Effects of Seeding on the Energy of Systems

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  • 1 North American Weather Consultants, Santa Barbara, Calif
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Abstract

Maps of hourly precipitation have been prepared for storms during the 1957–1960 Santa Barbara randomized seeding program. In non-seeded storms, they showed that approximately N–S oriented precipitation bands could be tracked eastward across the area. Similar maps for seeded cases showed that the bands were obscured by a strong stationary E–W oriented orographic band (the mountain range is oriented E–W).

Hourly station reports were arrayed in a table for each hour where row averages revealed the amplitude of the orographic effect and column averages that of the band effect. Row variance is related to the energy of the orographic precipitation-producing circulations, column variance to the band energy, and the residual variance, obtained by subtracting row and column variances from the total variance, to the energy of smaller-scale convective circulations. Attention was confined to the 7 hours of heaviest precipitation in each system.

In comparing seeded to non-seeded periods, the mean precipitation rate was more than double, and the total variance was almost five times as great. The proportion of the total variance in orographic form was more than double, the band variance was essentially the same, while the convective variance was less than a third of the non-seeded proportion.

It is concluded that the distribution of energy was shifted from smaller to larger scale circulation systems in going from non-seeded to seeded cases. The practical implications with respect to cloud seeding are discussed and illustrated by the results from two seasons of single generator tests made in the 1957–1959 period in the San Gabriel watershed near Los Angeles.

Abstract

Maps of hourly precipitation have been prepared for storms during the 1957–1960 Santa Barbara randomized seeding program. In non-seeded storms, they showed that approximately N–S oriented precipitation bands could be tracked eastward across the area. Similar maps for seeded cases showed that the bands were obscured by a strong stationary E–W oriented orographic band (the mountain range is oriented E–W).

Hourly station reports were arrayed in a table for each hour where row averages revealed the amplitude of the orographic effect and column averages that of the band effect. Row variance is related to the energy of the orographic precipitation-producing circulations, column variance to the band energy, and the residual variance, obtained by subtracting row and column variances from the total variance, to the energy of smaller-scale convective circulations. Attention was confined to the 7 hours of heaviest precipitation in each system.

In comparing seeded to non-seeded periods, the mean precipitation rate was more than double, and the total variance was almost five times as great. The proportion of the total variance in orographic form was more than double, the band variance was essentially the same, while the convective variance was less than a third of the non-seeded proportion.

It is concluded that the distribution of energy was shifted from smaller to larger scale circulation systems in going from non-seeded to seeded cases. The practical implications with respect to cloud seeding are discussed and illustrated by the results from two seasons of single generator tests made in the 1957–1959 period in the San Gabriel watershed near Los Angeles.

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