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G. A. Isaac and R. S. Schemenauer

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G. A. Isaac and R. S. Schemenauer

Abstract

Measurements have been made of the concentration and phase of large particles (>70 μm) within the supercooled regions of northern Canadian cumulus clouds. During June and July, for the years 1975 and 1976, a total of 58 cumulus clouds near Yellowknife, N.W.T., were examined with a specially equipped Twin Otter aircraft. The cumulus clouds studied were mainly 1–3 km deep with most of the 130 cloud penetrations being made within 300 m of cloud top, at temperature levels between −1 and −11°C. The median penetration average (Johnson-Williams) liquid water content was 0.3 g m−3. The median penetration average concentration of particles >70 μm and >350 μm was 0.9 l −1 and 0.015 l −1, respectively. The concentration of large particles was not well correlated with J-W liquid water content or temperature, and considering all the clouds, no consistent change in the concentration was observed in successive cloud penetrations. These large particles were predominantly water drops. Ice was only found in clouds with summit temperatures colder than −8°C. Clouds containing ice had significantly higher concentrations of large particles than did all-water clouds. The data suggest that both cold and warm rain precipitation formation mechanisms were present in some of these clouds.

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John L. Walmsley, William R. Burrows, and Robert S. Schemenauer

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This paper represents a stage within a larger project to estimate acid ion deposition from cloud impacting on high-elevation forests. Acid ion deposition depends principally on three factors: the liquid water content (LWC), the ion concentration(s) in fog or cloud water, and the efficiency of the deposition process. In the present paper, the objective is to estimate LWC on Roundtop Mountain in southern Quebec from routine meteorological measurements at the Sherbrooke weather station.

After describing preliminary efforts, the methodology that was found to work best is presented. This scheme was a hybrid of applications of two statistical nonlinear regression schemes. First, the classification and regression trees (CART) algorithm was applied to predict the occurrence or nonoccurrence of fog at Roundtop. The algorithm produced by this application permitted the elimination of a large proportion of the data records for which fog was very unlikely to occur at Roundtop. The remaining data were then processed by a second application of CART to determine the predictors that are important for estimating LWC at Roundtop. Finally, these same remaining data were processed by the neuro-fuzzy inference systems (NFIS) algorithm to derive the final prediction algorithm. This hybrid method (CART–CART–NFIS) achieved a correlation coefficient of 0.810, with accuracies of 0.962 and 0.664 for the no-fog and fog events, respectively. (Corresponding threat scores were 0.916 and 0.530, respectively.) These measures of skill were significantly better than those obtained from initial estimates or from schemes that used CART alone.

Although optical cloud detector and LWC data are necessary for derivation of the fog-occurrence and LWC prediction algorithms, in the end those algorithms are applied to only the predictor data. Fog-occurrence and LWC data are not required, except for verification purposes. The algorithms and list of predictors still need to be tested to determine how widely applicable they are.

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R. J. Valente, R. K. A. M. Mallant, S. E. McLaren, R. S. Schemenauer, and R. E. Stogner

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In May 1987 a two-week field intercomparison study of ground-based cloud liquid water content (LWC) and cloud detector instruments was performed at the Tennessee Valley Authority research station at the summit of Whitetop Mountain, Virginia. The objectives were to better understand the precision and accuracy of the methods and to assist instrument developers in assessing the field readiness of the instruments for operation in clouds impacting a mountain summit. About 80 h of cloud impaction occurred during the study providing ample opportunity for instrument testing.

LWC instruments tested included four optical instruments (the Atmospheric Environment Service of Canada Fog Monitoring Device, the Engineering Design, Inc. Beeman Instrument, the Gerber Scientific PVM-100, and the, Particle Measuring Systems, Inc. FSSP-100), a psychrometric instrument (ASRC-McLaren), and a gravimetric instrument (TVA-Valente). Correlation coefficients for all pairings of properly operating LWC instruments ranged from 0.85–0.94 and linear regression slopes ranged from 0.90–1.02. Root-mean-square deviations from the mean LWC ranged from 0.026–045 g m−3 for the properly operating LWC instruments. Some of the LWC instruments experienced difficulty during their first exposure to field conditions and have been modified accordingly.

Results for cloud detectors indicated that the ECN-Mallant optical cloud detector agreed with the reference reflectometer 98 and 92 percent of the time during cloud and nonclond periods, respectively. Relative humility sensors exhibited agreement ranging from 84 to 90 percent with the reference method provided that a carefully selected threshold was established after reviewing the data.

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G. A. Isaac, J. W. Strapp, R. S. Schemenauer, and J. I. Macpherson

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A summer (June and July) cumulus cloud seeding experiment was conducted in Canada near Yellowknife in 1975 and 1976, and Thunder Bay in 1977 and 1978. Microphysical and dynamical measurements were made with three instrumented aircraft, flying in the vicinity of and within 58 clouds near Yellowknife and 66 clouds near Thunder Bay. Using wing-mounted AgI pyrotechnic flares, a total of 25 of these clouds were seeded as an aircraft penetrated each cloud at the −5 to −10°C temperature level ∼ 300 m below cloud top. The microphysical properties of each cloud before and after seeding were compared with a statistical summary of the microphysical characteristics of natural or non-seeded cumuli at the same temperature level. Higher concentrations of ice particles were observed after seeding in ∼50% of the seeded clouds, with the magnitude of the increase and the post-seeding concentration being abnormally high in most cases. Approximately 40% of the Yellowknife cumuli produced rain after seeding and examples are given of how this rain could have been produced by an artificially stimulated Bergeron-Findeisen process. Near Thunder Bay, no rain was generated by the AgI. Even though Thunder Bay cumuli contained more cloud water and fewer large particles than Yellowknife cumuli, seeding was less successful because cloud lifetimes were too short for artificial precipitation to form.

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G. A. Isaac, R. S. Schemenauer, C. L. Crozier, A. J. Chisholm, J. I. MacPherson, N. R. Bobbitt, and L. B. MacHattie

Abstract

A cloud seeding technique is proposed which has the objective of stimulating rainfall from cumulus clouds drifting over forest fires. Preliminary tests of the ice crystal production capability of the cloud seeding technique were conducted on five cumulus clouds near Yellowknife, N.W.T., Canada, during July 1975. These clouds were over forest but not near forest fires. A T-33 turbulence research aircraft performed the seeding by burning wing-mounted TB1 AgI flares while flying through the clouds at the −5 to −10°C level. The T-33 turbulence measurements enabled estimates to be made of the rate of dispersion of the AgI. Microphysical measurements were made before and after seeding by an instrumented DHC-6 Twin Otter aircraft flying at the seeding level, and these were compared with measurements in six untreated cumulus clouds. High concentrations of ice crystals appeared after seeding in four of the five seeded cumulus clouds, and on two occasions precipitation-sized particles appeared at the seeding level. The evidence indicates that the AgI aerosol produced large quantities of ice crystals.

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