Formation and release of symmetric instability following Delta‐M adjustment

Abstract

SUMMARYConditional Symmetric Instability (CSI) and Delta-M adjustment have both been suggested as possible mechanisms to explain the slantwise circulations that lead to two-dimensional precipitation bands near midlatitude cold fronts. Previous work has shown how the initial state required for CSI to occur is similar to the state produced as a result of Delta-M adjustment. An idealized dry numerical model is used to test the hypothesis that slantwise circulations can occur due to the release of symmetric instability formed by the Delta-M adjustment process. The numerical simulation shows the coexistence of two pairs of slantwise circulations: one due directly to Delta-M adjustment, and the other which occurs in a region of symmetric instability which was formed by the Delta-M adjustment process. The simulated circulation is qualitatively similar to some high-resolution Doppler radar observations of vertically stacked slantwise circulations. It is argued that a combined Delta-M/CSI mechanism may also be responsible for the creation of multiple slantwise circulations in the moist atmosphere.

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

“…This duality in the mechanisms leading to slantwise circulations was also found in the idealized dry twodimensional numerical-model simulations of Morcrette and Browning (2006). They found that small regions of symmetric instability formed below the M-adjustment updraught, and that a second updraught formed in the region where this instability was being released.…”

“…The 861 model is run with 64 vertical levels; this gives a grid spacing of 200-300 m in the mid-troposphere, with higher resolution near the surface in the boundary layer. This horizontal resolution is coarser than the optimal resolution for resolving small-scale multiple slantwise circulations (according to Lean and Clark (2003) and Morcrette and Browning (2006)), but was chosen to make these large-domain simulations manageable. Morcrette and Browning (2006) found that simulations with 4 km horizontal grid boxes produced circulations similar to those obtained with finer resolutions, although they took longer to evolve.…”

Medium-Range Quantitative Precipitation Forecasts from Canada’s New 33-km Deterministic Global Operational System

“…In this model, the number of computational levels has been increased from 58 to 80, a nonorographic gravity wave drag based on Hines (1997a,b) has been implemented, and a new radiation scheme from Li and Barker (2005) using a correlated-k distribution for gaseous transmission is included. Other improvements to the model physics, related to the representation of deep convection [e.g., inclusion of the Bechtold et al (2001) scheme], of slantwise convection (see Ma 2000;Morcrette and Browning 2006), and of land surface processes (inclusion of the Canadian Land Surface Scheme, CLASS; Verseghy 1991; Verseghy et al 1993) are currently being explored.…”

Numerical study of slantwise circulations in a strongly sheared prefrontal environment

“…M-adjustment was put in a chronological sequence with CI and SI by Morcrette and Browning (2006), hereafter referred to as MB, to form a consistent picture that accounts for the widespread simultaneous presence of those three forms of unstable motion in the atmosphere. 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.…”