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Christopher M. Fuhrmann Ph.D. Student Department of Geography University of North Carolina at Chapel Hill A Closer Look at Ice Storm Severity in the Southeast.

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Presentation on theme: "Christopher M. Fuhrmann Ph.D. Student Department of Geography University of North Carolina at Chapel Hill A Closer Look at Ice Storm Severity in the Southeast."— Presentation transcript:

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

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 amountProportion 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) Labels along the horizontal axis represent the minimum value for each bin in inches Heaviest Event: 4 Feb 1975 1.18 in

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? WAA PVA WAA PVA (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)… Thermal/microphysical environment assessed from 25 soundings for events at GSO Light events: 0.18-0.49 Heavy events: 0.50-1.18 HeavyLight CTTPhaseSoundings > 0No ice211 With elevated cloud layer < 020 0 > CTT > -10Supercooled42 < -10Ice present21 < -15Max depositional growth10 n = 11 14

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 Convection above cloud top with modest ascent (125-400 hPa) Embedded convection with shallow ascent (50-300 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 Whats next? Use synoptic climatological techniques to determine how ice storm ingredients are assembled

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