On the Impact of Cloud Phase on High-Latitude Cloud-Climate-Feedbacks and Arctic Amplification (Invited Presentation)

Despite the growing effort in improving cloud microphysical schemes in global climate models (GCMs), little effort has so far focused on improving the ability of GCMs to accurately simulate phase partitioning in mixed-phase clouds.  Here, we argue that getting the relative proportion of liquid droplets and ice crystals in clouds right in GCMs is critical for the representation of cloud radiative forcings and cloud-climate feedbacks.  We first present satellite observations of cloud phase obtained by NASA’s CALIOP instrument, and report on robust statistical relationships between cloud phase and several aerosols species that have been demonstrated to act as ice nuclei (IN) in laboratory studies. We then report on results from model intercomparison projects that reveal that GCMs generally underestimate the amount of supercooled liquid in clouds. For a selected GCM (CAM5), we thereafter show that the underestimate can be attributed to two main factors: i) the presence of IN in the mixed-phase temperature range, and ii) the Wegener-Bergeron-Findeisen process, which converts liquid to ice once ice crystals have formed. We also show that adjusting these two processes such that the GCM’s cloud phase is in agreement with the observed has a substantial impact on the cloud-climate feedbacks and ultimately the climate sensitivity simulated by the GCM. Intriguingly, bringing the modeled cloud phase into agreement with observations increases climate sensitivity, but decreases the arctic warming amplification. The latter is mediated by modifications to low arctic clouds, and their impacts on the surface radiation budget.