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.