A Quick Review Of Snow Microphysics And Its Relation to Heavy Snow Forecasting

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A Quick Review Of Snow Microphysics And Its
Relation to Heavy Snow Forecasting

• Greg DeVoir
• NWS CTP Winter Weather Workshop
• November 7, 2002

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Forecasting Snow Amounts

• Forecasters generally employ the following
  formula for predicting snowfall totals
• (Intensity) (Duration) Snow Amount
• Dynamical processes which lead to the production
  of precipitation are generally well- resolved by
  numerical models, therefore we usually have a
  good idea of Duration for a given event.
• Intensity is a different story!

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Why is Determining Snow Intensity so Difficult?

• Many factors influence the amount/distribution of
  snow over a given area.
• Snow-to-water ratio (density), precipitation
  phases, terrain/orographic effects, convective
  stability, and surface temperatures are commonly
  considered by forecasters.
• But the character (type, size, shape) of
  snowflakes themselves, determined by the
  intensity of lift, variations in moisture, and
  snow microphysics (vertical thermal profile), can
  play a crucial role in the resulting
  accumulations.

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Why is Determining Snow Intensity so Difficult?

• Snow-to-water ratios (i.e. snow density) vary
  GREATLY.
• The standard 101 ratio commonly used in
  operations is only correct 25.8 of the time.
• From observational studies, snow ratios of
  freshly fallen snow vary from 31 to 1001!!!

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Distribution of Snow Ratios 1973-1994
Source Roebber et al., 26 March 2002,
Improving Snowfall Forecasting by Diagnosing
Snow Density.
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Snow Ratio Distribution
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In a Nutshell

• Even with accurate and precise QPF forecasts,
  large errors in snowfall forecasts will still
  occur (by factors of 2 to 10!) unless we do a
  better job at diagnosing and anticipating snow
  density/ ratios.
• Can we do this and how?
• Examining snow microphysics operationally may
  help us improve snow density/ratio assessments,
  leading to more accurate forecasts of snow
  amounts.

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References

• Dan Baumgardt - ARX SOO
• Wintertime Cloud Microphysics Review
• www.crh.noaa.gov/arx/micrope.html
• Jeff Waldstreicher former BGM SOO now at ER
  SSD
• The Importance of Snow Microphysics for Large
  Snowfalls
• www.erh.noaa.gov/er/hq/ssd/snowmicro/sld001.html
• Roebber, Schultz et al. (Collaboration - UW
  Milwaukee and NSSL)
• Improving Snowfall Forecasting by Diagnosing
  Snow Density
• www.nssl.noaa.gov/schultz/snowdensity/paper.shtm
  l

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Snow Microphysics

• Top-down approach (Steve D. talk)
• Ice crystals form by heterogeneous nucleation,
  and grow by deposition
• These ice crystals can then grow into larger
  snowflakes by aggregation and riming.
• The temperatures at which these processes occur
  determine not only precipitation type, but
  snowflake type, and resulting snow density.

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Snow Microphysics

• Heterogeneous Nucleation
• Cloud droplets commonly exist in super-cooled
  state.
• Nucleation of ice occurs when water molecules
  collect and freeze onto a foreign particle (dust,
  clay, existing ice crystal).
• For most particles, temperatures of less than
  -10C are required for heterogeneous nucleation of
  ice to occur.
• ICE MUST BE IN THE CLOUD FOR SNOW TO BE IN THE
  FORECAST!

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Got Ice?

• Baumgardt (1999) found
• -4CNO ICE in clouds
• -10C.60 chance ice is in the cloud (Warm
  cutoff)
• -12C.70 chance ICE is in the cloud a good
  operational discriminator
• -15C.90 chance ICE is in the cloud
• -20C.ICE is in there!
• Temperatures at top portion of cloud

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Ice Crystal (Snow) Growth

• Occurs in Three Ways
• Accretion Growth of an ice particle when it
  captures super-cooled liquid droplets
• Deposition Growth by water vapor depositing on
  the ice particle in a liquid form and immediately
  freezing, or directly depositing as a solid
• Aggregation Merging of multiple ice particles to
  form one main snowflakei.e. snowflakes sticking
  together. Surfaces of ice crystals become sticky
  at temperatures above -5 C. Aggregation
  maximizes near 0 C.
• Deposition is the dominant snow growth process

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Deposition

• The change from water in the vapor form to water
  in the solid form.
• Growth by deposition is greater at colder
  temperatures.
• Deposition growth is also dependent on pressure.

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Ice Crystal Growth as a Function of Temperature
and Pressure
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Snow Crystal Growth - Dendrites

• Crystal growth maximizes around -15oC with
  dendrites the preferred crystal type.
• Dendrites are efficient accumulators of snow
  because of the extra space within each crystal.

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Dendritic Snow Growth Zone-12 to -18C
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Can We Use This Operationally?

• Waldstreicher (2002) Assessed the viability of
  using NWP to forecast periods of efficient
  (rapid) snow accumulation.
• Ice crystals maximize near the greatest rising
  motion (if air mass saturated).
• Dendrites will be favored where omega maximums
  intersect dendrite-favored temperatures (-12 to
  -18 C).

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Omega/Dendrite Intersection (Shown using Bufkit)
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Omega/Dendrite IntersectionWaldstreicher Study

• Question
• Is the intersection of model omega and dendrite
  temperatures a consistent signature for heavy
  snowbursts and/or large snowfall accumulations?

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Waldstreicher Study

• Days where snowfall exceeded warning and advisory
  criteria were identified.
• Eta and MesoEta hourly forecast soundings were
  examined, using Bufkit, for the presence of the
  omega/dendrite intersection zone.

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Waldstreicher Study

• Omega Maximum of at least -10 µbs-1 arbitrary
  value sufficient to assume moderate liftwill
  vary by model and resolution.
• Temperatures favorable for dendrite formation
  (-12 to -18 C).
• Coincident signatures needed to be present in 2
  of 3 successive model runs.

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Waldstreicher Study

• Results
• 76.4 of the Warning Events exhibited the
  cross-hair signature.
• ONLY 9.4 of Advisory Events had the cross-hair
  signature.
• This signature may have utility distinguishing
• Warning vs. Advisory Events!

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Waldstreicher Study

• Omega max below the dendrite zone
• Warm atmospheric environment.
• Indicative of shallow lift.
• Snow is more likely to have low snowwater
  ratio.
• These events are not likely to produce warning
  criteria.

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Waldstreicher Study

• Omega max above the dendrite zone
• Cold Atmospheric environment.
• Snow more likely to have higher snowwater
  ratios.
• Major Virga Potential (look for lower layer
  subsidence or dry layers).
• Could be a sign of sloped lift in the model if
  upstream locations depict omega maximum at
  lower elevations. This could give a good
  estimate of the north/west extent of heavy
  snowfall.

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CTP Variation on Waldstreicher Cross-Hair Study

• Will examine this years snowfalls, and write
  routine summaries with regards to the snow
  microsphysics associated with each event to help
  us learn and improve.
• Will further attempt to classify false alarm
  cases, and cross reference with quantifiable
  parameters such as precipitable water (false
  alarms were not considered in the Waldstreicher
  study).

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Latest Directions in Snow Microphysics Research
The future of Snow Forecasting?

• Roebber, Schultz et al. have created an
  Artificial Neural Network (ANN) for predicting
  snow densities.
• Currently, their ANN correctly diagnoses better
  than 60 of cases (vs. only 25 using the
  standard 101 ratio typically used in
  operations). This should only improve in time.

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Summary

• Things to remember
• Need ice! -12C Cloud Top Temp is a good
  operational discriminator
• Check location of favored dendritic snow growth
  zones with regard to max model omega. Look for
  at least moderate lift maximum.
• Consider potential for aggregation (snowflakes
  sticking together to form bigger flakes)
• Size matters!