The 'Slant' of Slantwise Convection: An Idealized Modeling Study

Symmetric instability has received considerable attention for its role in multiple meteorological phenomena, particularly its potential role in the development of mesoscale banded structures in extratropical cyclones. Perturbations to a symmetrically unstable environment may result in slantwise convection, with "updrafts" and "downdrafts" that are neither purely vertical nor purely horizontal. In cases of moist symmetric instability, these slantwise circulations may produce banded precipitation.

One metric for diagnosing symmetric instability is slantwise convective available potential energy (SCAPE), a slantwise analog of CAPE. The potential utility of SCAPE as a tool for forecasting slantwise motions has led to the development of methods to calculate SCAPE, both from atmospheric soundings and from NWP model fields. SCAPE, like CAPE, is limited by its basis in parcel theory. Two symmetrically unstable environments may exhibit identical SCAPE but produce vastly different circulations.

This study investigates the role of an environment's theoretically-favored updraft slope in the release of its symmetric instability. A two-dimensional, quasi-compressible, dry numerical toy model is used to perform an array of idealized experiments. For a given value of SCAPE, multiple simulations are conducted, each with a different theoretically-favored updraft slope. The release of symmetric instability in each simulation is assessed, and analyses are performed to diagnose the physical mechanisms giving rise to the observed trends.