Measurement and Data Analysis

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Measurement and Data Analysis

• Snowfall is the depth of fresh snow which falls
  during a given recent period.
• Snowfall measurements are summed to determine the
  total for any time period a single storm, a day,
  a month or a year.
• Total precipitation is the sum of the vertical
  depth of all liquid precipitation and of the
  water equivalent (depth) of all forms of solid
  precipitation, including snowfall.

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• Proper utilization of snowfall data depends on
  the users requirements, assuming that the user
  understands how the data were obtained, realizes
  the problems of measuring and processing the data
  and is aware that errors may exist in the data.

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Point snowfall measurements

• A graduated ruler inserted vertically into the
  snow is the most direct method for measuring the
  depth of freshly-fallen snow.
• Where the snow has not drifted the mean depth of
  snowfall is determined from measurements made at
  several points.

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• To ensure that old snow is not sampled, the
  measurement is made on a patch or a snow board
  whose surface has been kept free of snow before
  the snowfall.
• A snow board is a piece of plywood or lightweight
  metal at least 40 cm by 40 cm, painted white or
  covered with white flannel which provides a
  reference level for measurement.

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• To obtain a representative mean depth of new
  snow under drifting conditions requires careful
  judgement by the observer.
• A large number of measurements must be taken in
  both drifted and exposed areas.
• The water equivalent of fresh snowfall from ruler
  measurements may be estimated by using an
  approximate relation between depth and swe.

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• Commonly, the average density of newly-fallen
  snow is accepted as 100 kg m-3 that is 1 cm of
  snow is taken 1 mm swe.
• In Canada, Environment Canada uses this method to
  estimate swe for more than 85 of the observing
  stations.
• In reality, the density of newly-fallen snow
  varies with region, with individual storm events
  and often throughout the duration of a storm.

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Snow Gauges

• Snow gauges measure snowfall water equivalent
  directly.
• Essentially, any open cylinder in which snow can
  accumulate and be measured can serve as a snow
  gauge.
• The cylinder is generally shielded to reduce wind
  turbulence around the orifice and is mounted far
  enough above the snow surface to minimize the
  accumulation of blowing snow in the gauge.

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• One of the biggest challenges in monitoring cold
  climates is measuring frozen precipitation with
  snow gauges.
• In high wind conditions, precipitation gauges
  disrupt the boundary layer atmospheric flow,
  causing frozen precipitation to preferentially
  blow over and around, rather than into, the
  gauge.
• Liquid precipitation is less susceptible to this
  undercatch problem because it is denser and has a
  faster falling velocity.

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Source Gray and Male (1981)
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• Estimates of snowfall undercatch for some types
  of gauges are as high as 70 or more.
• Because many northern regions have below-freezing
  surface air temperatures for 910 months of the
  year, a large percentage of annual precipitation
  is frozen.
• However, because the undercatch problem is so
  severe, it is difficult to estimate the total
  amount, let alone the percentage of precipitation
  that falls in each phase or in which season.

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• In addition to undercatch caused by disrupting
  the wind flow near the gauge, other systematic
  errors in measuring solid precipitation include
  evaporation/sublimation, wetting losses from
  water sticking inside the gauge, blowing snow,
  the tendency of observers to ignore trace events,
  and gauge location which is unrepresentative of
  the catchment.
• All of these systematic biases lead to
  underestimation of precipitation, with the
  exception of biases associated with measurements
  in areas of blowing snow deposition.

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Mechanical failure of unattended precipitation
gauge
WMO SPIR, 1998
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Impact of a heavy snowpack
Winter 2006/07
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Impact of strong winds (gt40 m/s)
Winter 2006/07
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Bear Mauling
Cherry, June 2006
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Impact of a bear attack
Spring 2007
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• In Canada, the Nipher shielded snow gauge is
  designated as the official Canadian instrument
  for measuring snowfall water equivalent.
• It has the shape of an inverted bell and is
  usually constructed of aluminum or fiberglass.
• Wind tunnel tests by NRC indicated that this
  shield design is effective in minimizing
  disturbances to the airflow over the gauge
  orifice.

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Source Gray and Male (1981)
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Weighting-type precipitation gauges

• Weighting-type precipitation gauges measure all
  forms of precipitation.
• They use the principle of a simple spring
  balance.
• Precipitation is collected in a catch bucket
  mounted on a spring, which becomes compressed and
  activates a recording mechanism.

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• The capacities of weighting-type gauges range
  from 300 to 600 mm swe.
• Some can operate unattended for up to one year
  their time resolution capabilities can vary from
  5 min. to several hours.
• In snowy climates, long-duration gauges require
  an antifreeze charge to prevent freezing of
  precipitation in the collector.

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Source Gray and Male (1981)
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Source Gray and Male (1981)
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Reconstructing snowfall from snow depth
measurements
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SR50
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Comparison of point snowfall measurement
techniques

• The ruler provides a measurement of snow depth
  from which the snowfall water equivalent can be
  estimated, whereas a snow gauge provides a
  measurement of snowfall water equivalent from
  which the depth can be estimated.
• The causes of errors in these point measurements
  and estimates are known.

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• However, the magnitudes of the errors or the
  differences between measured and true catches
  are not well known, largely because of the
  difficulty in determining true snowfall.

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Source Gray and Male (1981)
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Source Gray and Male (1981)
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Statistical Correction Factor
January
Yang et al., 2005
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Source Gray and Male (1981)
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Source Gray and Male (1981)
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Snowfall measurement with radar

• Weather radar (radio detection and ranging) is
  another very useful remote sensing tool used in
  meteorological forecasting.
• Microwave radar was developed in early World War
  II to aid in spotting distant ships and
  airplanes. It was noticed early on that during
  adverse weather conditions widespread
  interference often appeared on the radar screen
  and obscured the military objects of interest.
• A large body of theoretical and experimental work
  in the 1940s showed that this weather clutter
  arose from the scattering of radar waves by
  precipitation.

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• These early findings have been refined and
  elaborated to the point that most of the
  measurable properties of radar signals -
  amplitude, phase, polarization, and frequency -
  can be interpreted in terms of the sizes, shapes,
  motions or thermodynamic phase of the
  precipitation particles.
• Because of their ability to observe and measure
  precipitation quickly, accurately, and from great
  distances, radars have become essential in
  weather observation and forecasting.

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• Heavier precipitation reflects more microwave
  energy back to a radar than lighter snow.
  However, more distant snow also gives a weaker
  return signal. A range-corrected and
  equipment-calibrated measure of reflectivity from
  rain is given by
• log(Z) log(received power) 2 log(range)
  constant
• where Z is the radar reflectivity factor. Because
  Z has such a wide range of values, the
  reflectivity is usually expressed as decibels dB
  of Z.

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• dBZ 10 log(Z)
• Larger and more numerous drops or snowflakes
  reflect more radar energy
• Z SD6/ V
• where D is (melted) drop diameter, V is volume of
  air holding the drops, and the sum is over all
  precipitation within that volume.

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• But the number and diameter of drops also
  determines the precipitation rate.
• When the above three equations are combined and
  empirically tuned to the observations, the result
  is a formula for converting radar echo intensity
  in dBZ to rainfall rate R
• R cR100.0625dBZ
  Eqn 1
• where cR 0.036 mm h-1 .
• Six discrete levels of radar echo intensity are
  often used, corresponding to descriptive rainfall
  categories.

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Figure 8-12 Rainfall intensity chart (Stull 2000)
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• Owing to their particular shapes, snowflakes
  produce echoes of different intensity than
  raindrops of the same size.
• The snowfall rate S is therefore often inferred
  from the radar reflectivity factor from
• S cS100.05dBZ
• where cS 0.018 cm h-1 .
• This relationship assumes that 1 cm of melted
  snow equates 1 mm of water, i.e. that the falling
  snow has a density of 100 kg m-3 .

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Source Gray and Male (1981)
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Point measurements of snow depth

• Most simply, depth measurements of snow (snow
  accumulated on the ground) are made with a snow
  ruler or similar graduated rod which is pushed
  through the snow to the ground surface.
• Representative measurements by this method may be
  difficult to obtain in open areas since the
  snowcover undergoes drifting and may have
  embedded ice layers that limit penetration with a
  ruler.

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Measurements of snow depth

• At each observing station a number of
  measurements are made and averaged.
• In remote regions, graduated snow stakes or
  aerial markers may be used.
• The snow depth at the stake or marker is observed
  from a distant point through binoculars or
  telescopes.
• However, rulers, stakes and aerial markers do not
  provide swe information.

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Snow depth measurement
Device doesnt disrupt the measured quantity as
much as a gauge does
Source J. Cherry