The NOAA/FAA/NCAR Winter Precipitation Test Bed: How Well Are We Measuring Snow?

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The NOAA/FAA/NCAR Winter Precipitation Test
BedHow Well Are We Measuring Snow?
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
t
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Winter Weather Nowcasting for transportation
requires real-time liquid equivalent
measurements!
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January
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How will snowfall rates change in the future?
April
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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 90s to
evaluate snow measuring instrumentation for
climate purposes.
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• Challenges of automatic snow fall rate
  measurements
• Wind under-catch
• - Gauge acting as obstacle to the flow,
  generating updrafts
• Cap over of the orifice by snow accumulating on
  the gauge
• Minimum detectable signal often large (to
  overcome noise)
• Minimum detectable signal impacted by wind speed
  (higher the wind, the larger the minimum
  detectable signal)
• Eliminating blowing snow false accumulations
• High maintenance
• Need to empty the bucket after snow fills up and
  refill bucket with glycol and oil.

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Updraft generated upstream of gauge
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• Methods devised to solve the challenges
• Wind effect
• - Wind shields used to prevent updrafts from
  forming over weighing gauges.
• Orifice blocking effect
• - Heaters used to prevent snow build up on the
  body of the gauge.
• 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)

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Deployed multiple Double Fence Inter-comparison
Reference (DFIR) shields as truth gauge
Insert image of the Marshall site with DFIR
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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
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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
Hotplate
DFIR
Small DFIR
Wind speed
Double Alter
Single Alter
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Established transfer functions for various shields
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Established transfer functions for various
shields and gauges
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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)
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Developed and tested the Liquid Water Equivalent
system for ground deicing use
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Precipitation Type sensor (HSS)
Precipitation Type sensor (Vaisala PWD-22)
Snow Liquid Water Equivalent System
WXT temperature, humidity, and wind sensor
(Vaisala)
Hotplate (Yankee)
Liquid Equivalent snowfall rate determination
Weighing Snowgauge (GEONOR)
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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
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Disdrometer Observations

• 2DVD Specifications
• Measurement area 10 cm x 10 cm
• Scan rate 51.3 kHz
• Horizontal resolution 0.15 mm
• Vertical resolution 0.03 mm for snowflakes,
  0.1 mm for raindrops
• Particle Characteristics
• Height and width
• Volume
• Terminal velocity

Front view
Side view
4 mm
mm
mm
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Rain Period 1230 (17 March)-0200 UTC (18 March)
Terminal Velocity vs Equ. Diameter
2100-2400 UTC 17 March
Hydrometeor Size Distribution
2225-2300 UTC 17 March
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Mixed Phase Period 0200-0630 UTC Decreasing
temperature
Terminal Velocity vs Equ. Diameter
0200-0600 UTC
Hydrometeor Size Distribution
0515-0520 UTC
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Partially-Melted Snow Period 2020 UTC-
Temperature gt0oC Temporal maximum temperature
Terminal Velocity vs Equ. Diameter
2200-2300 UTC
Hydrometeor Size Distribution
Crystal Types Irregulars (hvy) 1-2
mm Spatial dendrites /snow
grains (hvy) lt1-2 mm Plates (lgt-mod)
lt1-2 mm Needles (mod) 2-4
mm Stellars (mod) lt1-2
mm Aggregrate sizes 2-8 mm
2125-2130 UTC
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Snow Period -2020 UTCTemperature slightly above
0oC Small crystals
Terminal Velocity vs Equ. Diameter
1100-1200 UTC
1900-2000 UTC
Hydrometeor Size Distribution
Crystal Type Irregulars (hvy) 1-2
mm Aggregrate sizes 3-4 mm
1950-1955 UTC
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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
• Under-catch of snow as a function of shield type
  and the development of transfer functions
• Develop and test new wind shields
• Evaluate the use of various gauge/shield
  combinations for both real-time and climate snow
  measurements.
• Develop and test new precipitation instruments
  (hotplate)
• Real-time measurement of snow for aircraft ground
  deicing purposes
• The use of visibility to measure snow intensity
• Snow size distributions and terminal velocity
• 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 dont 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!