The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2

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 mixedphase 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.

Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

“…Until recently, this bias has been common to many climate models, due to the lack of supercooled liquid cloud (Bodas‐Salcedo et al, , ). The improvement in the simulation of supercooled liquid cloud is expected to reduce the strength of the negative optical depth feedback in the Southern Ocean (Tan et al, ), although the effect of the reduction of this bias may depend on the climatology of cloud vertical structure of each model (Bodas‐Salcedo, ; Lohmann & Neubauer, ).…”

“…liquid cloud (Bodas-Salcedo et al, 2014, 2016a. The improvement in the simulation of supercooled liquid cloud is expected to reduce the strength of the negative optical depth feedback in the Southern Ocean , although the effect of the reduction of this bias may depend on the climatology of cloud vertical structure of each model (Bodas-Salcedo, 2018;Lohmann & Neubauer, 2018). Figure 5 shows the zonal means of the net cloud feedback and its decomposition into SW and LW components for the two baseline experiments and for combinations of the Mic_On, Aer_On, and Erf_On packages.…”

“…GA7.1 shows a substantial increase in the climatology of liquid cloud fraction, bringing it closer to the observations, which supports the hypothesis that a better representation of supercooled liquid cloud should lead to a reduction in the negative SW feedback in this region. However, this response is not necessarily robust across models (Lohmann & Neubauer, ).…”

Evidence of Strong Contributions From Mixed‐Phase Clouds to Arctic Climate Change

“…We note that the conclusions drawn from the analysis of the results in this study were based on a single climate model and the extent to which our results hold across different climate models has not yet been tested. A recent study investigating the impact of cloud thermodynamic phase partitioning on ECS using the ECHAM6.3‐HAM2.3 model found that the importance of the cloud phase feedback on ECS may be model‐dependent (Lohmann & Neubauer, ). We also point out that we focus on the long‐term (century and longer time scale) impact of SLF on Arctic climate change in our study that occurs after ocean heat adjustment.…”

The global aerosol-climate model ECHAM6.3-HAM2.3 – Part 2: Cloud evaluation, aerosol radiative forcing and climate sensitivity

“…Changes and improvements in E63H23 are also described in Lohmann and Neubauer (2018) and Tegen et al (2018) and are 25 repeated here shortly for the convenience of the reader. From ECHAM6.1 to ECHAM6.3 the following improvements were made:…”