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

utilize the NCAR Queen Air over the San Juan Mountains. Based on the results of
Elliott et al. (1974) and Marwitz (1974) the decision was made to conduct an exten-
sive investigation over the San Juan Mountains of the transport, diffusion and
cloud physics processes. It was apparent from the preliminary results from 1973
and 1974 that the airborne results should be placed in the context of the time
dependent storm period. Two quite fortunate advances in aircraft instrumentation
made this effort most attractive in 1975. First, the NCAR Queen Air was equipped
with an inertial navigation system during 1975 and, secondly, the Wyoming Oueen
Air was equipped with a Doppler navi.gation system for measuring horizontal winds
and an extremely reliable and rather complete cloud physics instrumentation package.
This package has made it possible to obtain the kind of reliable and complete
cloud physics measurements needed in order to assess the seeding potential of
these storms.
, The two aircraft were flown OVE!r the San Juan Mountains for 45 hours in 1975
and ~ total of 60 hours during the three years. It is estimated that there was
a precipitating cloud over the San Juans during at least 50 of the 60 hours.
During the other 10 hours a cloud was either in the process of forming or had
recently dissipated. Twelve separate storms were flown in the three years.

IV. Present Status and Preliminary Results

A preliminary synthesis of each of the storm periods has been completed.
The data from the NCAR Queen Air fat' the March 1975 storms has been delayed due
to software development and excessively long computer turn-around time. The
Wyoming Queen Air data has only become available at the time of this writing and
an indepth analysis is just beginning. Our present impressions of the cloud
processes are primarily based upon data a"ITailable to the onboard scientists who
monitored the realtime computer presentations.
Based on detailed analysis of the serial rawinsonde data obtained at Durango,
a typical storm sequence is presented in :Figure 1. Figure 1 is a vertical-time
cross section of equivalent potential temperature (8e), cloud base and cloud top
boundaries, region of conditional thermodynamic instability (a8e/aZ < 0) and level
below which the winds are not from 1600 through 2800. The typical storm sequence
is as follows:
28/05 to 28/15: The atmosphere is absolutely stable (a8e/aZ > 0) with clear
skies. The surface based nocturnal inversion forms each night and dissipates each
morning. The winds below 3.0 kIn are not from 1600 through 2800 due to blocked
flow below the mountain in the stable atmosphere. The 500 mb temperatures are
well above the -23C specification for seeding. \
28/15 to 28/21: Cold air advection begins above 6 kIn (ae~/dt < 0) and high
level clouds form. In this case the clouds are altocumulus but they are typically
cirrus. The blocked flow continues below mountain level.
28/21 to 28/23: The next occurrence in the sequence is a rapid development
and lowering of the cloud base as midlevel moisture arrives. The cloud tops remain
high and hence, cold during this period. Our preliminary cloud physics data indi-
cate that the concentration of ice crystals is at least 100/t and often exceeds
1000/t during this period. The preliminary data also indicate that supercooled
water and aircraft icing is quite rare. The winds above mountain top level (3.5 km)
begin to increase and the first precipitation begins. The 500 mb temperature
decreases significantly during this period but the atmosphere remains stably
stratified. Since the winds below 3 km are not from 1600 through 2800 and the
turbulence level above 3 kIn is low, any AgI released into this stably stratified
cloud cannot possibly seed (or overseed) this cloud.

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