ABSTRACT
Ice storms are among the most hazardous, disruptive, and costly meteorological
phenomena in the northeastern United States. The accretion of freezing rain during ice
storms endangers human safety, compromises public infrastructure, and causes economic
losses on local and regional scales. Furthermore, ice storms present a major operational
forecast challenge due to the combined influence of synoptic, mesoscale, and
microphysical processes on precipitation type. In consideration of these socioeconomic
impacts and forecast issues, we have identified three primary objectives for this thesis: 1)
create long-term climatologies of freezing rain and ice storms in the northeastern U.S., 2)
identify antecedent environments conducive to ice storms and dynamical mechanisms
responsible for freezing rain, and 3) increase situational awareness of the synoptic and
mesoscale processes that govern the evolution of ice storms.
The climatology portion of this thesis examines the temporal and spatial
variability of freezing rain and ice storms during the 1975–2010 and 1993–2010 periods,
respectively. Individual ice storms are also partitioned by five characteristic synopticscale weather patterns. Synoptic composite maps for the two most common event types
(Type G and Type BC) illustrate how large-scale circulation patterns and associated
quasi-geostrophic (QG) forcing, thermal boundaries, and moisture transport establish
environments favorable for freezing rain. Composite cross sections also offer critical
insights into synoptic–mesoscale linkages that influence the duration and intensity of
freezing rain. The composite analysis for both event types suggests that ice storms occur
in association with QG ascent and low-level frontogenetical forcing beneath the
equatorward entrance region of an upper-level jet. Moreover, low- to midlevel warm
ii
advection and moisture transport, in conjunction with near-surface ageostrophic cold
advection on the poleward side of a surface warm front, helps prolong freezing rain
events by maintaining a thermodynamic profile conducive to freezing rain.
Case studies of the 3–4 Jan 1999 (Type G) and 11–12 Dec 2008 (Type BC) ice
storms investigate the synoptic evolution of each event and describe the physical
mechanisms that prolonged the duration of freezing rain and resulted in significant ice
accretions. These case studies not only reinforce the results of the composite analyses,
but also demonstrate how mesoscale processes such as terrain–flow interactions and
frontogenesis modify synoptic-scale circulations and thermodynamic environments on
regional and local scales. Based on our findings from the composite and case study
analyses, we propose conceptual models illustrating the key synoptic ingredients and
dynamical processes associated with Type G and Type BC ice storms.
iii