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

of a water drop moving through the flow field around a freely

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falling graupel is the system's mixed Froude number.

Third, a

drop-equivalent approach, using the collection efficiencies for

ice spheres derived by Beard and Grover (1974). Fourth, by using

a unity collection efficiency.

It was concluded after canparisons

of calculated with experimental graupel growths that' the mixed

Froude number approach was more accurate than other' methods. A

parameterization was developed which when combined with the impact

veloci ty parameterization of Rasmussen and Heymsfield

(1985)

predicted the observed accretional density accurately.

2.2 REVIEW OF AOOREX;ATION THIDRY AND AOOREX;ATION MOOELS

2.2.1 Aggregation theory

Aggregation is a process of snowflake growth due to collision

and subsequent adhesion of ice crystals with ice crystals, ice

crystals with snowflakes, and snowflakes with other snowflakes.

Early radar observations have shown that aggregation is an

important precipitation process (Austin and Bemis, 1950; Wexler

and Atlas, 1956).

Ice crystal aggregation is a complicated

multifaceted hydrodynamic and electrostatic process canplicated by

the myriad shapes and sizes of crystals.

Laboratory observations

of models of ice crystal determined the conditions for two

crystals to aggregate (Podzimek, 1968).

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