Abstract
Abstract. How clouds change in a warmer climate remains one of the largest uncertainties for the equilibrium climate sensitivity (ECS). While a large spread in the cloud feedback arises from low-level clouds, it was recently shown that mixed-phase clouds are also important for ECS. If mixed-phase clouds in the current climate contain too few supercooled cloud droplets, too much ice will change to liquid water in a warmer climate. As shown by Tan et al. (2016), this overestimates the negative cloud-phase feedback and underestimates ECS in the CAM global climate model (GCM). Here we use the newest version of the ECHAM6-HAM2 GCM to investigate the importance of mixed-phase and ice clouds for ECS. Although we also considerably underestimate the fraction of supercooled liquid water globally in the reference version of the ECHAM6-HAM2 GCM, we do not obtain increases in ECS in simulations with more supercooled liquid water in the present-day climate, different from the findings by Tan et al. (2016). We hypothesize that it is not the global supercooled liquid water fraction that matters, but only how well low- and mid-level mixed-phase clouds with cloud-top temperatures in the mixed-phase temperature range between 0 and −35 ∘C that are not shielded by higher-lying ice clouds are simulated. These occur most frequently in midlatitudes, in particular over the Southern Ocean where they determine the amount of absorbed shortwave radiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over the Southern Ocean is only significantly overestimated if all clouds below 0 ∘C consist exclusively of ice. Only in this simulation is ECS significantly smaller than in all other simulations and the cloud optical depth feedback is the dominant cloud feedback. In all other simulations, the cloud optical depth feedback is weak and changes in cloud feedbacks associated with cloud amount and cloud-top pressure dominate the overall cloud feedback. However, apart from the simulation with only ice below 0 ∘C, differences in the overall cloud feedback are not translated into differences in ECS in our model. This insensitivity to the cloud feedback in our model is explained with compensating effects in the clear sky.
Highlights
Changes in clouds remain one of the largest uncertainties for the calculation of the response of the climate system to a given radiative forcing F (Dufresne and Bony, 2008)
In order to prove our hypothesis for the different relationship between SLF and equilibrium climate sensitivity (ECS) in ECHAM6-HAM2, we evaluate the changes in cloud fraction as a function of cloud-top www.atmos-chem-phys.net/18/8807/2018/
In this study we used the newly developed ECHAM6.3HAM2.3 coupled global aerosol–climate model to assess the influence of different cloud processes for ECS
Summary
Changes in clouds remain one of the largest uncertainties for the calculation of the response of the climate system to a given radiative forcing F (Dufresne and Bony, 2008). ECS is obtained from coupled atmosphere– ocean simulations that are abruptly exposed to a CO2 doubling relative to preindustrial concentrations and run until a new equilibrium of the climate system has been established This requires coupling of the atmosphere GCM to a fully coupled dynamic ocean model (e.g., Gregory et al, 2004; Flato et al, 2013). In addition, the cloud water path increases in a warmer climate, τ will be further enlarged All of these aspects result in a negative COD feedback. They found SLF to be systematically underestimated with respect to CALIOP observations, i.e., too much condensate to be in the form of ice at these temperatures This led to a too negative cloud-phase feedback and in the CAM5 GCM to a too low ECS. Stand how universal the findings of the change in ECS using extreme assumptions about the liquid water/ice phase in mixed-phase clouds of Tan et al (2016) are, here we use the ECHAM6-HAM2 GCM to calculate the impact of mixedphase and ice clouds on ECS
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