The impact of deformation strain on the formation of banded clouds in idealized modelling experiments

Abstract

Experiments are performed using an idealized version of an operational forecast model to determine the impact on banded frontal clouds of the strength of deformational forcing, low-level baroclinicity, and model representation of convection. Line convection is initiated along the front, and slantwise bands extend from the top of the line-convection elements into the cold air. This banding is attributed primarily to M adjustment. The cross-frontal spreading of the cold pool generated by the line convection leads to further triggering of upright convection in the cold air that feeds into these slantwise bands. Secondary low-level bands form later in the simulations; these are attributed to the release of conditional symmetric instability. Enhanced deformation strain leads to earlier onset of convection and more coherent line convection. A stronger cold pool is generated, but its speed is reduced relative to that seen in experiments with weaker deformational strain, because of inhibition by the strain field. Enhanced low-level baroclinicity leads to the generation of more inertial instability by line convection (for a given capping height of convection), and consequently greater strength of the slantwise circulations formed by M adjustment. These conclusions are based on experiments without a convective-parametrization scheme. Experiments using the standard or a modified scheme for this model demonstrate known problems with the use of this scheme at the awkward 4 km grid length used in these simulations.

Numerical study of slantwise circulations in a strongly sheared prefrontal environment

“…Numerical experiments with an idealized version of an operational forecast model were performed by Gray and Dacre (2008) to reproduce the hypothesized sequence of events in an idealized environment.…”

Precipitation Banding in Idealized Baroclinic Waves

“…This study follows up studies of multiple vertical-velocity banding in twodimensional (e.g., Knight and Hobbs 1988;Bénard et al 1992a,b;Xu 1992;Lafore et al 1994;Pizzamei et al 2005) and three-dimensional (e.g., Zhang and Cho 1995;Gray and Dacre 2008) idealized models. This study follows up studies of multiple vertical-velocity banding in twodimensional (e.g., Knight and Hobbs 1988;Bénard et al 1992a,b;Xu 1992;Lafore et al 1994;Pizzamei et al 2005) and three-dimensional (e.g., Zhang and Cho 1995;Gray and Dacre 2008) idealized models.…”