The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2
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 i...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-06-01
|
| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/8807/2018/acp-18-8807-2018.pdf |
| Summary: | 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.<br><br>
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. |
|---|---|
| ISSN: | 1680-7316 1680-7324 |