Secondary Cyclogenesis—Comparison of Observations and Theory

Abstract

A case study of a small-scale polar front cyclone observed during the Canadian Atlantic Storms Program (CASP) is presented. The cyclone forms along an essentially two-dimensional front, which is in approximate thermal wind balance. This cyclogenetic event occurs where the Richardson number near the surface is small. The storm appears to grow in response to favorable low-level thermal advection rather than to any significant upper-level forcing. As the wave amplifies, the initially two-dimensional frontal zone develops a three-dimensional structure. The cyclone has a horizontal wavelength of 1200 km and a vertical scale of 3?4 km. The wind field associated with the evolved system is strongly unbalanced in comparison with that in the initial state. A strong southerly low-level jet exists in the warm sector and a moderate northerly jet is observed in the air behind the cold front. The low-level warm sector is a region of reduced static stability. Results will be compared with a frontal zone stability theory that describes how a two-dimensional primary frontal zone will evolve into a three-dimensional structure with secondary fronts. As we shall see, the three- dimensional structure of the observed system compares favorably with the theory.