The Influence of Open Boundary Conditions and Model Resolution on Shallow Cloud Organization in Atmospheric Large Eddy Simulations

Abstract An open boundary setup is presented in which a high‐resolution (high‐res) large eddy simulation (LES) is one‐way nested in a low‐resolution (low‐res) LES. The high‐res nested LES is compared to the periodic LES from Savazzi et al. (2023, https://doi.org/10.1175/jas‐d‐23‐0098.1 ). Both simulations are forced by the regional weather model HARMONIE‐AROME: the periodic LES via domain‐averaged tendencies, and the open boundary setup via the boundaries of the low‐res nested LES. The open boundary simulations inherit the full atmospheric state from the larger domains through frequent boundary updates, including developed cloud structures and their environmental states. Cloud structures are refined as they transition to higher‐resolution simulations, with clouds breaking into smaller fragments while retaining their large‐scale distribution. This results in larger, more organized clouds in the high‐res nested LES compared to the periodic LES when cloud fractions are similar. The periodic LES has a stronger daily cycle in cloudiness, with days starting very cloudy and ending with clear skies, producing deeper and more intermittent clouds accompanied by more intense rainfall. This leads to greater variation in cloud structures, ranging from large clouds during cloudy periods to fewer, smaller clouds during low cloud cover. In contrast, the high‐res nested LES maintains more constant cloud cover, with cloud top and size varying more gradually. The intermittent behavior of the periodic LES is explained by the applied horizontally averaged tendencies, which drive the domain toward stable or unstable conditions. Inheritance of the full atmospheric state allows the high‐res nested LES to maintain larger, more organized clouds.