On the use of Liquid Water Equivalent (LWE) for estimating snowfall intensity and freezing drizzle intensity versus the use of visibility Roy Rasmussen

On the use of Liquid Water Equivalent On the use of Liquid Water Equivalent (LWE) for estimating snowfall intensity (LWE) for estimating snowfall intensity

and freezing drizzle intensity versus and freezing drizzle intensity versus the use of visibilitythe use of visibility

Roy Rasmussen National Center for Atmospheric Research

Presentation to OFCM meetingFeb. 14, 2008

Background

• Rough ice accumulation on a wing, the thickness of a thumbnail, leads to a loss of lift and increase in drag of 25%, requiring that aircraft be deiced with Type I (deicing), and/or Type II or IV anti-icing fluids (both either ethylene or propylene glycol based) to remove ice and protect against re-icing before takeoff.

• Holdover time is the period that deicing/anti-icing fluids protect against re-icing.

BackgroundBackground

Outdoor and indoor testing of deicing fluids Outdoor and indoor testing of deicing fluids for endurance time has shown that the liquid for endurance time has shown that the liquid equivalent snowfall rate is the main cause for equivalent snowfall rate is the main cause for fluid failure:fluid failure:– Dilutes the fluid, leading to runoffDilutes the fluid, leading to runoff– Causes the freeze temperature of the fluid to Causes the freeze temperature of the fluid to

increaseincrease

Holdover times are also sensitive to fluid Holdover times are also sensitive to fluid temperaturetemperature

Deicing fluid endurance time as a function of liquid equivalent rate

Regression analysis that provides a power law relationship is used for each fluid and each dilution for each weather condition

0

10

20

30

40

50

60

70

80

90

100

110

120

0 5 10 15 20 25 30 35 40 45Plate Pan

Rate of Precipitation (g/dm²/h)

Flui

d Fa

ilure

Tim

e on

Pla

te (m

in)

0 to -3°C-3 to -14°CCurrent SAE HOT (0 to -3°C)Current SAE HOT (-3 to -14°C)Current SAE HOT (Below -14°C)-3-14-25

# of Tests = 37

FAA TYPE I HOLDOVER TIME GUIDELINE

Table 1. FAA Guideline for Holdover Times SAE Type I Fluid Mixtures as a Function of Weather Conditions and OAT.

CAUTION: THIS TABLE IS FOR DEPARTURE PLANNING ONLY AND SHOULD BE USED IN CONJUNCTION WITH PRE-TAKEOFF CHECK PROCEDURES.

Outside Air

Temperature

Approximate Holdover Times Under Various Weather Conditions (hours: minutes)

Snow/Snow Grains Degrees Celsius

Degrees Fahrenheit

Active Frost

Freezing Fog Very

Light Light Moderate

Freezing Drizzle*

Light Freezing

Rain

Rain on Cold Soaked Wing**

Other‡

-3 and above

27 and above 0:45 0:11 - 0:17 0:18-0:22 0:11 - 0:18 0:06 - 0:11 0:09 - 0:13 0:02 - 0:05 0:02-0:05

below -3 to -6

below 27 to 21

0:45 0:08 - 0:13 0:14-0:17 0:08 - 0:14 0:05 - 0:08 0:05 - 0:09 0:02 - 0:05

below -6 to -10

below 21 to 14

0:45 0:06 - 0:10 0:11-0:13 0:06 - 0:11 0:04 - 0:06 0:04 - 0:07 0:02 - 0:05

CAUTION: No holdover time guidelines exist

below -10

below 14 0:45 0:05 - 0:09 0:07-0:08 0:04 - 0:07 0:02 - 0:04

THE RESPONSIBILITY FOR THE APPLICATION OF THESE DATA REMAINS WITH THE USER. * Use light freezing rain holdover times if positive identification of freezing drizzle is not possible ** This column is for use at temperatures above 0 degrees Celsius (32 degrees Fahrenheit) only ‡ Heavy snow, snow pellets, ice pellets, moderate and heavy freezing rain, hail TO USE THESE TIMES, THE FLUID MUST BE HEATED TO A MINIMUM TEMPERATURE OF

60C (140F) AT THE NOZZLE AND AT LEAST 1 LITER/M2 ( 2 GALS/100FT2) MUST BE APPLIED TO DEICED SURFACES SAE Type I fluid/water mixture is selected so that the freezing point of the mixture is at least 10 C

(18 F) below OAT. CAUTIONS:

THE TIME OF PROTECTION WILL BE SHORTENED IN HEAVY WEATHER CONDITIONS. HEAVY PRECIPITATION RATES OR HIGH MOISTURE CONTENT, HIGH WIND VELOCITY, OR JET BLAST MAY REDUCE HOLDOVER TIME BELOW THE LOWEST TIME STATED IN THE RANGE. HOLDOVER TIME MAY BE REDUCED WHEN AIRCRAFT SKIN TEMPERATURE IS LOWER THAN OAT.

SAE TYPE I FLUID USED DURING GROUND DEICING/ANTI-ICING IS NOT INTENDED FOR AND DOES NOT PROVIDE PROTECTION DURING FLIGHT. August 2006

2.5 – 4 g/d2/hrVery Light

4 – 10 g/d2/hrLight

10-25 g/d2/hrModerate

> 25 g/d2/hrHeavy

0

20

40

60

80

100

120

0 10 20 30 40 50

Octagon 50% data

Endurance Time (min) T = 0 CEndurance Time (min) T = -5 C Endurance Time (min) T = -10 C

En

du

ran

ce T

ime

(min

)

Precipitation Rate (g/d2/hr)

Ver

y L

igh

tL

ig ht

Mo

de

rat

e

He

av

y

FAA TYPE IV HOLDOVER TIME GUIDELINE TABLE 4 - FAA Guideline for Holdover Times SAE Type IV Fluid Mixtures as a Function of

Weather Conditions and OAT.

CAUTION: THIS TABLE IS FOR DEPARTURE PLANNING ONLY AND SHOULD BE USED IN CONJUNCTION WITH PRE-TAKEOFF CHECK PROCEDURES.

Outside Air Temperature Approximate Holdover Times Under Various Weather Conditions (hours: minutes)

Degrees Celsius

Degrees Fahrenheit

Type IV Fluid Concentration

Neat-Fluid/Water (Volume %/Volume %)

Active Frost

Freezing Fog Snow/Snow Grains

Freezing Drizzle*

Light Freezing Rain

Rain on Cold Soaked Wing**

Other‡

100/0 12:00 1:15-2:30 0:35-1:15 0:40-1:10 0:25-0:40 0:10-0:50

75/25 5:00 1:05-1:45 0:20-0:55 0:35-0:50 0:15-0:30 0:05-0:35

-3 and above

27 and above

50/50 3:00 0:15-0:35 0:05-0:15 0:10-0:20 0:05-0:10

100/0 12:00 0:20-1:20 0:20-0:40 ***0:20-0:45 ***0:10-0:25 below

-3 to -14

below

27 to 7 75/25 5:00 0:25-0:50 0:15-0:35 ***0:15-0:30 ***0:10-0:20

CAUTION: No holdover time guidelines exist

below

-14 to -25

below

7 to -13

100/0 12:00 0:15-0:40 0:15-0:30

below -25 below -13 100/0 SAE Type IV fluid may be used below -25 °C (-13 °F) provided the freezing point of the fluid is at least 7 °C(13 °F) below the OAT and the aerodynamic acceptance criteria are met. Consider use of SAE Type I when SAE Type IV fluid cannot be used.

THE RESPONSIBILITY FOR THE APPLICATION OF THESE DATA REMAINS WITH THE USER. * Use light freezing rain holdover times if positive identification of freezing drizzle is not possible ** This column is for use at temperatures above 0 °C (32°F) only *** No holdover time guidelines exist for this condition below -10 C (14 F) ‡ Snow pellets, ice pellets, heavy snow, moderate and heavy freezing rain, and hail

CAUTIONS:

THE TIME OF PROTECTION WILL BE SHORTENED IN HEAVY WEATHER CONDITIONS. HEAVY PRECIPITATION RATES OR HIGH MOISTURE CONTENT, HIGH WIND VELOCITY, OR JET BLAST MAY REDUCE HOLDOVER TIME BELOW THE LOWEST TIME STATED IN THE RANGE. HOLDOVER TIME MAY BE REDUCED WHEN AIRCRAFT SKIN TEMPERATURE IS LOWER THAN OAT.

SAE TYPE IV FLUID USED DURING GROUND DEICING/ANTI-ICING IS NOT INTENDED FOR AND DOES NOT PROVIDE PROTECTION DURING FLIGHT.

Moderate Light

0

20

40

60

80

100

120

0 10 20 30 40 50

Octagon 50% data

Endurance Time (min) T = 0 CEndurance Time (min) T = -5 C Endurance Time (min) T = -10 C

En

du

ran

ce T

ime

(min

)

Precipitation Rate (g/d2/hr)

Upper range of snow cell

at -5 C

Lower range ofsnow cell

at -5 C

Type IV

Snowfall Intensity by Visibility

NWS Category

Light

Moderate

Heavy

Visibility Liquid Equivalent Snowfall Rate

> 5/8 mi < 1.0 mm/hr

5/8-5/16 mi 1.0 to 2.5 mm/hr

1/4 mi or less > 2.5 mm/hr

- SN

SN

+ SN

Background

• An analysis of five aircraft ground deicing accidents showed that the snowfall rate as determined by a weighing snowgauge was nearly identical for each accident (near 2.5 mm/hr), but that the snowfall intensity reported to the pilot from the METAR using visibility ranged from light to heavy.

“Common Snowfall Conditions Associated with Aircraft Takeoff Accidents”, 2000: R.

Rasmussen, J. Cole, R.K. Moore, and M. Kuperman. J. Aircraft, Vol. 37, 110 – 116.

Liq

uid

-eq

uiv

alen

t P

reci

pit

atio

n R

ate

(in

/hr)

LightS-

ModS

HeavyS+

Snow Intensity Based on Visibility Alone

.004 in/hr

.04 in/hr

.08 in/hr

.10 in/hr

.4 in/hr

x*

+

Accidents Plotted Against Visibility AndLiquid-Equivalent Precipitation Rate

Background• Since the current method to estimate snow intensity is based

on visibility, I decided to do a follow up study to examine this relationship in more detail.

• Study based on observations from well shielded weighing snowgauges and calibrated automatic visibility sensors. Visibility sensor calibrated during snow conditions by a manual observer.

• Theoretical study also performed and compared to the observations.

“The Estimation of Snowfall Rate Using Visibility”, 1999: R. Rasmussen, J. Vivekanandan, J. Cole, B. Myers, and C. Masters. Journal of Applied Meteorology, Vol. 38, 1542 – 1563.

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty [k

m]

Snowfall Rate [mm/hr]

'94-'95

SN (Light)

SN (Moderate)

+SN (Heavy)

Boston

LaGuardia

Washington, D.C.

Denver Newark

Observations of Visibility and Liquid Equivalent Snowfall Rate from the NCAR Marshall Field Site

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty [k

m]


'94-'95

SN (Light)

SN (Moderate)

+SN (Heavy)

Boston

LaGuardia

Washington, D.C.

Denver Newark


Light Mod Heavy

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty [k

m]


'94-'95

SN (Light)

SN (Moderate)

+SN (Heavy)

Boston

LaGuardia

Washington, D.C.

Denver Newark


Light Mod Heavy

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty [k

m]


'94-'95

SN (Light)

SN (Moderate)

+SN (Heavy)

Boston

LaGuardia

Washington, D.C.

Denver Newark


Light Mod Heavy

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty (

km)

Snowfall Rate (mm/hr)

Wet Snowflakes

Dry snowflakes(aggretated snowcrystals)

Theoretical Relationship between Visibility andLiquid Equivalent Snowfall Rate for Dry and Wet snow

Theoretical Relationship between Visibility and Liquid Equivalent Snowfall Rate for various crystal types and degree of riming.

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty (

km)

Snowfall Rate (mm/hr)

Wet Snow

DrySnow

1/17 - 1/18/96

0.1

1

10

0.01 0.1 1 10

Vis

ibili

ty (

km)

ETI Snowfall Rate (mm/hr)

N1a R3b

C1h

P1a

Wet

R1a Dry

P1e

94-95

R2a Key

Wet - Wet SnowflakesDry - Dry SnowflakesP1e - DendritesP1a - Hexagonal platesC1h - Thick platesN1a - Elementary needleR2a - Densely rimed plateR3b - Heavily rimed dendriteR1a - Rimed elementary needle

Boston

LaGuardia

Washington, D.C.

DenverNewark

Theory, Observations, and Accident Data Combined

LOG LOG PLOTVISIBILITY VS. RATE (1995-96 to 2001-02)

1.0 (100%)

2.5 (100%)

0.5 (85%)

1.0 (94%)

253

0.7 (~95%)

1.1 (~95%)

1.9 (~95%)

0.1

1

10

1 10 100

Plate Pan Rate (g/dm2/h)

AE

S V

isib

ility

(Sta

tute

Mile

s)

TC

FAA

Captures >95%

Night, <-1ºC

Data Points = 3816Days Represented = 89Hours of Data = 381.6

Courtesy of APS Aviation

Seven years of data from Montreal

LOG LOG PLOTVISIBILITY VS. RATE (1995-96 to 2001-02)

1.0 (100%)

2.5 (100%)

0.5 (85%)

1.0 (94%)

253

0.7 (~95%)

1.1 (~95%)

1.9 (~95%)

0.1

1

10

1 10 100

Plate Pan Rate (g/dm2/h)

AE

S V

isib

ility

(Sta

tute

Mile

s)

TC

FAA

Captures >95%

Night, <-1ºC

Data Points = 3816Days Represented = 89Hours of Data = 381.6

Courtesy of APS Aviation

Seven years of data from Montreal

Light

Moderate

Heavy

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3

Day versus Night Visibility using Vd=-3Vn/ln(Vn*0.00336)

A

Nig

httim

e V

isib

ility

, Vn

(mile

s)

Daytime Visibility, Vd (miles)

Day vs Night Visibility

• Visibility at night is twice as good as the visibility during the day for

the same snowfall rate

• Means that going from day to night decreases the snowfall rate from

heavy to moderate, or from moderate to light.

• The LaGuardia accident (3/22/92) occurred at night

– Occurred under wet snow conditions

– Snow intensity of light

– Occurred at night

Summary• Both observations and theory show that a given visibility is not

uniquely related to only one snowfall rate due to:

– a variety of crystal types– varying degrees of riming – varying degrees of aggregation– wet snow– difference in visibility between day and night for a given

extinction coefficient

• Above suggests that the variation in visibility during the previous deicing accidents is real and potentially misleading.

FREQUENCY OF DE/ANTI-ICING OPERATIONS

(ALL AIRPORTS) – SURVEY 2000-03

Type I / IV 2000-2003

Frost33%

Snow 56%

ZP7%

Other 4%

Visibility Criteria for Snow Intensity

Time of Day

Temp.Visibility (Statute Mile)

(C) (F) 2 1/2 2 1 1/2 1 3/4 1/2 1/4

Day

colder/equal-1

colder/equal 30VeryLight

Very Light

Light Light Moderate Moderate HeavySnowfall Intensity

warmer than-1

warmer than 30VeryLight

Light Light Moderate Moderate Heavy Heavy

Night

colder/equal-1


Light Moderate Moderate Heavy Heavy Heavy

warmer than-1


Light Moderate Heavy Heavy Heavy Heavy

NOTE: Based upon technical report, “The Estimation of Snowfall Rate Using Visibility,” Rasmussen, et al., Journal of Applied Meteorology, October 1999 and additional in situ data.

HEAVY = Caution - no holdover time guidelines exist

Modified Visibility Criteria for Snow Intensity Based on Temperature and Day or Night

Time of Day

Temp.Visibility (Statute Mile)

(C) (F) 2 1/2 2 1 1/2 1 3/4 1/2 1/4

Day

colder/equal-1


Very Light

Light Light Moderate Moderate HeavySnowfall Intensity

warmer than-1


Light Light Moderate Moderate Heavy Heavy

Night

colder/equal-1


Light Moderate Moderate Heavy Heavy Heavy

warmer than-1


Light Moderate Heavy Heavy Heavy Heavy

NOTE: Based upon technical report, “The Estimation of Snowfall Rate Using Visibility,” Rasmussen, et al., Journal of Applied Meteorology, October 1999 and additional in situ data.

HEAVY = Caution - no holdover time guidelines exist

PROBABILITY OF SNOW IN EACH HOT TABLE TEMPERATURE RANGE – TYPE I FLUIDS

Temperature (°C)

Very Light Snow

Light Snow

Moderate Snow

Heavy Snow Total

-3 and above 20.1% 8.0% 9.9% 1.2% 39.2%

below -3 to -6 12.4% 5.2% 6.1% 0.8% 24.4%

below -6 to -10 11.6% 4.1% 4.9% 0.6% 21.2%

below -10 9.0% 2.6% 3.1% 0.5% 15.2%

Total 53.2% 19.9% 24.0% 3.0% 100.0%

• The liquid equivalent rate of snow, freezing rain, snow pellets, ice pellets, and freezing drizzle is the primary factor causing aircraft deicing fluids to fail.

• The current NWS estimates of snowfall and drizzle intensity available to pilots via METARS through ASOS and ATIS is based on visibility, not liquid equivalent.

• The use of a modified visibility table is only a stop-gap measure. Need real-time liquid equivalent!

The lack of liquid equivalent and precipitation type real-time information updated every minute is having a significant impact on aircraft deicing operations and is a significant safety factor

Hotplate (Yankee)

Weighing Snowgauge (GEONOR)

Liquid WaterEquivalent Measurements

ASOS weighing precipitation gauge (AWPAG)Manufactured by OTT

CHANGE PARAGRAPH 2.6.6 TO INCLUDE CHANGES APPROVED BY ACCB AND APMC?CHANGE PARAGRAPH 2.6.6 TO INCLUDE CHANGES APPROVED BY ACCB AND APMC? 2.6.6 Algorithms Used by Automated Stations. 2.6.6 Algorithms Used by Automated Stations. Automated stations shall use algorithms that Automated stations shall use algorithms that

conform with the latest conform with the latest Federal Standard Algorithms for Automated Weather Observing Systems Federal Standard Algorithms for Automated Weather Observing Systems published by the OFCM, published by the OFCM, or or subsequently approved by the ASOS Configuration Control subsequently approved by the ASOS Configuration Control Board and ASOS Program Management Council.Board and ASOS Program Management Council. These algorithms do not apply to previously These algorithms do not apply to previously authorized systems, which may continue to operate until replaced or modified.authorized systems, which may continue to operate until replaced or modified.

8.4 Present Weather Observing Standards8.4 Present Weather Observing Standards 8.4.1 Qualifiers. 8.4.1 Qualifiers. Present weather qualifiers fall into two categories: intensity or Present weather qualifiers fall into two categories: intensity or

proximity and descriptors. Qualifiers may be used in various combinations to describe proximity and descriptors. Qualifiers may be used in various combinations to describe weather phenomena.weather phenomena.

a. a. Intensity/Proximity. Intensity/Proximity. The intensity qualifiers are: light, moderate, and heavy. The The intensity qualifiers are: light, moderate, and heavy. The proximity qualifier is vicinity.proximity qualifier is vicinity.

(1) (1) Intensity of Precipitation. Intensity of Precipitation. When more than one form of precipitation is occurring at When more than one form of precipitation is occurring at a time or precipitation is occurring with an obscuration, the intensities determined shall a time or precipitation is occurring with an obscuration, the intensities determined shall be no greater than that which would be determined if any forms were occurring alone. be no greater than that which would be determined if any forms were occurring alone.

The intensity of precipitation shall be identified as light, moderate, or heavyThe intensity of precipitation shall be identified as light, moderate, or heavy in in accordance with one of the following:accordance with one of the following:

SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS?SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS? (b) (b) Intensity of Snow and Drizzle. Intensity of Snow and Drizzle. The manually-derived intensity of snow and drizzle shall The manually-derived intensity of snow and drizzle shall

be based on the reported surface visibility in accordance with Table 8-6, when occurring be based on the reported surface visibility in accordance with Table 8-6, when occurring alone. Where possible, the intensity of snow or drizzle may be automatically determined alone. Where possible, the intensity of snow or drizzle may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated by sensors operating in accordance with the Federal Standard Algorithms for Automated Observing Systems Used for Aviation Purposes using a liquid water equivalent rate. Observing Systems Used for Aviation Purposes using a liquid water equivalent rate.

Table 8-4. Table 8-4. Intensity of Snow Based on Liquid Water Equivalent RateIntensity of Snow Based on Liquid Water Equivalent Rate IntensityIntensity CriteriaCriteria Light Less than 1 mm*/hr Moderate 1 – 2.5 mm*/hr Heavy Greater than 2.5 mm*/hr Table 8-6. Estimating Intensity of Snow or Drizzle Based on Visibility Intensity Criteria Light Visibility > 1/2 mile. Moderate Visibility > 1/4 mile but < 1/2 mile. Heavy Visibility < 1/4 mile.

Proposed change #2:Proposed change #2: 2.5.2 Aviation Selected Special Weather Report (SPECI) 2.5.2 Aviation Selected Special Weather Report (SPECI) a. Criteria for SPECI a. Criteria for SPECI CHANGE CHANGE (6) PRECIPITATION. (6) PRECIPITATION. (a) hail begins or ends. (a) hail begins or ends. (b) freezing precipitation begins, ends, or changes intensity. (b) freezing precipitation begins, ends, or changes intensity. (c) ice pellets begin, end, or change intensity. (c) ice pellets begin, end, or change intensity. TOTO (6) PRECIPITATION. (6) PRECIPITATION. (a) hail begins or ends. (a) hail begins or ends. (b) freezing precipitation begins, ends, or changes intensity. (b) freezing precipitation begins, ends, or changes intensity. (c) ice pellets begin, end, or change intensity. (c) ice pellets begin, end, or change intensity. (d) snow begins, ends or changes intensity.(d) snow begins, ends or changes intensity.

Freezing drizzle

•

Intensity

FMH-1 Criteria for Drizzle Intensity1

FZDZ Accretion Rate Criteria2

Light Visibility > ½ mile Rate ≤ 0.25 mm/hr

(0.01 in/hr)

Moderate Visibility > ¼ mile but ≤ ½ mile

Rate > 0.25 mm/hr but ≤ 0.5 mm/hr

Heavy Visibility ≤ ¼ mile Rate > 0.5 mm/hr

(0.02 in/hr)

Criteria for Determining Freezing Drizzle Intensity

1 Federal Meteorological Handbook

2 Glossary of the American Meteorological Society

Remarks on Current Methods to Estimate Freezing Drizzle Intensity

• Currently used method to estimate drizzle intensity (light, moderate, or heavy) reported on METAR’s is

based on visibility. This is true both with the U.S. National Weather Service, and also internationally.

•

Remarks on Current Methods to Estimate Freezing Drizzle Intensity

• Report by Allan Ramsay prepared for the U.S. National Weather Service ASOS program (Sept. 21, 1999) has shown that the use of visibility

to estimate drizzle intensity is highly inaccurate in terms of the actual ice accretion rate on an exposed surface.

•

Freezing Drizzle

Heatingcycles

Frequencydrop due to ice accretion on the sensor

Ramsay Algorithm for FZRA/FZDZIce Detector LEDWI

Present Wx Type

Temp Visibility Sky Cover

Present Weather Reported

AccretionFrequency < 39,967 Hz

and

15 min Accretion Rate>13 Hz in 15 min

RA,UP <2.8 C

(<37°F)

Any Any FZRA

SN Any Any Any SN


and

15 min Accretion Rate>6 Hz in 15 min

No Precip

< 0 C

(<32°F)

Any OVC FZDZ

Not OVC

None


No Precip

< 0 C

(<32°F)

> 7 miles CLR or

SCT

FROST

Icing Rates

• Generally based on rate per 15 minutes• .01 inch/hour = 66 Hz/hour = 16.5 Hz/15 min

FZFG .001 - .004 in/hr 1 – 5 Hz/15 min

Light FZDZ .004 - .01 in/hr 6 – 16 Hz/15 min

MDT FZDZ .01 - .02 in/hr 17 – 33 Hz/15 min

Heavy FZDZ > .02 in/hr > 33 Hz/15 min

0

2000

4000

6000

8000

10000

12000

14000

16000

Light Moderate Heavy

Visibility baseintensity

Accretaion basedintensity

One minute freezing drizzle observations from ASOS siteswith freezing rain sensors at various locations in the U.S. from 1998-99 (Analysis by Al Ramsey, formerly of Raytheon).

Freezing Drizzle Intensity

Min

ute

s

0

10

20

30

40

50

60

70

80

90

100

Light Moderate Heavy

Visibility baseintensity

Accretaion basedintensity


Per

cen

t of

tim

e

One minute freezing drizzle observations from ASOS siteswith freezing rain sensors at various locations in the U.S. from 1998-99 (Analysis by Al Ramsey, formerly of Raytheon).


Quotes from the Al Ramsey report to the U.S. National Weather Service:

“It is clear that visibility-based criteria provide little or no information on the intensity of ice accretion from freezing drizzle.”

“…data collected in this project clearly indicate that METAR/SPECI reports of FZDZ intensity, based on visibility, do not accurately represent the rate of ice accretion, and they may mislead users into selecting incorrect holdover times.”

Freezing drizzle ingest into engines can cause significant damage to fan blades (light freezing drizzle reported by METAR, heavy freezing drizzle actually occuring based on liquid equivalent)

SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS?SHOULD WE CHANGE INTENSITY OF SNOW AND DRIZZLE AS FOLLOWS? (b) (b) Intensity of Snow and Drizzle. Intensity of Snow and Drizzle. The manually-derived intensity of snow and drizzle shall The manually-derived intensity of snow and drizzle shall

be based on the reported surface visibility in accordance with Table 8-6, when occurring be based on the reported surface visibility in accordance with Table 8-6, when occurring alone. Where possible, the intensity of snow or drizzle may be automatically determined alone. Where possible, the intensity of snow or drizzle may be automatically determined by sensors operating in accordance with the Federal Standard Algorithms for Automated by sensors operating in accordance with the Federal Standard Algorithms for Automated Observing Systems Used for Aviation Purposes using a liquid water equivalent rate. Observing Systems Used for Aviation Purposes using a liquid water equivalent rate.

Table 8-5. Table 8-5. Intensity of Freezing Drizzle Based on Liquid Water Equivalent RateIntensity of Freezing Drizzle Based on Liquid Water Equivalent Rate IntensityIntensity CriteriaCriteria LightLight Less than 0.254 mm*/hrLess than 0.254 mm*/hr ModerateModerate 0.254 – 0.508 mm*/hr0.254 – 0.508 mm*/hr HeavyHeavy Greater than 0.508 mm*/hrGreater than 0.508 mm*/hr Table 8-6. Table 8-6. EstimatingEstimating Intensity of Snow or Drizzle Based on Visibility Intensity of Snow or Drizzle Based on Visibility Intensity Intensity CriteriaCriteria Light Light Visibility > 1/2 mile.Visibility > 1/2 mile. Moderate Moderate Visibility > 1/4 mile but < 1/2 mile.Visibility > 1/4 mile but < 1/2 mile. Heavy Heavy Visibility < 1/4 mile.Visibility < 1/4 mile.

Documents

On the use of Liquid Water Equivalent (LWE) for estimating snowfall intensity and freezing drizzle intensity versus the use of visibility Roy Rasmussen