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Forecasting Aircraft Icing
Jonathan LefflerChicago CWSU
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Agenda Definitions (review)
Considerations
Synoptic Conditions
Methodology
Mitigation Procedures
Icing Products & Reporting
Final Thoughts
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Definitions Types
• Rime: Whole droplet rapidly freezes, traps air, forms brittle, opaque, milky-colored ice. Most common form.
• Clear: Small part of drop freezes, rest smears along surface, less opaque, denser than rime. Greatest threat.
• Mixed: Combination of rime + clear. Can appear as layers of opaque and clear. Accumulates like clear, more difficult to remove than rime.
Clear 0 C to -10 C
Mixed -10 C to -15 C
Rime -15 C to - 40* C
Common Temperature Ranges* Icing typically not found< -20 C to -25 C due to the minimal availability of supercooled liquid
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Definitions Rates of Accumulation
• Trace: Typically not hazardous to flight. De-icing, anti-icing and altitude change normally not required < 1 hr.
• Light: Occasional use of de-icing or anti-icing is required to remove or prevent accumulation. Could become problematic if duration > 1 hr.
• Moderate: Short encounter can be hazardous. Use of de-icing or anti-icing equipment is necessary and heading/altitude change normally required.
• Severe: De-icing or anti-icing equipment cannot control or remove hazard. Immediate heading/altitude change required.
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Considerations Seasonality: Fall through Spring, Summer TS
Supercooled large drops (solid ↔ liquid, e.g. FZRA)
Liquid water content, not ice content (hard to measure)
Temperature advection (warm/cold), 0 C & -20/-25 C
Droplet Size (rain/drizzle/cloud)
Depth & duration of moisture (horizontally/vertically)
PIREPS! What has already been reported?
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Considerations
Frequency of icing type occurrence
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Synoptic Conditions
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Synoptic Conditions
Cold Front Usually not widespread Faster moving Clear more than Rime Heavier ice w/Cu vs St
Warm Front More widespread clouds Longer residency Rime more than Clear Heaviest in FZRA
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Synoptic Conditions
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Synoptic Conditions
Freezing Precip = Highest Threat Snowflake to raindrop phase transition
Generates supercooled large drops (SLDs)
Greater moisture availability
Clear ice freezing process (dense, hard to remove)
Rapid accumulation → Severe
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Synoptic Conditions
40-45 hr/yr
30-35 hr/yr
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Methodology Satellite: Visible & Infrared
• Surface moisture contamination (snow cover)• Cloud type (stratus vs cumulus)
Observations• Surface precipitation type (DZ, RA, SN)• Heavy moisture in dendritic “snow” zone tends to scrub supercooled droplets (more freezing process)• Cloud base & height
Radar + Sounding: Bright Band Height
AWIPS 4-Panel Interrogation (NWS) - Initial Areas
BUFKIT Soundings (Work/Home) - Refined Areas
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Methodology
~ 050-070SLD Threat
MOD or greater potential
0 C
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SFC-020 020-050
050-100 100-140
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SFC-020
?
?
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020-050
?
Lake Effect
?
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050-100
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100-140
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RUC 18Z - KORDT + 4 (22Z)*Forecast*
075
015
Sampling Locations
Snow Zone
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RUC 18Z - KORDT + 9 (03Z)*Forecast*
015
075
Snow Zone
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RUC 22Z - KORDT + 0*Analysis*
055
035Snow Zone
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ISOL LGT ICGICIP AOB 070.
The Forecast
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ISOL LGT ICGICIP AOB 070.
The Results
What does MOD ICG look like?
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RUC 18Z - KORDT + 4 (22Z)FWA 01 Feb/21Z
130
010
180
MOD
LGT Snow Zone
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CMI 01 Feb/21Z
130
005
180
MOD
LGT
SLD Threat
Snow Zone
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Mitigation Procedures Ascend into colder layer (< -20 C), typically safest based on accumulation potential
Descend into warmer layer (> 0 C), but caution...
Lateral into FEW/SCT deck vs BKN/OVC deck
Cold soaked skin = delayed warming and susceptible to icing with increased moisture
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So where do I go now ?
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Icing Products & Reporting
http://aviationweather.gov/adds
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Icing Products & Reporting
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Icing Products & Reporting
Graphical AIRMETS
http://aviationweather.gov/products/gairmet
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Icing Products & Reporting
AIRMETS/SIGMETS
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Icing Products & Reporting
PIREPS Java Tool
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Icing Products & Reporting
http://aviationweather.gov/adds
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Icing Products & Reporting
Current Icing Potential (CIP)Forecast Icing Potential (FIP)
CIP / FIP = Help pages on how icing analyses and forecasts are done
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Icing Products & Reporting
Experimental Icing Products
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Icing Products & Reporting
Experimental Icing Products
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Icing Products & Reporting
Experimental Icing Products
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Final Thoughts Icing can be a deadly aviation hazard if not planned for in advance.
Icing is subjective, however there are objective ways to forecast potential. We need your PIREPS!
Lots of web-based tools to help you flight plan
Graphics and literature courtesy AFWA TN-98/002
Special thanks to Robb Kaczmarek (Chicago CWSU) and Paul Merzlock (NWS Romeoville) for technical advice and support
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Questions ?