Orogenic Propagating Precipitation Systems over the United States in a Global Climate Model with Embedded Explicit Convection

Abstract

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 superparameterized (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 explicit convection model approach [i.e., 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 in nature. The simulated convective systems exhibit spatial scales, phase speeds, and propagation speeds comparable to radar 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 “metastructures” are shown to transcend individual cloud-resolving model arrays. In reconciling how this new mode of diurnal convective variability is admitted in SP-CAM despite the severe idealizations in the cloud-resolving model configuration, an updated discussion is presented of what physics may transcend the re-engineered scale interface in MMFs. The authors 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.

Effects of explicit convection on global land‐atmosphere coupling in the superparameterized CAM

“…These effects of superparameterization on rainfall have been noted before [Khairoutdinov et al, 2005;DeMott et al, 2007;Somerville, 2009a, 2009b;Pritchard et al, 2011;Dirmeyer et al, 2012] and are improvements given the observed nocturnal peak of rainfall in the U.S. Midwest [Dai et al, 1999] and the persistent drizzle problem in conventional GCMs [Dai, 2006]. It is reassuring that similar improvements of diurnal rainfall associated with superparameterization occur even in the micro-CRM configuration that we have employed for computational efficiency, which emphasizes that mesoscale organization on the 32-128 km scale is not a critical factor for the diurnal cycle, as has also been noted for SPCAM's Madden-Julian Oscillation [Pritchard et al, 2014].…”

Tropical continental downdraft characteristics: mesoscale systems versus unorganized convection

“…Accurate simulation of mesoscale convective systems (MCSs) in continental regions (Pritchard et al, 2011) was also shown to be sensitive to downdraft-boundary layer interactions, with significantly improved representation of MCS propagation in the central USA once such interactions were resolved. Additionally, representing the effects of downdrafts and cold pools in models has been shown to have positive effects on the representation of the diurnal cycle of precipitation (Rio et al, 2009;Schlemmer and Hohenegger, 2014).…”

Impacts of the mountain–plains solenoid and cold pool dynamics on the diurnal variation of warm-season precipitation over northern China

“…In addition, existing observation networks or analysis datasets are unable to resolve the MPS structure and evolution, as well as the interactions of the MPS and the diurnal precipitation, in detail. High-resolution simulations have already been used to analyze the evolution of the MPS circulation near different mountain ranges (e.g., Cotton, 1986, 1989;Wolyn and McKee, 1994;Zhang and Koch, 2000;Koch et al, 2001;Carbone and Tuttle, 2008;Trier et al 2006Trier et al , 2010Li and Simith, 2010;Pritchard et al, 2011;Laing et al, 2012).…”