A NEW FRONTAL INSTABILITY - THEORY AND ERICA OBSERVATIONS

A short-wave instability theory is applied to secondary waves on a nar row cold-front rainband observed during the Experiment on Rapidly Inte nsifying Cyclones over the Atlantic (ERICA). The basic mean state is a pproximated by the parabolic, geostrophically balanced interface betwe en two layers of homogeneous density. The observed wavelength of pertu rbations along the ERICA cold front is about 20-30 km and their doubli ng time is about 2 hours. The observed wavelength is well within the s hort-wave regime of the theory, which yields a growth rate in good agr eement with the ERICA observations. The spatial patterns of both the h orizontal and vertical velocity components observed during ERICA are c onsistent with the model-derived patterns.