Three-dimensional simulation of frontal rainbands and conditional symmetric instability in the Eady-wave model

Abstract

The structure and evolution of frontal rainbands in relation to conditional symmetric instability (CSI) are presented with a high-resolution, three-dimensional (3-D) simulation of an Eady wave. This work extends an earlier 2-D study of Knight and Hobbs by use of the same mesoscale model and similar initial conditions. The model simulates numerous updraft bands that correspond to the warm-sector (WSF), the surface cold frontal (SCF), the wide cold frontal (WCF) and the post-frontal (PCF) bands as observed. It is shown that the WSF band develops in a convectively near-neutral environment, while the WCF band forms under a convectively stable but slantwisely unstable condition and it interacts closely with the PCF and SCF bands. It is found that the present 3-D results conform in many respects to CSI theory and support the earlier 2-D interpretations of the rainbands. However, some differences exist, which include the development of deep and intense rainbands without downshear tilt, the deviation of the rainbands from the direction of thermal winds, strong low-level jets and intense across-frontal thermal gradients. By introducing the third spatial dimension, the model appears to show better the relationships between these rainbands and their environments, thus providing more realistic representation of the mesoscale structure and evolution of the rainbands. The importance of some physical parameters in determining the formation of various rainbands is examined. It is found that the surface friction, ice microphysics, static stability, baroclinicity and the magnitude of centrifugal forces all have important contributions to the formation, structure and dynamics of the frontal rainbands.