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A. I. Weinstein

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A. I. Weinstein and D. M. Takeuchi

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Two vertical-fall silver iodide pyrotechnics were inserted into a supercooled cumulus cloud near Flagstaff, Ariz. Within a few minutes, a turret was observed to extend over 1.35 km above the cloud top. The temperature within the main body of the cloud below the turret rose by 0.75C.

Formvar replicator film showed that the number of ice crystals in the cloud at the −5C level exceeded the number of ice nuclei effective at this temperature by three orders of magnitude.

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A. I. Weinstein and P. B. MacCready Jr.

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A randomized seeding experiment was conducted in Flagstaff, Ariz., in July and August of 1967 wherein the heights, rainfall amounts and rainfall durations of isolated cumuli were measured. Twenty-one clouds were studied, 10 seeded and 11 not seeded, on 11 days. On nine days there were paired seed and no-seed events. The choice of the test clouds and much of the analysis was performed with the aid of a simple numerical model of cumulus dynamics and microphysics. The project aims were to show distinct effects of seeding on cloud-top height, rainfall and duration; to verify and/or improve the model; and to use the model to interpret the importance of the seeding effect.

For the 21 test clouds, the average increases in radar tops, precipitation and duration were 5900 ft, 2.00 mm and 10 min, respectively, with student's t-test significance values being 96, 92 and 81%, respectively. On the nine days of paired seed and no-seed events, the t-test significance exceeded 98% for the three variables quoted above. On every one of the nine days, the seeded clouds showed increased height, rainfall and duration.

Using the model after it was calibrated against the 11 control clouds, it was found that the seeding increased the 10 cloud-top heights by an average of 6000 ft, with a maximum of 21,500 ft. The student t-test significance value for this increase was 96%.

Using the calibrated model on all of the 21 test clouds showed how much benefit could have been derived from seeding all of the clouds. The average potential increase in cloud-top height was 6500 ft. This represents a 52% increase assuming bases at 13,000 ft. Comparison with observations showed an rms prediction error of 3341 ft. The corresponding rainfall and duration increases were +2.86 mm (up from 2.99 mm) and 7 min (up from 11 min), respectively. These hypothetical increases are comparable to the observed values.

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Alan I. Weinstein and Bernard A. Silverman

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A two-dimensional Eulerian model of warm fog dispersal by airborne hygroscope particle seeding is used to evaluate some practical aspects of urea seeding at airports. It is found that although turbulence and wind shear reduce the effectiveness of single-line seeding to a point where it is of no practical value, seeding over a wide area (1−10 mi2) can result in practically useful visibility improvements in the approach zone and over the runway of airports.

The quantity of material and the cost of the wide-area seeding technique depend upon fog intensity, fog type, and cross-runway wind speed. For typical fog, approximately 80,000 lb hr−1 of urea costing $40,000 per hour are needed to keep the visibility above ½ mi. The figures are reduced to approximately 36,000–58,000 lb and $18,000–29,000 per hour, respectively, if the visibility needs only to be raised to ¼ mi.

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Bruce A. Kunkel, Bernard A. Silverman, and Alan I. Weinstein

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Thermal fog dispersal tests were conducted by the Air Force Cambridge Research Laboratories during July 1972 at Vandenberg AFB, California. The experimental heating system consisted of an array of passive liquid propane burners that were arranged in four lines perpendicular to the prevailing wind. An instrumented 200 ft tower and a lidar were used to monitor the effects of the tests on the foggy environment.

The heating tests were designed to simulate fog dispersal operations at an airport under cross-runway wind conditions. The effect of wind speed and heat output on the temperature, visibility and turbulence structure of the environment are discussed. The test results confirmed the earlier findings of the British FIDO program during WW II with respect to the characteristic temperature rise pattern for a cross-wind situation. The program documented visibility improvements in the heat plumes that could only be inferred from the published FIDO temperature rise data. Extrapolating the results of these experiments to a similar system installed at an airport, it appears that the visibility improvements that were achieved in the experiments would always be adequate for Category 2 (100 ft decision height, ¼ mi visibility) and 3A (no decision height, ⅛ mi visibility) landing operations. Category 1 (200 ft decision height, ½ mi visibility) landing conditions could be achieved by increasing the heat output of the burners.

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A. I. Weinstein, E. R. Reiter, and J. R. Scoggins

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Several time series of vertical wind velocity profiles obtained by tracking spherical superpressure balloons using l1–20 radars at Point Mugu, California and Cape Kennedy, Florida are analyzed. In the altitude range 11–20 km the measured wind data are an order of magnitude better than standard GMD-1 data. Below 11 km the smooth 2-m diameter spherical balloon used is aerodynamically unstable, producing spurious high frequency oscillations. Each series contains from 8 to 18 wind velocity profiles spaced over a period of time from 8–12 hr.

Every profile in each series contained mesoscale perturbations of from 5–10 m sec−1 and 5–20 deg (referenced to an arbitrary smooth profile), through depths up to 2 km, which persisted in recognizable form at approximately constant altitude throughout the series. A model to account for such perturbations is presented, picturing the atmosphere from 11–20 km as having a distinctly layered structure. Each layer covers a large horizontal area and contains air whose motion is principally controlled by a quasi-inertial oscillation. Wind profile perturbations result from the relative horizontal motion between layers. It is suggested that, in the presence of geostrophic wind shear, overall thermal stability, and low turbulence energy normally found in the stratosphere, quasi-inertial oscillations, once started through a deep atmospheric layer, would soon disperse into a series of shallow individual ones of different wavelengths, capable of maintaining themselves for long periods of time.

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