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

10

Jayaweera and Mason (1966) suggested that since snowflakes are

not often observed at

o
temper a tures colder than -5 C,

the

"cementing" of ice crystals by sintering* or by' capture of

supercooled drops may serve to keep warmer aggregates intact.

Aggregates which form in colder clouds contain less cloud

droplets; thus they are not cemented and may subsequently be

broken apart by coil isions or turbulence.

However, from

observations of Ohtake (1970), even large aggregates of dendritic

crystals did not exhibit any riming:

a direct contradiction with

Jayaweera and Mason (1966).

Ohtake suggested that the most

impor tant fac tor in crysta 1 agg regation is the mechan ical

interlocking between crystal branches.

Rogers (1974) documented that maximum snowflake diameters and

snowfall rates exhibited their greatest value near OOC. Snowflake

collection efficiencies were inferred to increase with surface

temperature to near uni ty at OOC. Ice crystal concentrations

necessary for aggregation to maximize snowflake diameters were

calculated to be in the range 103 to 105 m-3.

The number and

sizes of ice crystals which comprise aggregates were related to

the overall sizes of the aggregates.

Rogers suggested that large

dendri tes (5 rrm) exhibit unstable fall behavior, characterized by

horizontal excursions during descent; the sweep out volume,

consequently increased, estimated to be by a factor of 5.

* Sintering IS the diffusion transport of water molecules to
reg ions of large curvature; the transport is driven by the
gradient of the surface free energy.

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