The NOAA/FAA/NCAR Winter Precipitation Test Bed:

How Well Are We Measuring Snow?

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Roy Rasmussen1, Bruce Baker2, John Kochendorfer2, Tilden Myers2, Scott Landolt1, Alex Fisher3, Jenny Black1,

Julie Theriault1, Paul Kucera1, David Gochis1, Craig Smith3, Rodica Nitu3,Mark Hall2,Steve Cristanelli1 and Ethan Gutmann1

1. National Center for Atmospheric Research (NCAR) 2. NOAA

3. Environment Canada

Winter Weather Nowcastingfor transportation requires real-time liquid equivalent measurements!

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ESSLJanuary

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April

How will snowfall rates change in the future?

The NOAA/FAA/NCAR Winter Precipitation Test Bed was initially

established in 1991 at NCAR in Boulder, Colorado to address FAA

needs for real-time snowfall rates in support of ground deicing

The NOAA Climate Reference Network program started using the site in the late 90’s to evaluate snow measuring instrumentation for climate purposes.

Challenges of automatic snow fall rate measurements:

1. Wind under-catch - Gauge acting as obstacle to the flow, generating updrafts

2. Cap over of the orifice by snow accumulating on the gauge

3. Minimum detectable signal often large (to overcome noise)

4. Minimum detectable signal impacted by wind speed (higher the wind, the larger the minimum detectable signal)

5. Eliminating blowing snow false accumulations

6. High maintenance - Need to empty the bucket after snow fills up and refill bucket with

glycol and oil.

National Center for Atmospheric Research

Updraft generated upstream of gauge

Methods devised to solve the challenges:

1. Wind effect:- Wind shields used to prevent updrafts from forming over weighing gauges.

2. Orifice blocking effect- Heaters used to prevent snow build up on the body of the gauge.

3. Reduce minimum detectable signal by software and hardware:- Improved software to reduce false tips by vibration.- Improved hardware to eliminate vibrations and other noise.

4. Reduce the minimum detectable signals increase with wind speed- Use wind shields that have high efficiency (e.g. WMO Double

Fence Intercomparison Reference Shield)

Insert image of the Marshall site with DFIR

Deployed multiple Double Fence Inter-comparison Reference (DFIR) shields as “truth” gauge

Layout of site:

Flat and level site located 7 km south of Boulder, Colorado

NCAR owned and operated with security fence

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Aerial View of the NOAA/FAA/NCAR Test site

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View of test site to the South

15

View of test site towards the West

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Developed and tested

double Alter shield

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Developed and tested 2/3 DFIR

shield (CRN)

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Developed and tested

hotplate snowgauge

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Testing multiple hotplates

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Documented snow under-

catch behavior of

various shields and

gauges

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0

5

10

15

20

6 8 10 12 14 16

March 14, 2002

Original Hotplate Zeroed DFIR Zeroed NDblAlt Zeroed DblAlt Zeroed SngAlt Zeroed SmWyo Zeroed SmDFIR

10m Wind

Acc

umul

aton

(inc

hes) W

ind Speed (m

/s)

Time (Hrs)

Single Alter

Double Alter

Small DFIR

DFIRHotplate

Wind speed

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Established transfer

functions for various shields

Established transfer

functions for various shields and

gauges0.2

0.4

0.6

0.8

1

1.2

-2 0 2 4 6 8

y = 0.96676 - 0.082568x R= 0.92561

y = 1.059 - 0.10492x R= 1

Orig

inal

hot

plat

e ac

cum

/DFI

R

accu

m (1

hou

r per

iods

)

10 m wind speed (m/s)

Single Alter Catch Efficiency

Hotplate Catch Efficiency

Data used to develop

transfer function shows

significant scatter!

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Thank You!

Rasmussen et al. 2001

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Mapped airflow around

shields/gauges using sonic

anemometers and numerical modeling

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Established that visibility

is a poor method to

estimate the liquid

equivalent rate of snow

(light, moderate,

heavy)

NWS TABLE VISIBILITY (STATUE MILES) >0.50 >.25 -

<=.50 .25

Light Moderate Heavy

HVY

MOD

LGT

1.7 mm/hrModerate

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Developed and tested the Liquid Water Equivalent system

for ground deicing use

Precipitation Type sensor (HSS)

WXT temperature, humidity, and wind sensor (Vaisala)

Hotplate (Yankee) Weighing Snowgauge

(GEONOR)

Snow Liquid WaterEquivalent System

Liquid Equivalent snowfall rate determination

Moderate Snow

Precipitation Type sensor (Vaisala PWD-22)

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Developed method to heat the

orifice of a gauge using temperature controlled heat tape

(max temperature

2 ˚C)

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Accurate snow depth measurements remain a

challenge!

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Measured snow particle

size distribution using video disdrometer

Disdrometer Observations2DVD SpecificationsMeasurement area = 10

cm x 10 cmScan rate = 51.3 kHzHorizontal resolution =

0.15 mmVertical resolution = 0.03

mm for snowflakes, 0.1 mm for raindrops

Particle CharacteristicsHeight and widthVolumeTerminal velocity

Front view Side view

[mm] [mm]

~4 mm

Rain Period: 1230 (17 March)-0200 UTC (18 March)

2100-2400 UTC17 March

2225-2300 UTC17 March

Terminal Velocity vs Equ. Diameter

Hydrometeor Size Distribution

Mixed Phase Period: 0200-0630 UTC Decreasing temperature

0200-0600 UTC

0515-0520 UTC

Terminal Velocity vs Equ. Diameter

Hydrometeor Size Distribution

Partially-Melted Snow Period : 2020 UTC- Temperature >0oC; Temporal maximum temperature

2200-2300 UTC

2125-2130 UTC

Crystal Types:Irregulars (hvy) 1-2 mmSpatial dendrites /snow grains (hvy) <1-2 mmPlates (lgt-mod) <1-2 mmNeedles (mod) 2-4 mmStellars (mod) <1-2 mmAggregrate sizes 2-8 mm

Terminal Velocity vs Equ. Diameter

Hydrometeor Size Distribution

Snow Period: -2020 UTCTemperature slightly above 0oC; Small crystals

1100-1200 UTC

1950-1955 UTC

Crystal Type:Irregulars (hvy) 1-2 mmAggregrate sizes 3-4 mm

1900-2000 UTC

Terminal Velocity vs Equ. Diameter

Hydrometeor Size Distribution

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Measured vertical

profile of precipitation

using K-band radar

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Aircraft Deicing Fluid

testing

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Summary• The NOAA/FAA/NCAR Winter Precipitation Test Bed

has been used to investigate a number of important aspects of winter precipitation:

1. Under-catch of snow as a function of shield type and the development of transfer functions

2. Develop and test new wind shields3. Evaluate the use of various gauge/shield combinations

for both real-time and climate snow measurements. 4. Develop and test new precipitation instruments

(hotplate)5. Real-time measurement of snow for aircraft ground

deicing purposes6. The use of visibility to measure snow intensity7. Snow size distributions and terminal velocity8. Radar- reflectivity snowfall relationships

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SummaryHow well are we measuring snow?

• While advances in shields and gauges have been made, we still don’t fully understand the significant scatter in the data nor have we designed the perfect wind shield to reduce the scatter.

• Need to use direct measurements of the liquid equivalent rate of snow to estimate snow intensity in METARs rather than use visibility

• The automated measurement of precipitation type and snow depth remains a significant challenge.

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Thank You!