A Closer Look at Ice Storm Severity in the Southeast United States Using an “Ingredients-Based” Methodology

1. A Closer Look at Ice Storm Severity in the Southeast United States Using an “Ingredients-Based” Methodology Christopher M. Fuhrmann Ph.D. Student Department of Geography University of North Carolina at Chapel Hill

2. Ice Storms and Freezing Rain (FZ) • Responsible for traffic accidents, power outages, damaged communication lines, stalled transportation networks, and stressed ecosystems • From 1949-2000, insured property losses from ice storms in the contiguous US >$18 billion (USD) • Greatest percentage of US ice storm catastrophes (i.e., producing >$1 million in insured property losses) have occurred in the Southeast region • Average losses per event >$122 million (second highest amount behind Northeast region) Source: Changnon (2003)

3. Forecast Approaches & Challenges • Approaches - Trad. Synoptic Climatology (SC): linking broad-scale circulation to the surface environment using composites/analogs - Pattern recognition: predictions based on canonical scenarios that assume a particular set of conditions - Forecasters build conceptual models (organize features and processes) to show how weather events are assembled • Challenges - Energy exchanges between environment and precipitation - Local effects (surface conditions, topography, moisture) - Distinguishing the ordinary from the extraordinary event - When the prevailing patterns begin to deviate from the composite…

4. “Ingredients-Based” Methodology (IM) • Ingredient - Fundamental, physical component or process that contributes to the development of a meteorological event - Ascent, moisture, instability, efficiency, temperature • Methodology - Predictions based on the presence and sufficiency of the ingredients regardless of how they are assembled - The large-scale environment (SC approach) is the “setting” under which the necessary ingredients are assembled - Multiple sets of diagnostics can be used to identify ingredients - Predicated on an understanding of the processes related to precipitation formation, growth, and rate Sources: Janish et al. (1996); Wetzel and Martin (2001); Schultz et al. (2002)

5. Defining an Ice Storm • Hourly surface weather observations from Greensboro, NC FOS (1958-1995) used in conjunction with Storm Data reports Rationale, limitations, regional representation… • Winter weather event: measurable precipitation with at least one observation of a winter precipitation type (snow, sleet, FZ, FZDZ) • Event terminated if >24 hr lapse in conditions • “Ice storm” criteria: FZ amount Proportion of event duration 0.10-0.24 in (0.25-0.61 cm) 75 percent 0.25-0.49 in (0.64-1.24 cm) 50 percent 0.50+ cm (1.27+ cm) ---

6. Ice Storm “Ingredients” at GSO • Ascent - Quasi-geostrophic forcing (Ω) - Isentropic upglide • Moisture - Availability, amount, trajectory • Efficiency - Precipitation (ice) formation and cloud microphysics - Growth rate by deposition, riming - Evaporation, melting, freezing • Instability - Upright gravitational convection - Parcel ascent - “Seeder” clouds • Temperature - Warm layer above cold wedge - A persistent feature, or… - Maintained during heavy precipitation (shorter duration) - Diabatic effects (energy exchange)

7. The Spectrum of Ice Storm Severity • 46 ice storms identified at GSO (1.2 per year) Heaviest Event: 4 Feb 1975 1.18 in Labels along the horizontal axis represent the minimum value for each bin in inches

8. Relationships Between Ice Storm Attributes and FZ Severity • FZ duration a decent, but not perfect proxy for FZ severity • Greatest variability at higher FZ amounts and longer durations • ~45% of events have max hourly FZ rates > 0.10” • Compare with all hourly FZ observations in contiguous US – 70% have rates < 0.05”

9. Ingredients for a Heavy Ice Storm at GSO 1. Strong quasi-geostrophic forcing - greater PVA over ice storm region due to strong cyclone? PVA PVA WAA WAA (Top Quartile, n = 11, 0.57-1.18”) (Bottom Quartile, n = 13, 0.13-0.26”)

10. Ingredients for a Heavy Ice Storm at GSO 2. More efficient ice formation and growth - Maximum growth rate by deposition at -15°C - Supercooled cloud liquid condensing onto active ice nuclei - Note cloud top temperatures (CTT)… Light events: 0.18-0.49” Heavy events: 0.50-1.18” n = 11 14 Thermal/microphysical environment assessed from 25 soundings for events at GSO

11. Ingredients for a Heavy Ice Storm at GSO 3. Upright gravitational convection - Variable FZ rates suggests embedded convection - Convection is upright (advection along sloped isentropic sfc) - Either embedded in cloud layer or through cloud top - Ice crystals from top of convective cloud (seeder) advected over/supplied to stratiform cloud (feeder) – riming, deposition Embedded convection with shallow ascent (50-300 hPa) Convection above cloud top with modest ascent (125-400 hPa)

12. Summary and Future Work • Ingredients-based methodology provides new insight into factors controlling ice storm severity in the Southeast US • FZ durations and max FZ rates variable among events • Ice storm ingredients that control severity - Stronger PVA over icing region (surface cyclone?) - Cloud environment optimal for efficient ice growth - Additional ice introduced into cloud by elevated convection - Higher mixing ratios in region of max depositional growth (?) • Ice storm ingredients not related to severity - Depth and temperature of warm layer and cold wedge - Mid-level (850-700 hPa) moisture • What’s next? Use synoptic climatological techniques to determine how ice storm ingredients are assembled