Climate Change Feedbacks in Aquaplanet Experiments With Explicit and Parametrized Convection for Horizontal Resolutions of 2,525 Up to 5 km

Abstract

Earth's equilibrium climate sensitivity (ECS) is the long-term response to doubled atmospheric CO 2 and likely between 1.5 and 4.5 K. Conventional general circulation models do not convincingly narrow down this range, and newly developed nonhydrostatic models with relatively fine horizontal resolutions of a few kilometers have thus far delivered diverse results. Here we use the nonhydrostatic ICON model with the physics package normally used for climate simulations at resolutions as fine as 5 km to study the response to a uniform surface warming in an aquaplanet configuration. We apply the model in two setups: one with convection parametrization employed and one with explicit convection. ICON exhibits a negative total feedback independent of convective representation, thus providing a stable climate with an ECS comparable to other general circulation models, though three interesting new results are found. First, ECS varies little across resolution for both setups but runs with explicit convection have systematically lower ECS than the parametrized case, mainly due to more negative tropical clear-sky longwave feedbacks. These are a consequence of a drier mean state of about 6% relative humidity for explicit convection and less midtropospheric moistening with global warming. Second, shortwave feedbacks switch from positive to negative with increasing resolution, originating foremost in the tropics and high latitudes. Third, the model shows no discernible high cloud area feedback (iris effect) in any configuration. It is possible that ICON's climate model parametrizations applied here are less appropriate for cloud resolving scales, and therefore, ongoing developments aim at implementing a more advanced prognostic cloud microphysics scheme.

A Baseline for Global Weather and Climate Simulations at 1 km Resolution

“…Figures 4a to 4b). This finding is consistent with Retsch et al (2019), who find that an overall drier tropical atmosphere is simulated when deep convection is explicitly simulated in aqua‐planet experiments. However, when deep convection is resolved at 1.4 km, a moistening of the tropical upper troposphere and a drying of the lower tropical troposphere occur when compared to the 9 km parametrized simulation (Figure 4d).…”

“…The 9 km parametrized simulation better matches observations and ERA5. The increase in tropical precipitation in simulations with explicit representation of deep convection is consistent with what is found for the nonhydrostatic aqua‐planet simulations of Retsch et al (2019). Comparing the distribution of precipitation events over 24 hr intervals in DJF2019 (Figure 6b) shows that a higher probability of more extreme rainfall events occurs in the simulations with explicitly resolved deep convection.…”

“…Figure 3b to Figure 3d). The stronger subtropical jets are also observed in nonhydrostatic aquaplanet simulations with the ICON model when the deep convection parametrization—which is similar to the one used in IFS—is switched off (Retsch et al, 2019).…”

“…The ITCZ is also known to sensitively depend on the explicit versus parametrized representation of convection (e.g., Nolan et al, 2016; Retsch et al, 2019; Tomita et al, 2005). Average tropical precipitation rates are often taken as a proxy for the ITCZ.…”

“…Using global models with grid spacing down to 2.5 km, Hohenegger et al (2020) found improvements to the net shortwave radiation, which increases with resolution due to the reduction in low cloud amount over the subtropical oceans. The role of the level of parametrized convection in the context of climate change feedbacks was recently explored by Retsch et al (2019). They performed global aqua‐planet simulations with resolutions between 2,525 and 5 km grid spacing, finding a remarkably stable climate sensitivity regardless of the resolution.…”