Some Bulk Properties of Cumulus Ensembles Simulated by a Cloud-Resolving Model.Part I: Cloud Root Properties

Abstract

To provide a basis for developing an improved cloud model for use in cumulus parameterization (CMCP), a two-dimensional cloud-resolving model is used to simulate the bulk properties of cloud roots. Two idealized experiments are presented: one is based on an undisturbed trade wind situation and the other is based on a disturbed tropical situation. The simulated data are analyzed through extensive trajectory analysis to trace the thermodynamic properties and buoyancy of ascending air back into the subcloud layer. Particular efforts are placed on finding the relations between cloud root properties and large-scale variables that are valid for both deep and shallow clouds. It is found that the air below cloud base (in cloud roots) is usually moister than its environment and typically becomes saturated at levels closer to or below the mixed-layer top. Moreover, no matter at which levels cloud roots become saturated, they have very small buoyancy near the level of the mixed-layer top. Based on these two findings, it is suggested that cloud-root or in-cloud moist static energy at the level of the mixed-layer top can be approximated by the environmental saturation moist static energy at the same level. This assumption, when used in a CMCP, provides a possible solution for describing cloud-base conditions without knowing the details of the PBL inhomogeneity.