WMOD00403

• Title: Special Regional Weather Modification Conference Augmentation of Winter Orographic Precipitation in the Western US
• Date: 11/11/1975
• Weather Modification - Doc Type: Report

Effective 8 November 1972 changed to:

2d) Cloud-top temperature of -8 to -230C or,in the absence of this datum,
500 mb temperature, is forecast to exist over the Projeet target areas for at least
3 hours during the 1100 to 1100 MST period. The operat:Lonal criteria of cloud-top
temperature shall be applied to the lower precipitation--producing cloud mass. In
those cases where the upper clouds are present with a substantial clear air layer
between the tops of the lower layer and the base of the upper layer, the cloud-top
temperature of the upper cloud layer will not be used in the application of the
cloud-top temperature criterion.

Effective 11 October 1973 changed to:

2e) A 500 mb temperature of -230C or warmer in forecast to exist over the
Project target areas at least 3 hours during the 1100 to 1100 MST period.

3) 700 mb winds are forecast to be towards the mountain slopes of the target
area (1500 through 3000).

Effective 1 October 1971 changed to:

3a) Mean wind field from the surface to 700 ~) is forecast to be toward
the mountain slopes of the project area (approximately 1700 through 3000).

Effective 22 December 1971:

3b) The representative wind flow, usually 700 mb, is forecast to be towards
the mountain slopes of the Project target areas (approx:lmately 1700 through 3000).
The representative wind flow was determined by averaging the 9,000 through 11,000 feet
winds for direction and speed. A plot of the trajector:, of the winds from the sur-
face to 10,000 feet is used to determine the lateral of:Eset distance of the center of
the plume, between the surface source and the height of the representative wind flow
near 700 mb.

Changes in the precipitation criterion reflect a refin~nent in the verification of
the experimental day precipitation forecasts. Changes:ln the temperature criterion
reflect the uncertainty resulting from the concept that the cloud-top temperature
and 500 mb temperatures were approximately synonymous. Also reflected is the concern
of natural seeding from ice crystals falling from cirru:; clouds. Since the 500 mb
temperature was the actual operational criterion for thla Climax Project and was the
actual parameter in the temperature stratification analysis of the Project's data,
the decision was made by the Bureau of Reclamation to r,aturn to the 500 mb temperature
criterion. Changes in the wind direction criterion reflect attempts to define and use
the most representative wind flow for the targeting of seeding material.

Once an experimental day meeting the above criteria was forecast and determined by
the randomization to be a seed day, the area was seeded using a variable seeding rate
as based on the Climax model. The initial seeding rate was determined by the fore-
cast cloud top temperature or 500 mb forecast temperature. During the seeding period
the rate was controlled by the monitored cloud-top temperature or 500 mb temperature
according to the following criteria:

1) -140C and warmer produce up
generator site (up to 140 gm hr~l).

2) -150C to -240C produce 1016
site (20 gm hr-l).

16 nuclei -1 0
to 10 effective hr ^ at -15 C per
j'
,
effective nuclei -1 -200C ~er generator
hr at
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