base worldwide. Coupling them together is a great contribution to the modeling community. This reviewer recommends publication after minor revisions listed below.

1. In the text, spell out all the acronyms, e.g., WRF-GC, CMAQ, COSMO, SSA, and many more, when they first appear.
2. Line 45, replace “advances” with “advancement”.
3. Table 1, DST4 should be “dust bin 4”.
4. Line 389, change “over than 440 nm” to “over 440 nm”.
5. Line 400, why not include all the monitoring sites within a grid cell for model evaluation?
6. Figure 3: it appears that WRF-GC severely underestimates monthly AOD across most of the domain in comparison with the VIIRS observations. What are the possible reasons? Is it mainly due to emissions, or meteorology (e.g., humidity)? A little in-depth analysis may provide useful information for both model improvement and satellite retrieval.
7. Figure 4: why not converting the modeled AOD to the one at the observed wavelength for an apple-to-apple comparison?
8. Line 451, add “cloud” after “stratocumulus or stratus”.
9. Line 474, change “2.5” in PM2.5 from superscript to subscript.
10. Line 550, change “WRF-GC’se” to “WRF-GC’s”.
11. 27 km resolution has been applied in the study for model evaluation. This resolution is too coarse to resolve cloud processes to which the ACI is important. Some discussion may be necessary to clarify this.
12. The ARI and ACI have been specifically linked to RRTMG scheme and Morrison two-moment microphysics scheme, respectively, in the application. Have the authors considered a more generalized method so that they can easily link to other radiation and microphysics schemes available to WRF?
13. Has WRF-GC already been included in the community WRF release? Or will it be included in the standard WRF release in the near future?

The paper presents a new online two-way coupled model WRF-GC v2.0 based on WRF meteorological model coupled with the GEOS-Chem chemical model including aerosol-radiation interactions (ARI) and aerosol-cloud interactions (ACI) based on bulk aerosol mass and composition, and nesting capability for high-resolution simulations. The authors also analyze chemical feedbacks to meteorology considering ARI and ACI mechanisms.

The paper is interesting and can be suitable for publication, but several comments need to be improved and clarified.

The coupling structure modular, which allows the two parent models to be run off-line or online. This is definitely an advantage. However, it is difficult to classify this model as online-integrated coupled model. According to the definitions (Baklanov et al., 2014) it is still online-access coupling model, like WFR-CMAQ, because the equations of the meteo and chemical transport parts are solved separately, not on the same grid, not simulteneously  in each grid-cell and not on each time step (at least it is not clear from the model description). So, in such way of coupling it is difficult to guarantee the consistency and mass-conservation. Besides, most probably the convection numerical schemes are different and not consistent in WRF and GEOS-Chem models. 

From other side, the model uses a coupler for data transfer from one model to other. Transfer of 3D data on each time step for each grid-cell will take a lot of time, that makes the modelling system substantially slower in comparison with the fully online integrating approach. For example, ECMWF demonstrated that when they switched from the online-access version IFS-MOZART to the online integrated C-IFS (Huijnen et al., 2010), the modeling system became much faster. 

So, it would be important to demonstrate the consistency tests and the effectiveness of the suggested way of the coupling.