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William L. Woodley

A statistical study has been conducted investigating the possible cloud growth effects due to heat released when supercooled water is converted to ice. Calculations are made of the heights to which cloud tops would rise assuming that the supercooled water is converted to ice at − 10C (the seeded case) or at − 30C (the unseeded case). The clouds suffer a loss in buoyancy due to entrainment and the weight of the condensate. Sixty-two cases are treated for soundings from St. Martins Island during August of 1962. The results of this study are compared to those of a similar study for Flagstaff, Arizona, during the summers of 1961, 1962 and 1963. The results imply that spectacular height increases due to seeding can be expected, but that such cases are relatively infrequent. Specific observations of cloud growth due to seeding are noted, but can only be considered as consistent with these concepts rather than verifying them. The factors important for the seeding effect and the conditions which produce them are also examined. Despite the simplicity of the model, it is felt that the computed height differences qualitatively represent a reasonable measure of the potential for cloud dynamics changes from seeding.

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Joanne Simpson, William L. Woodley, and Robert M. White

A serious drought in the spring of 1971 occurred in South Florida. In view of NOAA's research experience in dynamic cumulus modification (massive silver iodide seeding to invigorate cumulus updrafts by means of release of latent heat of fusion), the state government sought our aid in combating this drought by means of a seeding effort. NOAA responded by an extension of its experimental program with both practical and research objectives.

Two of the NOAA Research Flight Facility aircraft were used for airborne pyrotechnic seeding from 1 April to 31 May 1971. A one-dimensional numercial cumulus model was run in real time on each day, eliminating 38 days in the period as unsuitable. Flights were conducted on 16 days, with actual seeding on 14. Seven suitable days were lost due to routine aircraft maintenance. Radars and raingages were used to calculate rain amounts from all seeded and many unseeded clouds.

Seeding was conducted in an attempt to promote merger by treating clouds close together in space and also the upshear towers of each previously seeded complex. A total of not less than 180,000 acre-ft of water was calculated to have fallen from the seeded clouds in two target areas totaling about 7200 n mi2. A conservative estimate attributed 100,000 acre-ft as due to seeding, although without randomization this evaluation cannot be made firm. Satellite and synoptic studies accompanied rainfall evaluation on all seeding days. Among the important scientific results is that some frontal conditions appeared suitable for dynamic seeding in Florida, offering hope for extension of the technique into dry periods.

Some aspects of NOAA's future policy in the rain enhancement aspects of weather modification are presented.

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Daniel Rosenfeld, William L. Woodley, Alexander Khain, William R. Cotton, Gustavo Carrió, Isaac Ginis, and Joseph H. Golden

Improving the forecasts of the intensity of tropical cyclones (TCs) remains a major challenge. One possibility for improvement is consideration of the effects that aerosols have on tropical clouds and cyclones. The authors have been pursuing this under the Hurricane Aerosol and Microphysics Program, supported by the U.S. Department of Homeland Security. This was done through observations of aerosols and resulting cloud microphysical structure within tropical cyclones and simulating their effects using high-resolution TC models that treat cloud internal processes explicitly. In addition to atmospheric aerosols, special attention was given also to the impact of the intense sea-spray-generated aerosols and convective rolls in the hurricane boundary layer (BL) under hurricane- force winds.

The results of simulations and observations show that TC ingestion of aerosols that serve as cloud condensation nuclei can lead to significant reductions in their intensities. This is caused by redistribution of the precipitation and latent heating to more vigorous convection in the storm periphery that cools the low levels and interferes with the inflow of energy to the eyewall, hence making the eye larger and the maximum winds weaker. The microphysical effects of the pollution and dust aerosols occur mainly at the peripheral clouds. Closer to the circulation center, the hurricane-force winds raise intense sea spray that is lifted efficiently in the roll vortices that form in the BL and coalesce into rain of mostly seawater already at cloud base, which dominates the microstructure and affects the dynamics of the inner convective cloud bands.

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William L. Woodley, John A. Flueck, Ronald Biondini, Robert I. Sax, Joanne Simpson, and Abe Gagin

The Florida Area Cumulus Experiment (FACE) is a long-term program to determine the potential of dynamic seeding for increasing convective rainfall over a fixed target area. The first phase of FACE (FACE-1) provided strong indications for increased, seeding induced rainfall. The second phase, FACE-2 (beginning in June 1978 and ending in August 1980), was conducted in an attempt to confirm these indications of a positive seeding effect. The criteria for confirmation in FACE-2 were published in a NOAA Technical Report prior to program commencement. A clarification and sharpening of these confirmatory criteria are discussed in this paper. In addition, a minority position of what is to constitute confirmation in FACE-2 involving the use of linear predictor models also is discussed. This paper was written and accepted for publication before the treatment decisions of FACE-2 were known.

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