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