Abstract
Marine low-level clouds have long been poorly simulated in models despite many studies and field experiments devoted to their improvement. The marine subtropical low stratiform cloud decks simulated in E3SMv1 AMIP runs are evaluated using the CALIPSO satellite simulator. It is found that the stratiform cloud decks are displaced away from where they are observed in the CALIPSO-GOCCP climatology. It is this displacement that causes the documented wide spread in model-simulated low-level cloud cover (LCC) and in the well-known strong relationship between LCC and lower tropospheric stability (LTS). Comparing with three other U.S. models, some models like CESM have the cloud decks restricted to near the coast, while others like E3SMv1 displace them further out into the open ocean. To gain further insights for model improvement, we consider 30° latitude ´ 35° longitude extended regions, focusing on those parts where low stratiform clouds dominate (as indicated by LCC > 45%). Model-simulated LCC and the LCC-LTS relationship agree better in the larger regions. The displacement of the cloud decks point to model biases in the interaction between model physical parameterizations and the large-scale dynamics, since simulated LTS is very similar to that from ERA-Interim. Large-scale dynamics biases alone appear likely to be responsible for the lack of sensitivity to ENSO in E3SMv1 and three other U.S. models. Large-scale dynamics biases may also play a role in the lack of sensitivity of LCC to LTS in E3SMv1’s fully coupled historical simulations in the northeast Pacific stratiform cloud deck.