Formation and release of symmetric instability following Delta-M adjustment

Abstract

Conditional 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 slantwis 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. © Royal Meteorological Society, 2006.