The NOAA/FAA/NCAR Winter Precipitation Test Bed:

How Well Are We Measuring Snow?

t

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

The NOAA/FAA/NCAR Winter Precipitation Test Bed:

How Well Are We Measuring Snow?

t

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

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 (Bruce Baker, lead).

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

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Insert image of the Marshall site with DFIR

To address these challenges, developed the NOAA/FAA/NCAR Solid Precipitation Test Site near Boulder, Colorado

Core to the site: 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

10

Aerial View of the NOAA/FAA/NCAR Test site

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View of test site to the South

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View of test site towards the West

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Developed and tested

double Alter shield

16

Developed and tested 2/3 DFIR

shield (CRN)

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Developed and tested

hotplate snowgauge

18

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

(in

ches

) Wind S

peed (m/s)

Time (Hrs)

Single Alter

Double Alter

Small DFIR

DFIRHotplate

Wind speed

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

Ori

gin

al h

otp

late

acc

um

/DF

IR

accu

m (

1 h

ou

r p

erio

ds)

10 m wind speed (m/s)

Single Alter Catch Efficiency

Hotplate Catch Efficiency

Data used to develop

transfer function shows

significant scatter!

22

Thank You!

Rasmussen et al. 2001

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Numerical studies of flow past various

shield/gauge combinations

help explain scatter

(Julie Therialt talk later in

this session)

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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

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Developed method to heat the

orifice of a gauge using temperature controlled heat tape

(max temperature

2 ˚C)

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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Aircraft Deicing Fluid

testing

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Accurate snow depth measurements remain a

challenge!

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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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Summary

How 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.

• The upcoming WMO Solid Precipitation Intercomparison Experiment mentioned by Rodica can help address these challenges.

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Thank You!