The Frontal Structure of an Explosive Oceanic Cyclone: Airbone Radar Observations of ERICA IOP 4

Abstract

Observation taken during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) have permitted analyses of explosive oceanic cyclogenesis of unprecedented detail. The most intense of the cyclones that occurred during this experiment was that of intensive observing period 4 (IOP 4). This storm not only contained the lowest sea level pressure ever observed south of 40°N in an extratropical cyclone over the Atlantic but was well-sampled by the specially deployed observing systems (aircraft, airbone Doppler radar, dropwindsondes, and buoys. This paper presents detailed analysis of this case. The primary issues addressed here are 1) the finescale structure of the fronts, and 2) the structure and organization of the associated precipitating features. Both of these issues have previously been investigated primarily through numerical simulation of various cases of intense cyclogenesis. Analysis of the resulting model output has indicated a structural evolution of such cyclones that departs significantly from that described by the Norwegian cyclone model. Diagnosis of the output has indicated that latent heat release plays a significant role, both in the cyclone intensification and in the evolution of the associated fronts. The detailed in situ observations in the present case allow for observational evaluation of the attendant conclusions of these prior modeling studies. Principal findings include: 1) Confirmation of the existence of a ?bent-back%rdquo; warm front wrapping to the west around the cyclone center; the frontal structure is very different from that of the occluded front that would here by analyzed according to the Norwegian model. 2) The presence of an extremely sharp warm front, with Kelvin-Helmholtz waves and an intense line of convection found along the front. 3) A continuous extension of convective activity along the cold front to the point of intersection with the warm front, with no evident "fracture? zone. 4) The presence of only scattered convective cells along and to the north of the bent-back warm front. 5) A significant displacement between the cold front and the main cloud band. The cold front lay along a narrow line of intense convection well to the rear of the main comma-shaped cloud mass evident in the satellite imagery.