Large-Eddy Simulations of the Sensitivities of Polar Clouds to Climate Change

Because of polar and in particular Arctic amplification of global warming, high latitudes are sensitive to climate change. How sensitive they are depends, among factors, on how sea ice, clouds, and boundary layer processes respond to climate change. For example, the uncertainties in the Arctic cloud fraction simulated by current GCMs contribute to the inter-model spread in sea ice states through their impact on the surface energy budget (Eisenman et al. 2007), and they potentially exert feedbacks on sea ice cover. It is questionable whether the widely used semi-empirical cloud parameterizations, developed primarily for low latitudes, can be applied to polar regions to capture their climate change response. Here we use a newly developed large-eddy simulation model (PyCLES) to study Arctic clouds and how they respond to a wide range of climate changes. The model has been successfully validated against field data from the Indirect and Semi-Direct Aerosol Campaign. PyCLES resolves motions that are relevant to cloud processes, but its domain size is limited. Therefore, we relax the free-tropospheric temperature and moisture profiles in PyCLES towards those from an idealized GCM at high latitudes. This allows the inclusion of large-scale dynamics. We vary the climate by changing the longwave optical thickness in both the GCM and PyCLES, and study the statistically steady states that eventually ensue to elucidate the processes governing Arctic clouds in different climates. A one-moment bulk mixed-phased microphysics scheme is implemented. Our primary results show that without a surface temperature inversion, low-cloud fraction decreases significantly as the climate warms. There is also an associated cloud regime transition from stratus to cumulus as the mean sea surface temperature increases from 242 K to 288 K. The low-cloud fraction changes can be mostly explained by changes in saturation deficit (the difference between saturation and total water specific humidity). Higher boundary layer temperature corresponds to a higher saturation deficit, which leads to decreased low-cloud fraction. Our sensitivity experiments shows that the lower tropospheric stability (LTS) does not always predict low cloud fraction at high latitudes.