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Alexis B. Long and Elizabeth J. Carter

Abstract

Two Australian winter mountain storm field research projects were conducted by the Commonwealth Scientific and Industrial Research Organisation Division of Atmospheric Research and the Desert Research Institute Atmospheric Sciences Center in the austral winters of 1988 and 1990. These projects gained information about winter storms in support of the ongoing Melbourne Water randomized cloud seeding experiment aimed at increasing runoff into Melbourne's main water supply, the Thomson Reservoir. This paper discusses some of the 1988 instrumentation data. One variable of interest is the precipitation augmentation potential π. It is the difference between (a) the horizontal supercooled liquid water flux in the clouds crossing the mountains and (b) the vertical precipitation flux at the surface from the clouds. These fluxes are based on calculations of supercooled liquid water depth in clouds with a microwave radiometer, Omegasonde wind velocity, and rates of precipitation from gauges. It was found that π varies systematically during a winter storm. The greatest potential occurs in the post-cold-frontal stage of a storm when the cloud-top temperature is warm and about −12°C and the wind direction of 240° is approximately orthogonal to the main southwest face of the predominant orographic feature, Baw Baw Plateau, of the study area. The potential is significantly less during the prefrontal and frontal stages, with cloud-top temperatures of about −35°C and a wind direction of about 3O0° parallel to the Baw Baw Plateau. The results show that cloud seeding would have the greatest benefit in the postfrontal stage.

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Edward E. Hindman, Elizabeth J. Carter, Randolph D. Borys, and David L. Mitchell

Abstract

Supercooled cloud droplets were inertially impacted onto “cloud-sieves” at a mountaintop location. The large cross-sectional areas of the sieve meshes permitted grams of cloud water to be passively collected in minutes. Each sieve was constructed from specific diameter cylindrical strands and collected all cloud droplets larger than a critical size. Procedures are developed to produce liquid water content (LWC) and chemical composition values as a function of droplet-size interval.

The sieve LWC measurements were compared with simultaneous LWC measurements obtained from a standard cloud droplet spectrometer. The sieve and spectrometer values were consistent for droplets between approximately 4 and 13 µm in diameter. The sieves overestimated the water contents of larger and smaller droplets in low LWC clouds (<0.1 gm−3). In high LWC clouds, the sieve LWC values for all droplet sizes closely approximated the spectrometer values.

Sources of error were investigated. Rime “feathers” and frost grew on the larger sieves in low-LWC clouds, capturing droplets smaller than the sieves critical size. Frost growth on the smallest sieve overestimated LWC values of the smallest droplets. Procedures are suggested to overcome these limitations.

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