The ERICA IOP 5 Storm. Part I: Analysis and Simulation

Abstract

The ERICA IOP 5 storm was the third strongest cyclone observed during the three-month Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) and the least successfully predicted by the operational models. This paper documents the storm development with use of nearly all available observational data and presents the results of a simulation of the storm carried out by the Pennsylvania State University-NCAR mesoscale Model MM4. The observations reveal that the storm formed in two stages: a first stage in which a weak, eastward-moving upper-level trough over the Gulf states excited the growth of two disturbances over the Gulf Stream, and a second stage in which a rapidly moving, moderately intense short-wave trough from the north-central states interacted with the more northerly of the two disturbances, producing rapid intensification. Maximum deepening rates were 11 mb (6 h)?1 and 33 mb (24 h)?1. At the mature stage a thermal gradient of 7°C (35 km)?1 was observed near the surface by a low-flying research aircraft that traversed the occluded frontal zone. A full-physics simulation, carried out on a movable 30-km grid embedded within a 90-km fixed grid, closely reproduced the storm development, as verified by surface ship and buoy observations, flight level and dropsonde data from research aircraft, and satellite infrared and microwave imagery. Sensitivity tests reported in a companion paper revealed that the development was highly sensitive to condensation heating and moderately sensitive to surface energy fluxes, grid size, and the location of the Gulf Stream. The companion paper also addresses the question of why in this case the MM4 outperformed the operational models of the National Meteorological Center.