Christopher M. FuhrmannPh.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

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)

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…

“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)

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) ---

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)

The Spectrum of Ice Storm Severity

• 46 ice storms identified at GSO (1.2 per year)

0

2

4

6

8

10

12

14

16

18

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Ab

solu

te F

req

uen

cy (

1958

-199

5)

Labels along the horizontal axis represent the minimum value for each bin in inches

HeaviestEvent:

4 Feb 19751.18 in

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”

Ingredients for a Heavy Ice Storm at GSO

1. Strong quasi-geostrophic forcing - greater PVA over ice storm region due to strong cyclone?

-88 -86 -84 -82 -80 -78 -76 -74 -72 -70

28

30

32

34

36

38

40

42

44

WAAWAA

PVAPVA

-88 -86 -84 -82 -80 -78 -76 -74 -72 -70

28

30

32

34

36

38

40

42

44

WAAWAA

PVAPVA

(Top Quartile, n = 11, 0.57-1.18”) (Bottom Quartile, n = 13, 0.13-0.26”)

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”

Heavy Light

CTT Phase Soundings Soundings

> 0 No ice 2 11

With elevated cloud layer < 0 2 0

0 > CTT > -10 Supercooled 4 2

< -10 Ice present 2 1

< -15 Max depositional growth 1 0

n = 11 14

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)

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