Shallow Cumulus Cloud Feedback in Large Eddy Simulations – Bridging the Gap to Storm Resolving Models

Abstract

Abstract. The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty to interpretations and projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Non-hydrostatic Large Eddy Model in a control and a perturbed 4 K warmed climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened the basic state cloud fraction increases substantially, especially at cloud base, lateral mixing is weaker and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of basic state cloud fraction, thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapor availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm resolving models may exaggerate the trade wind cumulus cloud feedback.

Strong cloud–circulation coupling explains weak trade cumulus feedback

“…In the spirit of the storyline approach for constraining equilibrium climate sensitivity 10 , our findings thus refute an important line of evidence for a strong positive cloud feedback and thus a large climate sensitivity. The EUREC 4 A observations therefore support recent satellite-derived constraints from observed natural variability 37,40 and climate-change experiments using idealized highresolution simulations 41,42 , which suggest that a weak trade cumulus feedback is more plausible than a strong one. Moreover, for the first time, we take into account all types of cloud present in the trades, including the optically thinnest ones that are usually missed in satellite observations 43 , and consider the full range of mesoscale variability that was not represented in idealized simulations of cloud feedbacks.…”

Satellite Observed Sensitivity of Tropical Clouds and Moisture to Sea Surface Temperature on Various Time and Space Scales: 2. Focus on Marine Low Level Clouds

“…We find that the greatest (smallest) reduction of C OPAQUE in the model warmer climate in regions where we have the greatest (smallest) C OPAQUE in the model current climate is consistent with the LES study by Radtke et al. (2021).…”

“…Meanwhile, C THIN decreases (−1.6%/K) while Z THIN rises (+60 m/K). The rising Z THIN is consistent with LES discussion about rising altitudes (Radtke et al., 2021; Rieck et al., 2012).…”

A Library of Large‐Eddy Simulations Forced by Global Climate Models

“…One advantage of the forcing framework is that it can be applied to generate LES of changed climates. While previous studies have used LES to simulate cloud responses to idealized climate perturbations (e.g., Blossey et al., 2013; Blossey et al., 2016; Bretherton et al., 2013; Radtke et al., 2021; Tan et al., 2017), driving LES with a GCM allows more realistic representation of changes in large‐scale forcings. In this study, we run a set of simulations with large‐scale forcings from the AMIP4K experiment, where SST is uniformly increased by 4 K.…”