Analysis and numerical modeling of convectively driven ageostrophic circulations and their role in the rapid cold-air cyclogenesis during ERICA IOP8.

摘要

The structure and evolution of a rapidly developing cold-air cyclone is investigated using observational and model data during Intensive Observing Period 8 (IOP8) of the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). The enhanced ERICA data set is used to verify the evolution of mesoscale structures seen in high resolution (20 km) numerical simulations utilizing the Mesoscale Atmospheric Simulation System (MASS) Model.; On 24 February 1989 a small synoptic-scale cyclone developed off the coast of North Carolina during an intense cold-air outbreak. Total surface heat fluxes into the air ingested by the stonn exceeded 1200 Wm{dollar}sp{lcub}-2{rcub}.{dollar} The mature storm exhibited a symmetric eye in satellite imagery, with spiral bands of moderate to heavy precipitation evident in airborne radar data. Cross sections through the low center revealed a warm-core structure, and low-level winds exceeding 30 ms{dollar}sp{lcub}-1{rcub}.{dollar}; Initially, intensification of the 850 mb vortex resulted from coupling with an upstream tropopause fold and strong quasi-geostrophic forcing. Thunderstorm activity was triggered in the immediate vicinity of the surface low center in response to the onset cyclogenesis at 850 mb. This deep convection perturbed the jet-stream adjustment process aloft through latent heating, producing an imbalanced flow and enhanced mass divergence over the developing low center, leading to rapid deepening. The central pressure decreased 12 mb during the 12 hours ending 1800 UTC and was nearly steady for several hours thereafter, yielding a 24 hour pressure fall of.91 Bergerons for the period ending 0000 UTC 25 February.; Model sensitivity studies confirm that convection produced a highly unbalanced and accelerative jet flow which substantially increased mass divergence above the surface low. Moreover, convection generated vorticity at low-levels via tilting, while eroding the tropopause fold.; On 25 February, in the absence of quasi-geostrophic forcing, the mature storm maintained its depth and intensity and tracked northeastward along the warm side of the Gulf Stream front. The physical characteristics of the mature system, and its propagation along the axis of maximum potential thickness anomaly, where fluxes have the greatest potential to modify the storm environment, are consistent with a leading role played by air-sea interaction instability and CISK in maintaining the storm.