A Convection Parameterization for Low-CAPE Environments

Abstract

Numerical models that are unable to resolve moist convection in the atmosphere employ physical parameterizations to represent the effects of the associated processes on the resolved-scale state. Most of these schemes are designed to represent the dominant class of cumulus convection that is driven by latent heat release in a conditionally unstable profile with a surplus of convective available potential energy (CAPE). However, an important subset of events occurs in low-CAPE environments in which potential and symmetric instabilities can sustain moist convective motions. Convection schemes that are dependent on the presence of CAPE are unable to depict accurately the effects of cumulus convection in these cases. A mass-flux parameterization is developed to respresent such events, with triggering and closure components that are specifically designed to depict subgrid-scale convection in low-CAPE profiles. Case studies show that the scheme eliminates the “bulls-eyes” in precipitation guidance that develop in the absence of parameterized convection, and that it can represent the initiation of elevated convection that organizes squall line structure. The introduction of the parameterization leads to significant improvements in the quality of quantitative precipitation forecasts, including a large reduction in the frequency of spurious heavy-precipitation events predicted by the model. An evaluation of surface and upper-air guidance shows that the scheme systematically improves the model solution in the warm season, a result that suggests that the parameterization is capable of accurately representing the effects of moist convection in a range of low-CAPE environments.

Long‐term single‐column model intercomparison of diurnal cycle of precipitation over midlatitude and tropical land

“…In the sensitivity study in Section 4, when the default ZM scheme is modified by unlimiting the launching level, the nocturnal precipitation in EAMv1.trigger is significantly improved compared to default EAMv1. CMC has a separated mid‐level convective scheme to capture elevated convection (McTaggart‐Cowan et al ., 2020). The mid‐level scheme exhibits a maximum activity during the night (Figure 11).…”

“…Wang et al ., 2007; Bechtold et al ., 2008; Stratton and Stirling, 2012), or the detection of mid‐level convection that is related to the nocturnal precipitation peak over regions such as the central USA (e.g. Lee et al ., 2008; Wang et al ., 2015; Xie et al ., 2019; McTaggart‐Cowan et al ., 2020). Other parametrizations (e.g.…”

Sowing Storms: How Model Timestep Can Control Tropical Cyclone Frequency in a GCM

“…It is known that models with CAPE‐dependent deep convective parameterizations which become “less active” will simulate enhanced resolved (i.e., near the grid scale) moist convective motions (McTaggart‐Cowan et al., 2020). The particular behavior outlined here is a fundamental response to the timescales over which moist processes are operating within the model.…”

Mid-level convection in a warm conveyor belt accelerates the jet stream

“…The results thus suggest that mid-level convection contributes to ridge building and questions its role in large-scale dynamics. As parameterization schemes often struggle to represent updrafts that do not start in the boundary layer (e.g., McTaggart-Cowan et al, 2020), the representation of mid-level convection may be a source of uncertainty…”