Orogenic propagating precipitation systems over the central U.S. in a global climate model with embedded explicit convection

Description

In the lee of major mountain chains worldwide, diurnal physics of organized propagating convection project onto seasonal and climate time scales of the hydrologic cycle; but this phenomenon is not represented in conventional global climate models (GCMs). Analysis of an experimental version of the Super-Parameterized- (SP-) Community Atmosphere Model (CAM) demonstrates that propagating orogenic nocturnal convection in the central U.S. warm season is however representable in GCMs that use the embedded cloud-resolving model (CRM) approach (Multiscale Modeling Frameworks [MMFs]). SP-CAM admits propagating organized convective systems in the lee of the Rockies during synoptic conditions similar to those that generate Mesoscale Convective Systems (MCSs) in nature. The simulated MCSs exhibit spatial scales, phase speeds, and propagation speeds comparable to observations, and the genesis mechanism in the model agrees qualitatively with established conceptual models. Convective heating and condensate structures are examined on both resolved scales in SP-CAM, and coherently propagating cloud “meta-structures” are shown to transcend individual CRM arrays. In reconciling how this new mode of diurnal convective variability is admitted in SP-CAM despite the severe idealizations in the CRM configuration, an updated discussion is presented of what physics may transcend the re-engineered scale interface in MMFs. We suggest that the improved diurnal propagation physics in SP-CAM are mediated by large-scale first-baroclinic gravity wave interactions with a prognostic organization life cycle, emphasizing the physical importance of preserving “memory” at the inner resolved scale.