Sensitivities of slantwise convection dynamics to model grid spacing under an idealized framework

Although the release of conditional symmetric instability (CSI) by slantwise convection is recognized as an important baroclinic process, the basic dynamics of these circulations and their representation in numerical models remain inadequately understood. To address this issue, a series of 2D idealized experiments of pure slantwise convection are performed in an initially statically stable environment using the non-hydrostatic Weather Research and Forecasting model, with the horizontal grid lengths varying between 1 and 40 km. The results show that the larger-scale feedbacks of the slantwise convection converge numerically when a cross-band grid length ( increment y) of 5 km is reached. The differences between the non-converged and converged results tie closely to the release of a shallow layer of conditional instability that inevitably accompanies the early development of the slantwise circulation due to differential advection of saturation equivalent potential temperature (theta e*). The resolved small-scale upright convection embedded within the slantwise band can energize the horizontal acceleration of the slantwise band at mid-to-upper levels by transporting low geostrophic momentum upward that results in localized inertial instability. The convective cell also enhances the large-scale CSI neutralization by advecting high theta e* downward with strong downdraughts that orient more vertically than coarser-gridded runs. Moreover, increment y <= 5 km also better resolves the horizontal pressure gradients for cross-band motions. This work suggests that global/climate numerical weather prediction models may not adequately resolve important characteristics of slantwise convection. As most cumulus schemes target only upright convection, the inclusion of parametrized slantwise convection may improve their performance.