The NOAA FAA NCAR Winter Precipitation Test Bed How Well

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, Rodica Nitu 3, Mark Hall 2, Steve Cristanelli 1 and Ethan Gutmann t 1. National Center for Atmospheric Research (NCAR) 2. NOAA 3. Environment Canada

Winter Weather Nowcasting for transportation requires real-time liquid equivalent measurements!

January ESSL

How will snowfall rates change in the future? April ESSL

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.

Updraft generated upstream of gauge National Center for Atmospheric Research

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)

Deployed multiple Double Fence Inter-comparison Reference (DFIR) shields as “truth” gauge Insert image of the Marshall site with DFIR

Layout of site: Flat and level site located 7 km south of Boulder, Colorado NCAR owned and operated with security fence

Aerial View of the NOAA/FAA/NCAR Test site 11

View of test site to the South 13

View of test site towards the West 15

Developed and tested double Alter shield 16

Developed and tested 2/3 DFIR shield (CRN) 17

Developed and tested hotplate snowgauge 18

Testing multiple hotplates 19

Hotplate Documented snow undercatch behavior of various shields and gauges DFIR Small DFIR Wind speed Double Alter Single Alter 20

Established transfer functions for various shields 21

Established transfer functions for various shields and gauges

Data used to develop transfer function shows significant scatter!

Thank You! Rasmussen et al. 2001 24

Mapped airflow around shields/gauges using sonic anemometers and numerical modeling 25

Established that visibility is a poor method to estimate the liquid equivalent rate of snow (light, moderate, heavy) 1. 7 mm/hr Moderate LGT MOD HVY 26

Developed and tested the Liquid Water Equivalent system for ground deicing use 28

Precipitation Type sensor (Vaisala (HSS) PWD-22) Snow Liquid Water Equivalent System Moderate Snow Hotplate (Yankee) Liquid Equivalent snowfall rate determination WXT temperature, humidity, and wind sensor (Vaisala) Weighing Snowgauge (GEONOR)

Developed method to heat the orifice of a gauge using temperature controlled heat tape (max temperature 2 ˚C) 30

Accurate snow depth measurements remain a challenge! 31

Measured snow particle size distribution using video disdrometer 32

Disdrometer Observations 2 DVD Specifications Measurement area = 10 cm x 10 cm Scan rate = 51. 3 k. Hz Horizontal resolution = 0. 15 mm Vertical resolution = 0. 03 mm for snowflakes, 0. 1 mm for raindrops Front view Side view ~4 mm Particle Characteristics Height and width Volume Terminal velocity [mm]

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

Mixed Phase Period: 0200 -0630 UTC Decreasing temperature Terminal Velocity vs Equ. Diameter 0200 -0600 UTC Hydrometeor Size Distribution 0515 -0520 UTC

Partially-Melted Snow Period : 2020 UTCTemperature >0 o. C; Temporal maximum temperature Terminal Velocity vs Equ. Diameter 2200 -2300 UTC Hydrometeor Size Distribution 2125 -2130 UTC Crystal Types: Irregulars (hvy) 1 -2 mm Spatial dendrites /snow grains (hvy) <1 -2 mm Plates (lgt-mod) <1 -2 mm Needles (mod) 2 -4 mm Stellars (mod) <1 -2 mm Aggregrate sizes 2 -8 mm

Snow Period: -2020 UTC Temperature slightly above 0 o. C; Small crystals Terminal Velocity vs Equ. Diameter 1100 -1200 UTC 1900 -2000 UTC Hydrometeor Size Distribution Crystal Type: Irregulars (hvy) Aggregrate sizes 1950 -1955 UTC 1 -2 mm 3 -4 mm

Measured vertical profile of precipitation using Kband radar 38

Aircraft Deicing Fluid testing 39

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 shields 3. 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 purposes 6. The use of visibility to measure snow intensity 7. Snow size distributions and terminal velocity 40 8. Radar- reflectivity snowfall relationships

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. 41

Thank You! 42