Preliminary Freezing Rain/DrizzleClimatology 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? □ 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
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
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
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
FZRA events peak in December/January. Kansas City December, January and February have an equal distribution of FZRA events.
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)
Kansas City Averages • (1971-2000) – 3.2 days/yr • (1948-1977) – 3.2 days/yr (not shown) • (1948-2000) – 3.2 days/yr
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.
Operational Application 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!!!
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.