The NOAA/FAA/NCAR Winter Precipitation Test Bed: How Well Are We Measuring Snow? t Roy Rasmussen 1 , Bruce Baker 2 , John Kochendorfer 2 , lden Myers 2 , Scott Landolt 1 , Alex Fisher 3 , Jenny Bla lie Theriault 1 , Paul Kucera 1 , David Gochis 1 , Craig S ica Nitu 3 ,Mark Hall 2 ,Steve Cristanelli 1 and Ethan Gu 1. National Center for Atmospheric Research (NCAR) 2. NOAA 3. Environment Canada
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The NOAA/FAA/NCAR Winter Precipitation Test Bed: How Well Are We Measuring Snow?
The NOAA/FAA/NCAR Winter Precipitation Test Bed: How Well Are We Measuring Snow?. Roy Rasmussen 1 , Bruce Baker 2 , John Kochendorfer 2 , Tilden Myers 2 , Scott Landolt 1 , Alex Fisher 3 , Jenny Black 1 , Julie Theriault 1 , Paul Kucera 1 , David Gochis 1 , Craig Smith 3 , - PowerPoint PPT Presentation
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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
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
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.