AN ADIABATIC SIMULATION OF THE ERICA-IOP-4-STORM - AN EXAMPLE OF QUASI-IDEAL FRONTAL CYCLONE DEVELOPMENT

Numerical experiments with dry, inviscid models started from small nor mal-mode perturbations in baroclinic jet Rows provide examples of idea l baroclinic cyclone development. This paper examines, with use of the Pennsylvania State University-National Center for Atmospheric Researc h Mesoscale Model, cyclone development under conditions that resemble the ideal experiments in lacking initial surface fronts and in neglect ing latent heating and surface fluxes but that differ from most ideal experiments in including surface friction and an initial large upper-l evel disturbance. The initial state of the simulation is that of the i ntense, 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 front al formation, the frontal structure, the airflow relative to the cyclo ne 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 su rface low center. Warm, cold, and occluded fronts formed simultaneousl y and were already sharp by 9-12 h of the simulation. The warm front w as ill defined above the boundary layer (900 mb). The thermal gradient in the cold frontal zone reached large values near the surface (10 de grees C in 40 km). A plume of strong updraft (30 cm s(-1)) appeared ab ove the nose of the front. A weakly connected middle- and upper-level frontal zone marked by elevated levels of potential vorticity (PV) als o sloped rearward from the surface cold front but with a lesser inclin ation. Rising, or risen, warm. air with low PV levels and sinking, or sunken, cold air with high PV levels were juxtaposed along the occlude d front at the mature stage. The near-surface warm-sector air converge d 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 Rows: an ascending warm flow that, depending on point of ori gin, spread either anticyclonically downstream or cyclonically upstrea m, and a corresponding descending cold Row that near the low center in tertwined 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 o f 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 form ed at the tip of the occluded front shortly after its appearance and w as subsequently collocated with a pool of large vorticity that broke o ff from the strip of intense vorticity that lay along the occluded fro nt. The warm pocket and vorticity maximum were carried along together in the Bow for a period of at least 21 h. Diffusive processes in the m odel were essential to the maintenance of the observed nearly steady-s tate 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.