christopher m. fuhrmann ph.d. student department of geography university of north carolina at chapel...
TRANSCRIPT
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