WMOD00402

• Project Name: Sierra Cooperative Pilot Project
• Title: Sierra Cooperative Pilot Project - Final Report for Period November 1984 - September 1986
• Date: 6/1/1987
• Weather Modification - Doc Type: Report

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Section IV.

Meteoroloqical Data Collection and Analysis

These tasks involve forecasting winter storm conditions and their
suitability for seeding, operating high-altitude weather stations for fore-
cast and suspension criteria purposes, and using the DRI snow gauge network
for measuring the precipitation amounts and distribution over a major por-
tion of the Tahoe-Truckee watershed. All of these functions were performed
during the two years of this agreement and the details of the work performed
are described in the main sections of the text.

Section V.

Air Mass Characterization

The program was designed to study the concentrations and supersatura-
tion spectra of CCN in air masses flowing over the Sierra Nevada during
non-storm and storm conditions.

It was. found that CCN concentrations in air at ground level were very
low during storm periods. This is indicative of modified maritime air where
the concentrations of small particles are enhanced relative to the larger
particles. It may also reflect the loss of larger particles caused by more
efficient cloud water droplet capture.

It was also found that CCN spectra taken at Norden (on the Sierra
summit) and at Stead (about 60 km east of the summit) were quite similar
indicating broad spatial homogeneity of these spectra. By measuring con-
centrations and spectra continually throughout a storm period, it was
possible to "identify" the different components of air masses constituting
the mesoscale flow across the project area. Examples of this are given
in the text of Section V.

Airborne sampling was also conducted using the Wyoming King Air air-
craft. Results showed that transitions from continental air to maritime
air occur very rapidly, indicating relatively shar~ interfaces between air
masses with different CCN characteristics. Such changes of course would be
expected to influence the microphysical features of the clouds and thereby
their seedability and precipitation efficiencies. Measurements of CCN were
also made as a function of altitude above sea level. The results show some
very interesting features particularly with low concentrations occurring
near the freezing level. In general the CCN concentration decreases with
altitude.

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