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Graeme K. Mather

Abstract

Determination of the habits (ice or water) and therefore the densities of particles whose images are acquired by 2D probes is often an ambiguous process. A Learjet's radar measurements of equivalent reflectivity factors from a range gate 1800 m ahead of the aircraft are compared to reflectivities calculated from images acquired by a 2D-C probe over a range of assumed particle densities from 0.2 to 1 g cm−3. Although the comparisons suffer from many uncertainties, such as the vast disparity between the volumes sampled by the 2D-C probe and the aircraft radar, the method does discriminate well between water drops or recently frozen riming water drops and low density graupel particles.

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Graeme K. Mather

Abstract

While conducting a randomized seeding experiment, a storm was selected whose microphysical characteristics were so far from what was expected, given the existing thermodynamic environment, that some explanation was needed to account for the apparent errant behavior of this storm. More than a decade of sampling at −10°C using the project Lear jet has led to a simple classification technique, based on cloud-base temperature and buoyancy, which predicts the absence or presence, and to some extent the degree of coalescence (coalescence-freezing) precipitation growth in local convective storms. The unusual nature of this storm was recognized against this extensive microphysical database. The probable cause is attributed to emissions from a Kraft paper mill 10 km south of the storm's position. The mill had recently undergone an expansion program that had quadrupled its output of paper products.

Using the radar characteristics of this unusual storm as a guide, a search of one season of radar data revealed the existence of five other similar storms, all within about 30 km of the paper win. These records indicated that the storms apparently modified by the paper mill tended to last longer, grow taller, and rain harder than any other storms recorded on that day.

The new awareness of this apparent inadvertent weather modification by the paper mill led to launching missions to intercept clouds growing in the vicinity of the mill. Storms, apparently altered by the mill, were sampled and compared to other nearby storms. The most singular feature of the modified storms was the appearance of lame (>4 mm) drops at the most common sampling level (−10°C), indicating an accelerated or enhanced coalescence precipitation formation process.

Measurements in a field of cumuli indicated a broadening of the cloud-base droplet spectra in clouds affected by the emission of the mill. It is the addition of this “long tail” to the cloud-base droplet spectra that is apparently turning on or at least enhancing coalescence in affected storms.

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Graeme K. Mather

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Graeme K. Mather

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Graeme K. Mather

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Graeme K. Mather

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A commercial hail suppression program has been in operation at Nelspruit, South Africa (25°S, 31°E) since December 1971. The seeding technique employed has been the “on-top” method, which consists of dropping silver iodide pyrotechnics into the tops of cloud turrets growing on the flanks of mature storms. Daily crop insurance records of hail damage to tobacco are compared for seeded and nonseeded days. The control (unseeded) days consist of historical data and of days during the operational period when storms in the area were definitely not seeded, usually for operational reasons. A “hail severity ratio”, defined as the ratio of the crop lost to the total crop planted in the areas hit by hail, is derived from the hail-damage assessments and appears to be the parameter most sensitive to the claimed treatment effect. A comparison of seeded days during the initial phases of the program, when propeller aircraft were used, with more recent days when one or two jets were used for the seeding, indicates significantly lower severity ratios for jet-seed days. The analysis of the seed versus the control days for three separate regions in the operational area show, on the average, statistically significant reductions in hail severity on the days on which jets were used for seeding. As a ratio, the severity parameter can be altered by changes in the numerator (total crop damage) and/or the denominator (total crop planted) in the areas hit by hail. An analysis of total damages and areas hit by hail indicates that the numerator appears to be affected most strongly by the treatment, and that there is no indication that the seeding is causing an increase in the area hit by hail.

It is unfortunate that the data were not collected within the framework of a randomization scheme planned before the initiation of the seeding; the statistics, however, do possess an internal consistency that lends support to the validity of the conclusions.

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Graeme K. Mather, E. K. Bigg, and Susan Renton

Abstract

Daily precipitation records during the period of the hail suppression seeding program in the Eastern Transvaal around Nelspruit, South Africa are examined for evidence of persistence effects from silver iodide iodine similar to those reported by Bigg and Turton in Australia. Using their method of analysis, a positive anomaly is found in the comparison of target-to-control rainfall commencing 11 to 12 days after a seed day and lasting 10 to 11 days. The Australian anomaly commences 9 to 10 days after a day and lasts about 9 days. Otherwise, the “peristence signatures” in the two areas are very similar. This repetition of apparent persistence effects in another area on a different continent raises serious doubts about conventional statistical assessments of the success of randomized seeding experiments when silver iodide is used.

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Griffith Morgan, Mark Schormann, Erika Botha, and Graeme K. Mather

Abstract

A technique has been developed for deriving estimates of condensed water mixing ratio in cumulus clouds from measurements of potential temperature in the air in the compressor of a jet engine. Condensate that enters the engine at low temperatures (near −10°C) is evaporated in the compressor, causing a cooling of the air that is proportional to the amount of condensate evaporated. An important element of the technique is a correction for the time response of the temperature measurement system mounted in the compressed air. This new technique is simpler and more robust than an earlier technique based on measurement of the vapor concentration in the compressed air by an optical extinction (at Lyman-alpha wavelength) method. The values of condensed water mixing ratio derived from the new system are shown to be similar to those from the optical extinction system, previously reported in the literature.

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Graeme K. Mather, Brian J. Morrison, and Griffith M. Morgan Jr.

Abstract

For the past three years, a Learjet has been making microphysical measurements in new cloud development on the flanks of multicellular storms in the eastern Transvaal area of South Africa. Data from an imaging probe and a forward scattering spectrometer have been averaged for each storm for all first cloud penetrations between −8° and −12°C. Clear images of drops of diameters greater than 300 μm are found in 40% of the 42 storms measured.

Most of the observed drops are associated with the more “maritime” droplet spectra. Also, the appearance of coalescence around −10°C appears to be related to cloud base temperatures and buoyancies, rather than changes in air masses, suggesting that cloud thermodynamics may play a dominant role in determining cloud microphysics in the Nelspruit area.

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William A. Cooper, Roelof T. Bruintjes, and Graeme K. Mather

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Some possible effects of hygroscopic seeding with flares are explored by calculating how such seeding would modify the initial size distribution of cloud droplets and the subsequent evolution of that size distribution by coalescence. To be representative of recent experiments in South Africa, the calculations emphasize the effects of hygroscopic particles that can be produced by flares, instead of the larger particles used in most past hygroscopic-seeding experiments. Parcel calculations representing simultaneous condensation and coalescence suggest that the formation of rain through the warm-rain process can be accelerated significantly by the addition of such hygroscopic particles. Some observations of the effects of hygroscopic material near cloud base support at least the early stages of the calculations. The results suggest that the positive effects being obtained in the South African experiment may occur through such acceleration of the warm-rain process. Possible cloud-seeding applications and climate implications are discussed.

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