A New Frontal Instability: Theory and ERICA Observations

Abstract

A short-wave instability theory is applied to secondary waves on a narrow cold-front rainband observed during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). The basic mean state is approximated by the parabolic, geostrophically balanced interface between two layers of homogeneous density. The observed wavelength of perturbations along the ERICA cold front is about 20?30 km and their doubling time is about 2 hours. The observed wavelength is well within the short-wave regime of the theory, which yields a growth rate in good agreement with the ERICA observations. The spatial patterns of both the horizontal and vertical velocity components observed during ERICA are consistent with the model-derived patterns.