The NOAA/FAA/NCAR Winter
Precipitation Test Bed:
How 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 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!
T║MES
ESSL
January
How will snowfall rates change in the future?
T║MES
T MES
ESSL
April
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
March 14, 2002
Original Hotplate
Zeroed DFIR
Zeroed NDblAlt
Zeroed DblAlt
Zeroed SngAlt
Zeroed SmWyo
Zeroed SmDFIR
0.4
0.35
Hotplate
DFIR
Small DFIR
0.3
Accumulaton (inches)
20
15
0.25
Wind
0.2
speed
Wind Speed (m/s)
Documented
snow undercatch
behavior of
various
shields and
gauges
10m Wind
Double Alter
10
Single Alter
0.15
0.1
5
0.05
0
0
6
8
10
12
Time (Hrs)
14
16
20
Established
transfer
functions for
various
shields
21
Original hotplate accum/DFIR
accum (1 hour periods)
Established
transfer
functions
for various
shields and
gauges
1.2
y = 0.96676 - 0.082568x R= 0.92561
y = 1.059 - 0.10492x R= 1
1
Single Alter Catch
Efficiency
0.8
Hotplate Catch
Efficiency
0.6
0.4
0.2
-2
0
2
4
10 m wind speed (m/s)
6
8
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)
NWS TABLE
VISIBILITY (STATUE MILES)
>0.50
>.25 .25
<=.50
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
(HSS) PWD-22)
(Vaisala
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
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
Front view
Side view
~4 mm
Particle Characteristics
Height and width
Volume
Terminal velocity
[mm]
[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 >0oC; 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 0oC; 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