Preprints
https://doi.org/10.5194/egusphere-2025-3384
https://doi.org/10.5194/egusphere-2025-3384
17 Jul 2025
 | 17 Jul 2025
Status: this preprint is open for discussion and under review for Weather and Climate Dynamics (WCD).

Entrainment and the tropical tropospheric thermal structure in global climate models

Lucinda Anne Palmer and Martin Simran Singh

Abstract. The observed relationship between stability and humidity in the tropical troposphere has been argued to be strongly influenced by moist convective entrainment (Palmer and Singh, 2024). In this study, we investigate this relationship in fourteen models from phase 6 of the Coupled Model Intercomparison Project with the aim of evaluating their representation of such entrainment processes.

We define a diagnostic of convective entrainment using the climatological slope of the relationship between measures of lower-tropospheric stability and humidity in precipitating regions of the tropics. While some models reproduce the sign of this slope as estimated from reanalyses, others produce weak or opposing relationships between stability and humidity, implying unphysical entrainment rates. We relate these contrasting behaviours to aspects of the models’ convection schemes; models that employ plume-based cloud models and traditional “CAPE” closures, where convection is assumed to remove cloud buoyancy over a specified timescale, tend to better reproduce reanalyses.

We also explore the use of the stability-humidity relationship to constrain projections of extremes in convective available potential energy (CAPE) and boundary-layer moist static energy (MSE). These quantities have been argued to be influenced by convective entrainment and are relevant to intense thunderstorms and humid heatwaves, respectively. We find that models that quantitatively reproduce the stability-humidity relationship in reanalyses tend to produce higher increases in CAPE and boundary-layer MSE under warming. However, due to observational uncertainties and model scatter, no strong emergent constraint is found.

Competing interests: At least one of the (co-)authors is a member of the editorial board of Weather and Climate Dynamics.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
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Lucinda Anne Palmer and Martin Simran Singh

Status: open (until 20 Sep 2025)

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  • RC1: 'Comment on egusphere-2025-3384', Anonymous Referee #1, 14 Aug 2025 reply
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Lucinda Anne Palmer and Martin Simran Singh
Lucinda Anne Palmer and Martin Simran Singh

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Short summary
This study investigates the relationship between stability and humidity in the tropical troposphere in global climate models. We develop a method for quantifying the relationship and find varying relationships that are similar or even opposite to observations. We theorise that measures of intense thunderstorms and humid heatwaves are influenced by the stability-humidity relationship. Models that have a relationship like observations project greater increases in these measures under warming.
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