WMOD00430

• Title: A Diagnostic Technique for Targeting During Airborne Seeding Experiments in Wintertime Storms over the Sierra Nevada
• Date: 7/7/1988
• Weather Modification - Doc Type: Report

JULY 1988

RAUBER ET AL.

819

interpreted cloud system structure. The general pro-
cedure used during SCPP is described below.
During storms, rawinsondes were launched at 3 h
intervals from Sheridan and Kingvale. Occasionally,
additional soundings were launched if the project di-
rector anticipated strong changes in the wind fields in
shorter time periods. These soundings and other avail-
able data were used to sekct an initial seeding altitude
and average liquid water content of the cloud system
and to initiate the targeting calculations. The results
of the targeting predictions were provided to the seeding
coordinator with specific information concerning the
location of the seeding line center point (SLCP) and
appropriate altitudes to relay to the two aircraft. The
research aircraft then departed from McClellan Air
Force Base, a Sacramento Valley airport, and com-
pleted a sounding to 4880 m over the valley (all heights
are above mean sea level). This updated sounding was
reported to the forecaster, entered into the data base,
and sometimes used with a special Kingvale sounding
to update the targeting calculations. The research air-
craft then completed a cloud profile which included
an eastward pass beyond the crestline and a westward
descent along the MOCA. The research and seeder air-
craft then proceeded to the SLCP.
At the SLCP, the research aircraft made a classifi-
cation pass along the seedline. Seeding commenced if
I) the aircraft was visually in cloud and 2) liquid water
content was above 0.05 g m-3 on the cloud physics
aircraft liquid water sensors for at least one-third of
the distance along the classification track (total track

length was usually 37 km), or the radiometer detected
liquid water depths above 0.1 mm for 10 min.
If the criteria were not met at this location, a new
SLCP was calculated for a lower altitude. The aircraft
proceeded to this location. When the criteria were sat-
isfied, seeding began. Later soundings (or observed
winds from aircraft) when they became available, were
used to update the seeding location (no more often
than 30 min). In this way, the seeding location was
periodically adjusted to account for mesoscale and
synoptic scale changes in the wind fields. These pro-
cedures are detailed in the SCPP fixed target experiment
design (Bureau of Reclamation 1985).

2) PROCEDURES FOR PYROTECHNIC OR CO2 SEED-
ING

During experiments where pyrotechnics or CO2 were
released by aircraft, targeting calculations were per-
formed to predict I) the location of the SLCP and the
seedline orientation required to target ice particles cre-
ated by seeding to Kingvale, and 2) the areal coverage
of the seeding effect on the ground.
Figure 6 illustrates the procedure developed to de-
termine the SLCP and the seedline orientation. Based
on aircraft and other available information, the user
specifIed the altitude of the seeder aircraft and the es-
timated average cloud liquid water content. A particle
was then initiated at the grid origin (0, 0), 300 m below
the se:eder aircraft altitude. The 300 m distance was
chosen because it was approximately the center of the

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FIG. 6. Adjustment procedure to determine the location of the seeding line center
point (SLCP). An initial trajectory (AB) is calculated from the origin (A). Point C is
determined by moving from A along error vector BD. A second trajectory (CF) and
associated error (FD) are calculated resulting in a shift to point G. A final trajectory
(GE) and associated error (ED) is determined, resulting in convergence to the SLCP.