The in-situ measurement of cloud microphysics parameters such as cloud droplet spectrum and particle number concentration, is of great significance to research in cloud and precipitation physics, weather modification, and optoelectronic engineering for national defense. Currently, various methods for observing cloud microscopic characteristics have been developed, combining space-based in-situ observation, ground-based remote sensing, and space-based remote sensing. Due to differences in data and the theoretical basis among different inversion methods, significant variations exist among different cloud inversion products. Scholars typically aim to validate the accuracy of various remote sensing inversion products using in-situ measurements of cloud parameters. The measurement of cloud particles using research aircraft is a relatively representative in-situ method. However, the high-speed motion of the aircraft can cause particle breakup, and the rapid changes in the flow field can lead to errors in particle spectrum measurement. Additionally, due to safety concerns, research aircraft cannot operate in harsh environments such as thunderstorm centers and supercells. Lightweight cloud particle detectors, mounted on balloons and UAVs, have garnered increasing attention as an important supplementary method for in-situ measurements. Therefore, we design a lightweight cloud particle imager (LCPI) based on forward scattered light imaging. The LCPI can meet the needs of both ground-based and balloon-borne observations.

LCPI primarily utilizes the Tyndall effect to capture particle images in dark-field conditions. LCPI is mainly composed of three parts: a ring-shaped light source, a magnifying lens, and an imaging unit (CMOS). Designed for field experiments, LCPI features a lightweight and miniaturized design, weighing less than 2 kg and measuring 110 mm (base diameter) ×310 mm (height). The CMOS has a resolution of 640 pixel×480 pixel with a pixel size of 3.75 μm. A magnifying lens with a magnification of 2.5 is used in front of the CMOS, which results in a theoretical resolution of 1.5 μm and an effective sampling field reduced to 960 μm×720 μm. The ring-shaped light source consists of 8 high-brightness white LEDs with uniform specifications, mounted on a circular base to minimize beam divergence. When there are no cloud particles in the sampling volume, the 8 illumination beams intersect at the front of the magnifying lens and illuminate the sampling volume. Since light cannot directly enter the CMOS, the image appears visually pure black in this scenario. As cloud particles enter the sampling volume with airflow, these 8 illumination beams are scattered by the cloud particles. Forward scattered light from particles forms bright images on the CMOS through the magnifying lens. To avoid repeated sampling of the same particles, sampled air flows out through small holes beneath the fan, which are located underneath the fan.

To verify the accuracy of microphysical characteristic parameters such as particle size, shape, and spectrum distribution of cloud particles detected by LCPI, we first design a high-precision calibration device for it and calibrate the instrument in the laboratory (Fig. 3). The magnification of the imaging system can be directly obtained through magnification calibration, and the actual magnification of LCPI is 2.5 (Fig. 4). Measurement accuracy calibration shows that LCPI can accurately measure particle sizes under sunny, cloudy, and night conditions (Fig. 5). The measurement error of LCPI is within ±10% (Fig. 6). Sampling volume calibration demonstrates that the sampling volume increases linearly with particle diameter (Fig. 7). To further evaluate LCPI’s performance in measuring actual cloud and fog conditions, we conduct a comparative experiment in Lushan Mountain using LCPI and FM-120. The experimental results show good agreement between the two instruments (Fig. 9). The consistency in particle number density, volumetric water content, and mean diameter obtained by the two instruments reach 0.9316, 0.8221, and 0.8645, respectively, which indicates that LCPI can accurately measure microscopic cloud characteristics (Fig. 11).

In this paper, we present the calibration and preliminary experimental results of a novel LCPI. Based on the dark-field imaging principle, the instrument uses a ring-shaped light source composed of 8 high-brightness white LEDs to improve the imaging effect of small particles, which effectively solves the problem where small particles are easily submerged by background light and cannot be imaged. To ensure measurement accuracy, we design and complete magnification, measuring accuracy, and sampling volume calibration using a high-precision calibration platform developed for this purpose. The calibration results show that the actual magnification of the detector is 2.5, the measurement error is within ±10%, and there is a linear relationship between sampling volume and particle diameter. The particle size can be accurately measured under different lighting conditions. Comparative observational experiments with LCPI and FM-120 are conducted at Lushan Mountain. The comparison reveals high consistency in cloud particle number density (0.9316), volumetric water content (0.8221), and average diameter (0.8645), which proves the LCPI’s accurate measurement of cloud particle microphysical characteristics. Compared to the FM-120, this instrument can also capture images of particles, which can subsequently be used to study cloud microphysical processes involving particles of various shapes, such as ice clouds and mixed-phase clouds.