Observations of the Early Evolution of an Explosive Oceanic Cyclone during ERICA IOP 5. Part I: Synoptic Overview and Mesoscale Frontal Structure

Abstract

The structural evolution of the extratropical cyclone that occurred during intensive observation period 5 (IOP 5) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) is presented. In the present paper, the focus will be on the synoptic-scale development of the cyclone and the mesoscale frontal structure in the vicinity of the surface low center at a time early in its period of rapid deepening. Within this context the much higher resolution analyses of the three-dimensional wind field of the circulation center are discussed in the companion paper. The surface low center appeared initially as one of at least two mesoscale circulation centers along a preexisting low-level frontal zone approximately aligned with the Gulf Stream. Development occurred in association with the approach of a synoptic-scale upper-level trough from the west. As in the ERICA IOP 4 cyclone, a bent-back warm front-representing the extension of the warm front to the west of its point of intersection with the cold front-developed early in the period of rapid deepening. The surface low center was found to lie along this front and was associated with a significant meso-β-scale circulation. A secondary cold front developed out of the bent-back front to the south of the low center, while the antecedent primary cold front showed evidence of fracture near its juncture with the warm front. This frontal weakening appeared to be associated with a diffluent low-level wind field and with the intrusion of a swath of warm and dry air.