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The 1998 Ice Storm—Analysis of a Planetary-Scale Event

2001; American Meteorological Society; Volume: 129; Issue: 12 Linguagem: Inglês

10.1175/1520-0493(2001)129 2.0.co;2

1520-0493

John R. Gyakum, Paul J. Roebber,

Tropical and Extratropical Cyclones Research

The ice storm of 5–9 January 1998, affecting the northeastern United States and the eastern Canadian provinces, was characterized by freezing rain amounts greater than 100 mm in some areas. The event was associated with a 1000–500-hPa positive (warm) thickness anomaly, whose 5-day mean exceeded +30 dam (+15°C) over much of New York and Pennsylvania. The region of maximum precipitation occurred in a deformation zone between an anomalously cold surface anticyclone to the north and a surface trough axis extending from the Gulf of Mexico into the Great Lakes. The thermodynamic impact of this unprecedented event was studied with the use of a four-dimensional data assimilation spanning an 18-day period ending at 0000 UTC 9 January 1998. A moisture budget for the precipitation region reveals the bulk of the precipitation to be associated with the convergence of water vapor transport throughout the precipitation period. The ice storm consisted of two primary synoptic-scale cyclonic events. The first event was characterized by trajectories arriving in the precipitation zone that had been warmed and moistened by fluxes over the Gulf Stream Current and the Gulf of Mexico. The second and more significant event was associated with air parcels arriving in the precipitation zone that had been warmed and moistened for a period of several days in the planetary boundary layer (PBL) of the subtropical Atlantic Ocean. These parcels had equivalent potential temperatures of approximately 330 K at 800 hPa as they traveled into the ice storm's precipitation zone. Analogs to this unprecedented meteorological event were sought by examining anomaly correlations (ACs) of sea level pressure, and 1000–925 and 1000–500-hPa thicknesses. Five analogs to the ice storm were found, four of which are characterized by extensive freezing rain. The best analog, that of 22–27 January 1967, is characterized by freezing rain extending from the northeastern United States into central Ontario. However, the maximum amounts are less than 50% of the 1998 case. An examination of air parcel trajectories for the 1967 case reveals a similar-appearing horizontal spatial structure of trajectories, with several traveling anticyclonically from the subtropical regions of the eastern Atlantic. However, a crucial distinguishing characteristic of these trajectories in the 1967 case is that the air parcels arriving in the precipitation zone had equivalent potential temperature values of only 310 K, as compared with 330 K for the 1998 ice storm trajectories. It was found that these air parcels had traveled above the PBL and, therefore, had not been warmed and moistened by fluxes from the subtropical oceans.