Parameterization of Slantwise Convection in Numerical Weather Prediction Models

Abstract

In two-dimensional motion (i.e., x−z direction ), absolute momentum (m = f x + v) and potential temperature ( equivalent potential temperature if the air is saturated) are conserved. It can be shown (e.g., Eliassen 1962) that the atmosphere may be stable to upright convection but unstable to perturbations in the direction of the sloping m surface provided that the slope of the m surface is less (more horizontal) than the slope of the potential temperature surface. Bennetts and Hoskins (1979) and Emanuel (1979, 1983a) suggested the importance of this instability in the formation of banded cloud and precipitation structures. It was named conditional symmetric instability (CSI). It manifests theoretically as roll vortices oriented along the shear vector (parallel to the thermal wind). Emanuel (1983b, 1988) presented synoptic evidence that suggests the presence of convection coexisting with atmospheric conditions that support the release of CSI. Shapiro (1982) and Emanuel (1983a) suggested that the effect of slantwise convection in a region where CSI is released is to adjust the atmosphere from a state of convective instability along an m surface to a state of neutrality along the m surface. Nordeng (1987) developed a scheme for parameterization of slantwise convection along these lines and tested it on real data in a mesoscale numerical weather prediction model (mesh size 50 km). A slightly different version of the scheme was implemented into the Pennsylvania State University (Penn State)-National Center for Atmospheric Research (NCAR) Mesoscale Model and tested on the case described in Emanuel (1983a) (Lindstrom and Nordeng 1992).