Collecting Supercooled Cloud Droplets as a Function of Droplet Size

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  • 1 Earth and Atmospheric Sciences Department, The City College, New York City, New York
  • | 2 Atmospheric Sciences Center, Desert Research Institute, Reno, Nevada
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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.

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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