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 smallnormal-mode perturbations in baroclinic jet Rows provide examples ofideal baroclinic cyclone development. This paper examines, with use ofthe Pennsylvania State University-National Center for AtmosphericResearch Mesoscale Model, cyclone development under conditions thatresemble the ideal experiments in lacking initial surface fronts and inneglecting latent heating and surface fluxes but that differ from mostideal experiments in including surface friction and an initial largeupper-level disturbance. The initial state of the simulation is that ofthe intense, explosive Experiment on Rapidly Intensifying Cyclones overthe Atlantic intensive observation period 4 storm. Issues highlightedare the timing and rate of deepening, the rapidity and intensity offrontal formation, the frontal structure, the airflow relative to thecyclone and fronts, and the nature of the occlusion and warm-coreseclusion processes. Principal findings are as follows:The deepening rate well exceeded the common criterion for rapiddeepening, and the period of most rapid deepening commenced only 3 hafter the appearance of the surface low center.Warm, cold, and occluded fronts formed simultaneously and were alreadysharp 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 nearthe surface (10 degrees C in 40 km). A plume of strong updraft (30 cms(-1)) appeared above the nose of the front. A weakly connected middle-and upper-level frontal zone marked by elevated levels of potentialvorticity (PV) also sloped rearward from the surface cold front but witha 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 atthe mature stage. The near-surface warm-sector air converged on thetriple point and ascended above the occluded front, primarily on theforward side.The motion relative to the cyclone consisted of two basic Rows: anascending warm flow that, depending on point of origin, spread eitheranticyclonically downstream or cyclonically upstream, and acorresponding descending cold Row that near the low center intertwinedwith the cyclonic branch of the warm flow. The flow pattern can becrudely likened to that of two interlocking fans.On the basis of the fully resolved instantaneous relative motions in thevicinity of the occluded front, the occlusion process, after frontalformation, can be described as a motion of the cold front forward alongthe warm front with the segment of the warm front adjacent to the triplepoint being transformed into an occluded front.The warm-core seclusion formed at the tip of the occluded front shortlyafter its appearance and was subsequently collocated with a pool oflarge vorticity that broke off from the strip of intense vorticity thatlay along the occluded front. The warm pocket and vorticity maximum werecarried along together in the Bow for a period of at least 21 h.Diffusive processes in the model were essential to the maintenance ofthe observed nearly steady-state frontal structures. At low levels (850mb), on the poleward edge of the occluded front, the diffusion generateda substantial amount of potential vorticity with a peak value of about 5PVU.