Flow Dynamics and Stability in a Severe Rainband

Abstract

Observations of a severe form of rainband, which is comprised of a line of strong but shallow convection, suggest that the environment into which it moves is nearly neutrally stratified with respect to moist convection. A simple two-dimensional model of a severe rainband has been developed to explore how the cold air at the base of the rainband modifies local stability as it underruns the saturated surface layer. This dynamic lifting is found to have two distinct effects. First, the lifted fluid contains regions of absolute and conditional instability. Then it also reorganizes the midtroposphere into a sequence of elevated inversions set between unstable layers. This latter effect results from the intense ?lee wave? response in the air flow. The elevated inversions would appear to cap the severe moist convection, establishing the rainband character rather than allowing the deep cellular convection of squall lines. The flow disturbances end abruptly at the steering level (a critical level for the waves), which appears to define the upper limit of the rainband activity.