Open Water in Sea Ice Causes High Bias in Polar Low-Level Clouds in GFDL CM4

Global climate models (GCMs) struggle to simulate polar clouds, especially low-level clouds that contain supercooled liquid and closely interact with both the underlying surface and large-scale atmosphere. Here we focus on GFDL's latest coupled GCM–CM4–and find that polar low-level clouds are biased high compared to observations. The CM4 bias is largely due to moisture fluxes that occur within partially ice-covered grid cells, which enhance low cloud formation in non-summer seasons. In simulations where these fluxes are suppressed, it is found that open water with an areal fraction less than 5% dominates the formation of low-level clouds and contributes to more than 50% of the total low-level cloud response to open water within sea ice. These findings emphasize the importance of accurately modeling open water processes (e.g., sea ice lead-atmosphere interactions) in the polar regions in GCMs.