# Presentation: 706369

## 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?
- □  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

- 	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 28-32F
_**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 

- 30 yr average (1948-1977) – 3.2 events/yr
- 53 yr average (1948-2000) – 3.2 events/yr

## FZRA events peak in
December/January.

- 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) 

- 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)

## (1971-2000) – 3.2 days/yr
(1948-1977) – 3.2 days/yr (not shown)
(1948-2000) – 3.2 days/yr

- (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)

- 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 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!!!

## Slide 14

## 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.