Abstract

Many of the severe and persistent hailstorms in Alberta propagate by means of new cloud development on the southern or western flank. A concept is proposed whereby these cumulus towers are seeded with freezing nuclei early in their development. In order to accomplish this a droppable pyrotechnic flare system was developed and tested.

A T-33 jet aircraft was used as the seeding platform. It was equipped with a Ku-band weather radar, a flare rack and firing control panel, a 14 channel recorder and a 3-cm transponder. Seven inch pyrotechnic flares were manufactured to the following specifications: delay burn 50 sec, silver iodide burn time 30 sec, and flare output 24 gm of silver iodide producing a total of 2.4 × 1014 freezing nuclei active at −10C. A unique feature was the incorporation of 10-cm radar chaff which was released at the flare burn-out and used as a position marker. Flare performance was evaluated using radar, visual and photographic tracking. Total fall distance as a function of release height was determined. For typical drop altitudes used to seed storms the flares fell 9500 ft with silver iodide being released during the last 2700 ft.

In the summers of 1970 and 1971 the seeding system was used on sixteen occasions in experiments which emphasized physical understanding rather than statistical inference. On two occasions turbulence measurements were made in cumulus towers with a second T-33 aircraft. The calculated dissipation rates indicate that there is sufficient diffusion to produce silver iodide nuclei concentrations in excess of 100 liter−1 active at −10C through cloud volumes of several cubic kilometers within a few minutes after seeding.

The operational logistics of this seeding system are quite straightforward and the system appears to be a practical one for applying the direct injection seeding technique to multicell hailstorms. By means of radio communication between the project control room and the seeding aircraft, it was always possible to unambiguously identify and seed the selected target storm. The radar chaff was limited usefulness as a marker of seeding location.

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