General Comments:



The paper outlines the developments under GEOS-Chem version 13 series to increase the

accessibility, accuracy, and capabilities of the global atmspheric chemistry model focusing on the new

generation high-performance GEOS-Chem (GCHP). Overall, a number of significant

improvements are described and quantified spanning the model performance, ease

of use, and resolution/accuracy.

The Introduction section provides an overview fundamental concepts with

respect to atmospheric composition modelling, with particular focus on

Chemical Transport Models and the development of GEOS-Chem.

In general there is a lot of repeition of listing the novelties at the end of

the Introduction, again in Section 4 (in particular in Table 1) and in the beginning of different

sections (e.g. Sec.7). The paper can be streamlined by making a single listing and then

directly going into the details in sub-sections.

Section 6 can perhaps benefit in clarity by being shortened and made more succinct, 

or even removed altogether/moved to a supplement as it contains software

engineering details that are esoteric and may not be of relevance to a general

audience, and elaborates on problems of previous releases.

The the end of Sec. 7, some details on what was removed, or a specific

reference to the relevant documentation section/site should be added.

Finally, it is proposed to restructure the manuscript to have the very

important Sections on the numerical calculations and model quality

developments and performance assessment first (Secs.8,9), 

and then follow with the software engineering (containers, build system) aspects.

Specific Comments:

Comments are prepended by page and line number of the pre-print.

p.4 l.101-104: Please consider rephrasing the sentences to make the meaning

more clear (in particular the transport processes).

p.6 l.6: Not clear what an "unsatisfied" import is. Please explain briefly in text.

p.6 l.159-161: The paragraph is not providing specific information. Authors

could elabborate on specific gaps in ESMF. Does MAPL presently only provide an

additional regridding method?

Sec. 3.2: Perhaps a table with a synopsis of the resolution, grid-type, time

span/step, for the different analyses would assist the reader.

p.8 l.211: Instead of just stating "20 model days on 2304 cores", the relative improvement

on equivalent hardware would better showcase performance gains. Or even better

refer to to the relevant Section 7.5 (Fig.3).

p.9. l.257: "The original version of GCHP was implemented as a code

repository" - meaning not clear, please rephrase: Do you mean implemented as separate code

base?

Fig.4: some results are cut/not visible in the left panel. Though it's clear

it's under a minute, the presentation can be improved.

Fig.5 (and Sec.8.1) Is the error in units of Pa or hPa?

p.18 l.485: Can the authors quantify the approximate magnitude of the

error/improvement regarding mositure in air mass flux?

Editorial comments:

Web addresses should be added as references instead of inline.

p.6 l.168: surface quantities -> surface variables

p.8 l.208: opportunities -> capabilities

p.8 l.212: actual time -> wall-clock time

p.8 l.226: use of winds -> use of wind fields

p.8 l.227:  to and from a latitude-longitude grid to and from cubed-sphere

grid -> between latitude-longitude and cubed-sphere grids; and similar for

restaggering

p.8 l.234: FlexGrid is only for latitude-longitude -> FlexGrid only supports latitude-longitude

p.9 l.237: Remove nimble

Sec. 6 title: Software collaboration -> emgineering

p.20 l.501: vector compnents typo

p.20 l.505: familiar -> straightforward/simple/elementary