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

Abstract

Abstract 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.