This short note discusses key deficiencies in two-dimensional (2D) cloud-resolving model (CRM) simulations. Results differ significantly from three-dimensional (3D) simulations in the low-level humidity structure and associated fields. These differences are consistent across two different CRMs that differ substantially in their thermodynamic and microphysical formulations. Our analysis suggests than the near-surface humidity structure depends on moisture transport in clouds, and we suggest that differences in entrainment between 2D and 3D simulations lead to substantial differences in both cloud amount and moisture transport by the clouds at low levels. When compared with 3D, less entrainment in 2D reduces the likelihood that convective updraughts terminate and moisten the lower troposphere. The differences between the 2D and 3D are significant if the CRM is to be used as a reference for comparison against numerical weather prediction (NWP) or climate models. Copyright © 2008 Royal Meteorological Society, Crown Copyright 2008
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