The 1998 ice storm - Analysis of a planetary-scale event

The ice storm of 5-9 January 1998, affecting the northeastern United States and the eastern Canadian provinces, was characterized by freezing rain amo unts greater than 100 mm in some areas. The event was associated with a 100 0-500-hPa positive (warm) thickness anomaly, whose 5-day mean exceeded +30 dam (+15 degreesC) over much of New York and Pennsylvania. The region of ma ximum precipitation occurred in a deformation zone between an anomalously c old surface anticyclone to the north and a surface trough axis extending fr om the Gulf of Mexico into the Great Lakes. The thermodynamic impact of thi s unprecedented event was studied with the use of a four-dimensional data a ssimilation spanning an 18-day period ending at 0000 UTC 9 January 1998. A moisture budget for the precipitation region reveals the bulk of the precip itation to be associated with the convergence of water vapor transport thro ughout the precipitation period. The ice storm consisted of two primary syn optic-scale cyclonic events. The first event was characterized by trajector ies arriving in the precipitation zone that had been warmed and moistened b y fluxes over the Gulf Stream Current and the Gulf of Mexico. The second an d more significant event was associated with air parcels arriving in the pr ecipitation zone that had been warmed and moistened for a period of several days in the planetary boundary layer (PBL) of the subtropical Atlantic Oce an. These parcels had equivalent potential temperatures of approximately 33 0 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 1000-50 0-hPa thicknesses. Five analogs to the ice storm were found, four of which are characterized by extensive freezing rain. The best analog, that of 22-2 7 January 1967, is characterized by freezing rain extending from the northe astern 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 subtropic al regions of the eastern Atlantic. However, a crucial distinguishing chara cteristic of these trajectories in the 1967 case is that the air parcels ar riving in the precipitation zone had equivalent potential temperature value s 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, there fore, had not been warmed and moistened by fluxes from the subtropical ocea ns.