WMOD00401

• Contract/Permit #: 2-07-81-V0256
• Title: Precipitation Development in Seeded Clouds Over the Sierra Nevada
• Date: 8/1/1986
• Weather Modification - Doc Type: Report

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Passarelli (1978) developed an approximate analytical model of
depositional and aggregational growth of snowflakes by assuming the
snow-size distribution to be always exponential. Passarelli showed
analyt ically that exponential snow-si ze distr ibutions in a
steady-state heterogeneous cloud tends to an equilibrium that is
independent of the parameters of the ini tial exponential
distribution. The model of La and Passarelli (1982) considered
the three-parameter gamma distribution to study the: snow-size
distribution. The physical processes represented in this model
were vapor deposition, sedimentation due to particle differential
fall speed, aggregation and colI isional breakup. The model
assumed steady-state precipitation thus eliminating all time
derivatives. The results from the model were compared with
airborne observational data which indicated collisional breakup to
be important in shaping snow-size distribution. The model
predicted that snow growth passed through three distinct stages,
where each of the processes: vapor deposition, aggregation and
collisional breakup, in turn, dominated.
Sasyo (1971) demonstrated the importance of relative
horizontal motions and fall stabil i ties on the initiation of
aggregation. Shafrir (1965) showed that horizontal motions could
significantly increase the collection efficiency for coalescence
of drops. Shafrir' s conclusion would seem to apply to aggregation
as well. Considering the importance of the horizontal motions in
addi tion to the differential terminal velocities of crystals to
aggregation, Heymsfield (1986) developed an aggregation model to

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