Download Chapter 12

Freezing Precipita.on and Ice Storms Chapter 12 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 Ice Storms •  Ice formed due to freezing precipita.on that is oCen undetectable •  What are the two types of ice? Supercooled Water •  Water that remains liquid at temperatures below freezing •  How does this happen? •  At what temperature will water freeze regardless? 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? Freezing Precipita.on Forma.on •  Formed by two processes –  Mel.ng process –  Supercooled warm rain process 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 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 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 Freezing Rain •  Need to have an inversion in the atmosphere for freezing rain processes to occur –  Also referred to as a “warm nose aloC” Precipita.on Across a Front Freezing Rain 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? 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? Freezing Drizzle •  Causes ice accre.on on roadway surfaces •  Biggest hazard is to aircraC both on the surface and aloC. Why? Freezing Precipita.on Weather Paberns •  Typically found east of the Rocky Mountains. Why? 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 Arc.c Front / Arc.c High 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 Warm Fronts (No High Pressure to the North) 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 Warm Front w/ Canadian High Pressure to the North 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 Western Quadrant of Arc.c High Pressure 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 Cold Air Damming Cold Air Damming Freezing Precipita.on Coverage •  Typically confined to narrow bands that move along with the weather systems •  What causes major ice storms? Detec.ng Freezing Precipita.on •  Two methods –  Sfc based –  Radar based Radar-­‐based Detec.on of FZDZ Radar-­‐Based Detec.on of FZDZ •  What is the drawback to using radar in detec.ng FZDZ? Radar-­‐based Detec.on of FZRA Surface based •  Freezing Precipita.on Detec.on Sensor Freezing Precipita.on Sensor •  How does it work? •  What types of Precipita.on can it detect? •  Drawbacks? Freezing Rain Sensor •  How is frequency change related to ice thickness on the sensor? Freezing Rain Distribu.on Freezing Drizzle Distribu.on