Observations of Mesoscale Circulations within Extratropical Cyclones over the North Atlantic Ocean during ERICA

Pseudo-dual-Doppler analyses of two mesoscale circulations (20–40 km in horizontal dimensions) that formed along a warm front within a rapidly deepening extratropical cyclone over the ocean are presented. The circulations were analyzed using airborne Doppler radar during ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) IOP (intensive observing period) 4 and are believed to be a type of frontal instability that has not been addressed in past theoretical studies of frontal waves and cyclones. These features may be the shallow circulations that have been documented to play a role in cyclogenesis when they become coupled with an upper-level baroclinic wave. The present case is compared and contrasted with the only other well-analyzed event during ERICA IOP 5. A retrieval of the perturbation pressure and buoyancy fields using the kinematic wind fields is performed in order to facilitate the interpretation of the dynamic structure of these vortices. In addition, a set of equations is derived in natural coordinates to examine the contributions to the total pressure by the cyclostrophic, buoyancy, updraft–vertical wind shear interaction, and the Bernoulli effects.