Forecasting Aircraft Icing

Jonathan LefflerChicago CWSU

Agenda Definitions (review)

Considerations

Synoptic Conditions

Methodology

Mitigation Procedures

Icing Products & Reporting

Final Thoughts

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

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.

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?

Considerations

Frequency of icing type occurrence

Synoptic Conditions

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

Synoptic Conditions

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

Synoptic Conditions

40-45 hr/yr

30-35 hr/yr

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

Methodology

~ 050-070SLD Threat

MOD or greater potential

0 C

SFC-020 020-050

050-100 100-140

SFC-020

?

?

020-050

?

Lake Effect

?

050-100

100-140

RUC 18Z - KORDT + 4 (22Z)*Forecast*

075

015

Sampling Locations

Snow Zone

RUC 18Z - KORDT + 9 (03Z)*Forecast*

015

075

Snow Zone

RUC 22Z - KORDT + 0*Analysis*

055

035Snow Zone

ISOL LGT ICGICIP AOB 070.

The Forecast

ISOL LGT ICGICIP AOB 070.

The Results

What does MOD ICG look like?

RUC 18Z - KORDT + 4 (22Z)FWA 01 Feb/21Z

130

010

180

MOD

LGT Snow Zone

CMI 01 Feb/21Z

130

005

180

MOD

LGT

SLD Threat

Snow Zone

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

So where do I go now ?

Icing Products & Reporting

http://aviationweather.gov/adds

Icing Products & Reporting

Icing Products & Reporting

Graphical AIRMETS

http://aviationweather.gov/products/gairmet

Icing Products & Reporting

AIRMETS/SIGMETS

Icing Products & Reporting

PIREPS Java Tool

Icing Products & Reporting

http://aviationweather.gov/adds

Icing Products & Reporting

Current Icing Potential (CIP)Forecast Icing Potential (FIP)

CIP / FIP = Help pages on how icing analyses and forecasts are done

Icing Products & Reporting

Experimental Icing Products

Icing Products & Reporting

Experimental Icing Products

Icing Products & Reporting

Experimental Icing Products

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

Questions ?