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Forecasting Aircraft Icing Jonathan Leffler Chicago 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. Clear0 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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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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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 ~ 050-070 SLD 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 - KORD T + 4 (22Z) *Forecast* 075 015 Sampling Locations Snow Zone
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RUC 18Z - KORD T + 9 (03Z) *Forecast* 015 075 Snow Zone
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RUC 22Z - KORD T + 0 *Analysis* 055 035 Snow 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 - KORD T + 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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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 ?
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