An Adiabatic Simulation of the ERICA IOP 4 Storm: An Example of Quasi-Ideal Frontal Cyclone Development

Abstract

Numerical experiments with dry, inviscid models started from small normal-mode perturbations in baroclinic jet flows provide examples of ideal baroclinic cyclone development. This paper examines, with use of the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model, cyclone development under conditions that resemble the ideal experiments in lacking initial surface fronts and in neglecting latent heating and surface fluxes but that differ from most ideal experiments in including surface friction and an initial large upper-level disturbance. The initial state of the simulation is that of the intense, explosive Experiment on Rapidly Intensifying Cyclones over the Atlantic intensive observation period 4 storm. Issues highlighted are the timing and rate of deepening, the rapidity and intensity of frontal formation, the frontal structure, the airflow relative to the cyclone and fronts, and the nature of the occlusion and warm-core seclusion processes. Principal findings are as follows: The deepening rate well exceeded the common criterion for rapid deepening, and the period of most rapid deepening commenced only 3 h after the appearance of the surface low center. Warm, cold, and occluded fronts formed simultaneously and were already sharp by 9?12 h of the simulation. The warm front was ill defined above the boundary layer (900 mb). The thermal gradient in the cold frontal zone reached large values near the surface (10°C in 40 km). A plume of strong updraft (30 cm s?1) appeared above the nose of the ftont. A weakly connected middle- and upper-level frontal zone marked by elevated levels of potential vorticity (PV) also sloped rearward from the surface cold front but with a lesser inclination. Rising, or risen, warm air with low PV levels and sinking, or sunken, cold air with high PV levels were juxtaposed along the occluded front at the mature stage. The near-surface warm-sector air converged on the triple point and ascended above the occluded front, primarily on the forward side. The motion relative to the cyclone consisted of two basic flows: an ascending warm flow that, depending on point of origin, spread either anticyclonically downstream or cyclonically upstream, and a corresponding descending cold flow that near the low center intertwined with the cyclonic branch of the warm flow. The flow pattern can be crudely likened to that of two interlocking fans. On the basis of the fully resolved instantaneous relative motions in the vicinity of the occluded front, the occlusion process, after frontal formation, can be described as a motion of the cold front forward along the warm front with the segment of the warm front adjacent to the triple point being transformed into an occluded front. The warm-core seclusion formed at the tip of the occluded front shortly after its appearance and was subsequently collocated with a pool of large vorticity that broke off from the strip of intense vorticity that lay along the occluded front. The warm pocket and vorticity maximum were carried along together in the flow for a period of at least 21 h. Diffusive processes in the model were essential to the maintenance of the observed nearly steady-state frontal structures. At low levels (850 mb), on the poleward edge of the occluded front, the diffusion generated a substantial amount of potential vorticity with a peak value of about 5 PVU.