Preliminary Freezing Rain/Drizzle Climatology for EAX. Mike July Winter Weather/Cool Season Seminar November 3, 2006. Why Focus on Freezing Rain and Drizzle?. □ To fill a time slot in the seminar?
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Winter Weather/Cool Season Seminar
November 3, 2006
□ To fill a time slot in the seminar?
□ They produce hazardous weather conditions which can have significant impact on the power, insurance, and transportation industries and on public safety.
□ Average annual loss nationally – $313 million
Jan 29 – 31, 2002 - $32 million (KC Metro – Boonville)….worst ice storm ever in KC….ice over 1” thick....
at its peak over 409,00 customers without power in CWA; some without power for 2 weeks.
□ Accounts for 20% of all weather related injuries
□ Challenge to forecast in time and space
precipitation (east of Rockies)
• Usually short lived
(Nationally……≤ 2 hours/~70%)
• Usually end by cessation
• Tied to the diurnal solar cycle. Max occurs just before sunrise and drops off sharply during the morning with a late afternoon minimum.
• Cloud-top temperatures almost always warmer than -10C….i.e. little if any ice nuclei available
• 850mb and 700mb winds show a strong bias from the southwest
….if a transition - usually rain during the day
- evenly distributed among several pcpn types at night
• Normally associated with the classic “melting” process
• Most frequent north of surface warm front/occlusion
• Depth of moist/cloud layer deeper than FZDZ soundings
• Surface winds peak from the northeast to east
• East of Rockies approximately 80% of FZRA events occur with sfc temps 28-32F
….if a transition - strongly dominated by snow
• Most cases form via collision-coalescence or supercooled “warm rain” process
• Most frequent with passage of Arctic fronts
• Relatively shallow cloud layer
• Surface winds most common from the north
• East of Rockies approximately 90% of FZDZ events occur with sfc temps 21-32F
1) Precipitation Rate
• Model showed increasing rain rate =
greater accretion rate.
2) Precipitation Amount (duration)
• Bennett (1959) showed amount of accretion on wires was 40-60% of rainfall. If correct 2 inch rain = ~ 1 inch of ice.
• ice accumulation > 3/8” starts significant damage to trees/wires
• ice accumulation > 1” will cause most wires to break
3) Droplet Sizes and Temperatures
• 2m temperatures < 32F do not affect how much ice will form
• accretion rate increases with increasing wind speeds (Simple Model)
• winds at right angles to ice loaded wires are more damaging….leads to
• > 15 mph often causes wire breakage
December, January and
February have an equal
distribution of FZRA
Studies by Bennett (1958), Changnon (2003)
and Rauber, et al (2001) came to very similar
3) Cyclone/Anticyclone – 26%
• Pattern 3 most severe due to heavy icing
plus high winds.
• Area of FZRA/FZDZ is typically narrow
and just north of 0C surface isotherm.
◊ Freezing Drizzle vs Freezing Rain
► Depth of moist/cloud layer
► Any chance of ice falling into the cloud layer
► What type of weather pattern expected
► How much rain forecast…..FZRA advisory, ice storm warning
► Surface winds > 15 mph?
► How much/strong is the vertical motion in cloud layer to enhance the collision-
► Time of day when precipitation is expected
◊ Model Forecast/Observed Soundings
► Rauber, et al (1999)…25 year study; super-cooled warm rain process
responsible for 75% of all freezing precipitation soundings east of
Rockies……~72% of them produced only FZDZ.
► Top-Down Approach is best tool to use. Is the model sounding correct?
Check the 12z/00z analysis!!!
FZDZ occurs much more frequently than
Super-cooled warm rain process highly
favors FZDZ formation.
Classic “melting” process highly favors
In KC freezing rain peaks in Dec/Jan.
whereas the vast majority of FZDZ events occur within a much broader range (21-32F).