freezing(precipitaon(and(ice( storms( · ice(storms(•...
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Freezing Precipita.on and Ice Storms Chapter 12
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Freezing Precipita.on
• Rain or drizzle that is liquid in form, but freezes immediately to surfaces when it contacts them
• Freezing Rain is a significant hazard to electrical u.li.es, airlines and road surface transporta.on
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Ice Storms
• Ice formed due to freezing precipita.on that is oCen undetectable
• What are the two types of ice?
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Supercooled Water
• Water that remains liquid at temperatures below freezing
• How does this happen?
• At what temperature will water freeze regardless?
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Ice Nuclei
• Soil, organic par.cles, pollutants, bacteria, etc • What is special about ice nuclei par.cles that makes water freeze to them?
• What is the difference between CCN and IN?
• What temperatures are ice nuclei most effec.ve at crea.ng ice? Marginally effec.ve? Not effec.ve at all?
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Freezing Precipita.on Forma.on
• Formed by two processes – Mel.ng process – Supercooled warm rain process
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Mel.ng Process
• Snow falls into a layer where the temperature exceeds O°C
• Snow melts in this layer and then falls back into a sub-‐freezing layer below
• If the depth of the subfreezing layer is sufficiently shallow, the melted snow won’t refreeze
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Supercooled Warm Rain Process
• Cloud droplets grow to precipita.on size by collision/coalescence process
• Snow/frozen precipita.on is not involved in this process
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Mel.ng Process vs Supercooled Warm Rain Process
• Precipita.on produced by mel.ng process is typically freezing rain
• Precipita.on produced by supercooled warm rain process is typically freezing drizzle
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Freezing Rain
• Need to have an inversion in the atmosphere for freezing rain processes to occur – Also referred to as a “warm nose aloC”
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Precipita.on Across a Front
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Freezing Rain
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Ice Pellets
• Formed when melted snow refreezes before hiVng the ground
• Also referred to as Sleet • Contrary to the book, they are a significant hazard to avia.on. Why?
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Freezing Drizzle
• Light precipita.on commonly mistaken as Mist • Drizzle drop diameters are typically (0.002 to 0.005 mm, 0.02 – 0.05 mm, 0.2 – 0.5 mm) in diameter
• Forms in cloud layers typically 1-‐3 km in depth • En.re cloud temperature is typically below freezing
• What are the common cloud top temperatures associated with freezing drizzle?
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Freezing Drizzle
• Causes ice accre.on on roadway surfaces • Biggest hazard is to aircraC both on the surface and aloC. Why?
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Freezing Precipita.on Weather Paberns
• Typically found east of the Rocky Mountains. Why?
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Arc.c Front/Arc.c High
• High Pressure over the Midwest • Stalled front along the Rockies, Gulf and East Coasts
• Warm air riding over the front creates shallow cloud layer
• Can produce either FZRA or FZDZ • Accounts for 1/3 of all freezing precipita.on events
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Arc.c Front / Arc.c High
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Warm Fronts (No High Pressure to the North)
• Freezing Precipita.on occurs north of warm fronts
• Typically oriented parallel to front • Warm fronts account for 1/3 of all freezing precipita.on events – ½ of those have no high pressure to the north – ½ of those do have a high pressure to the north
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Warm Fronts (No High Pressure to the North)
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Warm Front w/ Canadian High Pressure to the North
• Similar setup to a situa.on with no high pressure to the north
• Results in stronger temperature gradients – Results in stronger surface winds, which can increase accre.on rates
– Can lead to significant destruc.on of trees and power lines
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Warm Front w/ Canadian High Pressure to the North
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Western Quadrant of Arc.c High Pressure
• Southerly flow at surface and aloC typically occurs on western side of arc.c air masses
• If 0°C isotherm is located in this region, warm moist air being carried over the cold air below can lead to freezing precipita.on events
• Typically occurs in a more circular region • 1/10 of all freezing precipita.on events occur from this setup
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Western Quadrant of Arc.c High Pressure
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Cold Air Damming
• 15% of all freezing precipita.on events occur because of this
• Phenomena that occurs when boundaries (topography, density gradients, etc) trap air in a certain loca.on
• Develops in two ways
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Cold Air Damming
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Cold Air Damming
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Freezing Precipita.on Coverage
• Typically confined to narrow bands that move along with the weather systems
• What causes major ice storms?
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Detec.ng Freezing Precipita.on
• Two methods – Sfc based – Radar based
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Radar-‐based Detec.on of FZDZ
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Radar-‐Based Detec.on of FZDZ
• What is the drawback to using radar in detec.ng FZDZ?
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Radar-‐based Detec.on of FZRA
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Surface based
• Freezing Precipita.on Detec.on Sensor
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Freezing Precipita.on Sensor
• How does it work?
• What types of Precipita.on can it detect?
• Drawbacks?
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Freezing Rain Sensor
• How is frequency change related to ice thickness on the sensor?
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Freezing Rain Distribu.on
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Freezing Drizzle Distribu.on