Preprints
https://doi.org/10.5194/egusphere-2023-227
https://doi.org/10.5194/egusphere-2023-227
06 Mar 2023
 | 06 Mar 2023

Rainbows and Climate Change: A tutorial on climate model diagnostics and parameterization

Andrew Gettelman

Abstract. Earth System Models (ESMs) must represent processes below the grid scale of a model using representations (parameterizations) of physical and chemical processes. As a tutorial exercise to understand diagnostics and parameterization, this work presents a representation of rainbows for an ESM: the Community Earth System Model version 2 (CESM2). Using the 'state' of the model, basic physical laws, and some assumptions, we generate a representation of this unique optical phenomena as a diagnostic output. Rainbow occurrence and it's possible changes are related to cloud occurrence and rain formation which are critical uncertainties for climate change prediction. The work highlights issues which are typical of many diagnostics parameterizations such as assumptions, uncertain parameters and the difficulty of evaluation against uncertain observations. Results agree qualitatively with limited available global 'observations' of Rainbows. Rainbows are seen in expected locations in the sub-tropics over the ocean where broken clouds and frequent precipitation occurs. The diurnal peak is in the morning over ocean and in the evening over land. The representation of rainbows is found to be quantitatively sensitive to the assumed amount of cloudiness and the amount of stratiform rain. Rainbows are projected to have decreased, mostly in the Northern Hemisphere, due to aerosol pollution effects increasing cloud coverage since 1850. In the future, continued climate change is projected to decrease cloud cover, associated with a positive cloud feedback. As a result the rainbow diagnostic projects that rainbows will increase in the future, with the largest changes at mid-latitudes. The diagnostic may be useful for assessing cloud parameterizations, and is an exercise in how to build and test parameterizations of atmospheric phenomena.

Journal article(s) based on this preprint

01 Sep 2023
Rainbows and climate change: a tutorial on climate model diagnostics and parameterization
Andrew Gettelman
Geosci. Model Dev., 16, 4937–4956, https://doi.org/10.5194/gmd-16-4937-2023,https://doi.org/10.5194/gmd-16-4937-2023, 2023
Short summary

Andrew Gettelman

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-227', Anonymous Referee #1, 18 Apr 2023
  • RC2: 'Comment on egusphere-2023-227', Anonymous Referee #2, 23 May 2023
  • AC1: 'Replies to Reviewers', Andrew Gettelman, 27 Jun 2023

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-227', Anonymous Referee #1, 18 Apr 2023
  • RC2: 'Comment on egusphere-2023-227', Anonymous Referee #2, 23 May 2023
  • AC1: 'Replies to Reviewers', Andrew Gettelman, 27 Jun 2023

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload
AR by Andrew Gettelman on behalf of the Authors (27 Jun 2023)  Author's response   Author's tracked changes   Manuscript 
ED: Publish subject to technical corrections (03 Jul 2023) by Martine Michou
AR by Andrew Gettelman on behalf of the Authors (05 Jul 2023)  Author's response   Manuscript 

Journal article(s) based on this preprint

01 Sep 2023
Rainbows and climate change: a tutorial on climate model diagnostics and parameterization
Andrew Gettelman
Geosci. Model Dev., 16, 4937–4956, https://doi.org/10.5194/gmd-16-4937-2023,https://doi.org/10.5194/gmd-16-4937-2023, 2023
Short summary

Andrew Gettelman

Data sets

Data and Code used in this Manuscript Andre Gettelman https://doi.org/10.5281/zenodo.7391777

Andrew Gettelman

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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Short summary
A representation of rainbows is developed for a climate model. The diagnostic raises many common issues. Simulated rainbows are evaluated against limited observations. The pattern of rainbows in the model matches observations and theory about when and where rainbows are most common. The diagnostic is used to assess the past and future state of rainbows. Changes to clouds from climate change are expected to increase rainbows as cloud cover decreases in a warmer world.