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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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Preliminary freezing rain drizzle climatology for eax l.jpg

Preliminary Freezing Rain/DrizzleClimatology for EAX

Mike July

Winter Weather/Cool Season Seminar

November 3, 2006


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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


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Characteristics(common to both)

  • 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


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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 28-32F

Freezing 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


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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


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FZRA events peak in

December/January.

Kansas City

December, January and

February have an equal

distribution of FZRA

events.


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Kansas City Averages (Nov-Mar).

  • (1971-2000) – 3.2 days/yr

  • (1948-1977) – 3.2 days/yr (not shown)

  • (1948-2000) – 3.2 days/yr




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Primary Weather Patterns Associated with Freezing Precipitation

Studies by Bennett (1958), Changnon (2003)

and Rauber, et al (2001) came to very similar

conclusions………

  • Arctic Fronts account for 42%

  • 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.


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Operational Application Precipitation

Forecast 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!!!


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Summary Precipitation

 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.


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