Mechanisms for the Operation of Three Cloudwater Collectors: Comparison of Mountain-Top Results

Three different cloudwater collectors co-mounted on a mountain-top platform which often stayed immersed in clouds, were studied to determine the physical mechanisms which control their operation. These physical mechanisms may affect the chemistry of the sampled water. The cloudwater collectors used were the Desert Research Institute linear-jet impactor (DRI), the Daube California Institute of Technology active-string collector (Caltech), and the non-rotating passive Atmospheric Science Research Center string collector (ASRC). The tests showed that these instruments collected different amounts of cloudwater with the DRI yielding the least, and the ASRC the most under the same sampling environments. The DRI preferentially collects cloud droplets by bringing them toward a central nucleus upon which they accumulate and subsequently deposit into its sampling bottle; the mechanism being analogous to diffusional-condensation growth of activated cloud condensation nuclei at cloud base. The amount of water collected depends on the rate at which droplets are brought into the DRI towards the nucleus and seems to be affected by the wind speed. The Caltech and the ASRC transfer cloudwater into their sampling bottles analogously to the collision-coalescence process in precipitation initiation. Simply, cloud droplets accumulate on the strings of these collectors and are transferred to the collection bottle as they become large enough to fall acquiring smaller droplets along their path. The amount of water collected does not depend on wind speed once the collected droplets begin falling into the collection bottle. However at moderate wind speeds, the ratio of the ASRC collected water amount and wind speed during a given time period is linearly related to the liquid water content of the sampled clouds. The liquid water content of the clouds can be estimated from the ASRC measurements at moderate wind speeds exceeding 2.0 m s(-1).