Preliminary Freezing Rain/Drizzle
Climatology for EAX
Mike JulyWinter Weather/Cool Season Seminar
November 3, 2006
Why Focus on Freezing Rain and Drizzle?
□ 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
• Usually do not mix with other types of 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
Characteristics (common to both)
Characteristics(Differences)
Freezing Rain………… ….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 28-32FFreezing Drizzle………. ….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
Conditions Which Determine Ice Accumulation
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
4) Winds • accretion rate increases with increasing wind speeds (Simple Model) • winds at right angles to ice loaded wires are more damaging….leads to
“wire dancing”. • > 15 mph often causes wire breakage
• 30 yr average (1971-2000) – 3.2 events/yr• 30 yr average (1948-1977) – 3.2 events/yr• 53 yr average (1948-2000) – 3.2 events/yr
Seasonal Frequency 1948-2000
Dec-Feb72%
Rest of the Season
28%
FZRA events peak inDecember/January.
Kansas City
December, January andFebruary have an equaldistribution of FZRAevents.
Seasonal Frequency 1971-2000
Dec-Feb74%
Rest of the Season
26%
• 92% chance FZRA will occur in during the cold season (Nov-Mar).• 83% chance FZRA will occur on 1 to 5 days per cold season.• 58% chance FZRA will occur on 2 to 4 days (annual avg 3.2/cold season)
02468
101214
Number of
Seasons
0 1 2 3 4 5 6 7 8 9 10# of FZRA Events
KC Seasonal Distribution (1948-2000)
• (1971-2000) – 3.2 days/yr• (1948-1977) – 3.2 days/yr (not shown)• (1948-2000) – 3.2 days/yr
Kansas City Averages
Annual Mean Freezing Precipitation Days (FZRA & FZDZ)
Monthly Frequency of FZRA/FZDZ
Primary Weather Patterns Associated with Freezing Precipitation
Studies by Bennett (1958), Changnon (2003)and Rauber, et al (2001) came to very similarconclusions………
1) Arctic Fronts account for 42%2) Warm Front / Occlusion – 19%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.
Operational ApplicationForecast Challenges
◊ 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-
coalescence process?► 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!!!
Summary FZDZ occurs much more frequently than
FZRA. Super-cooled warm rain process highly favors FZDZ formation.
Classic “melting” process highly favors FZRA occurring. In KC freezing rain peaks in Dec/Jan.
Three primary weather patterns account for the vast majority of all freezing precipitation in Kansas City (possibly as high as 90%).
Between 1 and 5 freezing rain events per cold season with an annual average of 3.2/season.
Around 80% of all FZRA events occur with 2m temperatures within a tight range (28-32F)
whereas the vast majority of FZDZ events occur within a much broader range (21-32F).
Top-Down Method best tool to investigate soundings to determine precipitation type.
The Freezing Precipitation Frequency graphics from NCDC look suspect….especially for December.