WMOD00287

• Title: The National Weather Modification Plan
• Prepared For: CAO Subcommittee on Weather Modification
• Prepared By: The Working Group of the CAO Subcommittee on Weather Modification
• Date: 2/13/1981
• Weather Modification - Doc Type: Report

1963. Th.ese early seeding experiments were not very revealing. The seeding
of Hurricane Debbie in 1969, using a revised hypothesis, provided results
suggesting reductions of 15 percent or more in peak winds.

The cun-ent STOBMFUR.Y hypothesis was developed through numerical
experimentation with a simple axisymmetric hurricane model by the National
Hurricane Research Laboratorr in the late 1960 's, and the Hurricane Debbie
seeding was based on this hypothesis. It proposes that silver iodide seeding
in the first major cloud band outside the hurricane eyewell can initiate a
chain of events within the hurricane cloud system that will result in
redistribution of wind energy concentrated near the storm's center so that the
original eyewall will weaken, anew eyewall will form at a larger radius, and
maximum winds will be decreased.

Hurricane seeding experiments were discontinued in 1973 to permi t NOAA to
replace its obsolete research aircraft and instrumentation. The new equipment
was available by the 1977 hurricane season, but there have been no hurricanes
suitable for experimentation since then. To be eligible for seeding a storm
must meet certain meteorological criteria, be within reach of U.S. airfields,
and have a probability of 10 percent or less of reaching land within 24 hours
after seeding. Since the Atlantic region has provided few opportunities in
recent years, attempts have been made to initiate experiments in other regions
of the world (such as the western Pacific, the eastern Pacific, and Australia)
that are more prone to tropical cyclone activity.

STORMFURY still involves many scientific uncertainties, and intensive
studies of nonseeded storms (that can economically be carried out in the
Atlantic from the Miami base of operations) are needed to provide insight into
the microphys ical, cloud-scale, and mesoscale links of the STORMFURY
hypothesis. Research flights were conducted in hurricanes during recent
summers to gather information on storm characteristics and cloud processes.
These data form the basis for refining computer models to provide more
accurate and representative simulations of the structure and dynamics of
hurricanes.

2. Florida Area Cumulus Experiment (FY 1980--$1.9 Million)

The Florida Area Cumulus Experiment (FACE) has been conducted in t",JO
phases. The first, FACE-i, was an exploratory experiment designed to
determine whether dynamic seeding can augment convective precipitation over a
substantial area in South Florida by promoting the growth and organization of
summer cumulus systems. FACE-i, conducted during five summers between 1970
and 1977, was a double-blind, randomized experiment" in which the treatment
decision was unknown to the e."tperimenters. Seeding effect was determined
using rainfall in the 6 hours after initial seeding in the intensively treated
area (floating target) and in the entire target area (total target) as
estimated by calibrated radar and adjusted on a daily basis by rain gages.

Analyses of experimental days provided modest evidence that dynamic
seeding increased areal rainfall. For the entire FACE sample (39 seed days
and 36 control days) rainfall in the floating and total targets generally was

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