The Collection Efficiency of a Modified Mohnen Slotted-Rod Cloud-Water Collector in Summer Clouds

Y. J. Kim NOAA/Environmental Research Laboratories, Air Resources Laboratory, Aerosol Research Section, Boulder, Colorado

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J. F. Boatman NOAA/Environmental Research Laboratories, Air Resources Laboratory, Aerosol Research Section, Boulder, Colorado

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Abstract

A modified Mohnen slotted-rod collector was used to collect cloud-water samples in summer clouds over the northeastern United States. Cloud-droplet-size distributions were measured with a forward-scattering spectrometer probe (FSSP) mounted on the National Oceanic and Atmospheric Administration (NOAA) King Air research aircraft. Cloud-droplet-volume distributions and liquid water content were determined for each cloud-water sample through analyses of the FSSP data. The theoretical collection efficiency of the slotted-rod collector was calculated as a function of droplet size for flight conditions encountered during each cloud-water sampling. The mass ratio was then calculated for each cloud sample by ratioing the actual collected water mass to the maximum possible collectable water mass. Mass-ratio values higher than unity were obtained having an average of 1.40±0.27. This could be due to an underestimation of the liquid water content by the FSSP or to the collection of large hydrometeors by the slotted rod. The modified Mohnen slotted rod collected fast representative cloud-water samples in sufficient quantities for chemical analysis of the sampled cloud water.

Abstract

A modified Mohnen slotted-rod collector was used to collect cloud-water samples in summer clouds over the northeastern United States. Cloud-droplet-size distributions were measured with a forward-scattering spectrometer probe (FSSP) mounted on the National Oceanic and Atmospheric Administration (NOAA) King Air research aircraft. Cloud-droplet-volume distributions and liquid water content were determined for each cloud-water sample through analyses of the FSSP data. The theoretical collection efficiency of the slotted-rod collector was calculated as a function of droplet size for flight conditions encountered during each cloud-water sampling. The mass ratio was then calculated for each cloud sample by ratioing the actual collected water mass to the maximum possible collectable water mass. Mass-ratio values higher than unity were obtained having an average of 1.40±0.27. This could be due to an underestimation of the liquid water content by the FSSP or to the collection of large hydrometeors by the slotted rod. The modified Mohnen slotted rod collected fast representative cloud-water samples in sufficient quantities for chemical analysis of the sampled cloud water.

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