, Volume 88, Issue 1-2, pp 65-90
Date: 24 May 2004

Cold-air cyclogenesis along the Gulf-Stream front: investigation of diabatic impacts on cyclone development, frontal structure, and track

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Summary

On 24–25 February 1989 a storm brought high winds and moderate to heavy snow to the U.S. East Coast. The storm is noteworthy for its rapid mesoscale development within a polar air mass at relatively low latitudes and for the difficulty experienced by operational NWP models and forecasters in predicting the storm’s impact. This paper investigates the mesoscale structure and evolution of the cold-air cyclone through analysis of enhanced data sets collected during the \(\underline{\rm E}\) xperiment on \(\underline{\rm R}\) apidly \(\underline{\rm I}\) ntensifying \(\underline{\rm C}\) yclones over the \(\underline{\rm A}\) tlantic (ERICA). Results are presented from numerical sensitivity studies of the impact of diabatic heating on storm structure and track using the Mesoscale Atmospheric Simulation System (MASS) model.

The following conclusions are drawn from the research. Differential surface fluxes in the vicinity of the Gulf Stream led to the development of a well-defined baroclinic zone at low levels that extended parallel to the axis of the Gulf-Stream front. The baroclinic zone strengthened and assumed the characteristics of a shallow warm front as the cyclone matured. Enhanced cyclonic vorticity, moisture-flux convergence, clouds, and precipitation accompanied the front. Early in the event a series of shallow, thermally forced vortices of small wavelength (∼200 km) formed along the baroclinic zone in the area of maximum surface-heat fluxes offshore of the Carolinas. Baroclinic instability associated with a vigorous short-wave trough aloft resulted in the outbreak of deep convection surrounding and rapid intensification of the northernmost vortex.

Numerical sensitivity experiments were conducted to investigate the nonlinear response of the mass field to the convection. The results show that latent heating in deep convection surrounding the surface low produced a mesoscale height perturbation aloft. The subsequent acceleration of the flow aloft substantially increased the integrated mass divergence above the surface cyclone, leading to deepening on the scale observed. The observed track of the low followed the axis of the warm front, which in turn followed the axis of maximum SST gradient associated with the Gulf-Stream front. Accurate simulation of the storm track required a high-resolution, full-physics run that included high-resolution SST data in the initial condition and moisture nudging during the early hours of the simulation.

Current affiliation: NWS Office of Climate, Water, and Weather Services, 1325 East-West Highway, Silver Spring, MD 20910.
Current affiliation: NWS UCAR/COMET, Boulder, CO 80307-3000.