Monday, 6 May 2024: 9:30 AM
Shoreline AB (Hyatt Regency Long Beach)
The halo region, immediately outside a cloud, is moister than the air further from the cloud and is distinct from cloud shell in which the downward motions dominate. The moist halo region is critical for the interplay between the cloud and the large-scale environment and may have non-negligible impact on radiation. Previous studies found large uncertainties in estimated halo size and suggested a dependency of halo size on cloud size. However, these used relatively coarse resolution simulations or limited samplings from observations.
In this study, the moist buffering halo region of shallow maritime cumulus clouds is systematically investigated using large eddy simulations with various grid resolutions and numerical choices. Auto-correlation analyses of cloud liquid water and relative humidity suggest a converged size of 200-300 m for moist patches outside clouds when model resolution is below 50 m but may overestimate this size due to non-cloudy moist regions. Based on a composite analysis, the structure of the moist halo immediately outside individual clouds is examined. It is found that, regardless of model resolution, the distribution of relative humidity in the halo region does not depend on cloud size, but on the real distance away from the cloud boundary, indicating some size-independent length scales responsible for the halo formation. The relative humidity decays with distance more quickly with finer horizontal resolution, which is possibly related to the model resolution dependency of the cloud spectrum. The halo size near cloud base is larger than that within the cloud layer and this feature is robust across all simulations. Further analyses of backward and forward Lagrangian trajectories originating from the moist halo region reveal the possible role for sub-cloud coherent structures on the cloud-base halo formation. Possible mechanisms explaining cloud halo sizes and associated length scales are discussed.
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