Applicability of the VisiSize D30 shadowgraph system for cloud microphysical measurements

Abstract

Abstract. A commercial shadowgraph system, the Oxford Lasers VisiSize D30, originally designed to characterize industrial and agricultural sprays, was tested with respect to its application for measuring cloud microphysical properties such as droplet size distribution and number concentration. A laboratory experiment with a dense stream of polydisperse cloud-like droplets indicated a strong dependence of the depth of field, and thus also the sample volume, on particle size. This relationship was determined and a suitable correction method was developed to improve estimations of droplet number concentration and size distribution. The spatial homogeneity of the detection probability inside the sample volume and the minimum droplet diameter providing uniform detection were examined. A second experiment with monodisperse droplets produced by a Flow Focusing Monodisperse Aerosol Generator (FMAG) verified the sizing accuracy and demonstrated reasonable agreement between the instruments. Effects of collisions and the evaporation of droplets produced by the FMAG were observed. Finally, when the instrument was applied to sample atmospheric clouds at a mountain-based observatory, it performed reliably during a 3-week-long field experiment. Based on the laboratory and field tests, recommendations concerning the use of the instrument for cloud droplet measurements were formulated.

Comment on amt-2021-281

“…A wide range of particle/droplet information, such as droplet size, velocity, and shape provided by the VisiSize makes the device potentially suitable for atmospheric research purposes, especially cloud microphysical measurements. The instrument specifications and proper- ties for the two camera lens magnification settings recommended by Nowak et al (2021) for cloud droplet measurements are summarized in Table 1. Moreover, specifications of the CPI and the HoloBalloon (HOLIMO 3B) are also listed in Table 1.…”

“…Droplets detected inside the depth of field are then measured based on their shadow images, and the size distribution is built by analyzing a series of images. However, some important points have been revealed after laboratory experiments with the VisiSize by Nowak et al (2021), such as the necessity of developing a correction method for sample volume (SV) calculation or minimum detection size limits for different lens magnifications.…”

“…In addition to the diameter mode used in Nowak et al (2021), there is also a velocity mode available in the VisiSize software.…”

“…According to their laboratory experiments, the shadowgraph imaging and PDPA techniques agree very well for droplet sizes in the range from 10 µm up to ∼400 µm. Nowak et al (2021) investigated applicability of a shadowgraph imaging system called VisiSize D30 for cloud microphysical measurements. The reliability of detection and accuracy of sizing under conditions resembling atmospheric clouds have been verified in this research.…”

Comment on amt-2021-281

“…By default, correction of the size-dependent sample volume (SV) of the VisiSize was performed based on Nowak et al (2021).…”