Q. J. R. Meteorol. Soc., submitted 5/2008
This short note discusses key dciencies in two-dimensional (2D) cloud resolving model (CRM) simulations. Results differ signcantly from three-dimensional (3D) simulations in the low level humidity structure and associatedelds. These differences are consistent across two different CRMs which 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 to 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 signcant if the CRM is to be used as a reference for comparison against numerical weather prediction (NWP) or climate models.