Evaluation of hygroscopic cloud seeding in liquid-water clouds: a feasibility study

Abstract. An airborne cloud seeding experiment was conducted over the eastern coast of Zhejiang, China, on 4 September 2016 during a major international event held in Hangzhou. In an attempt to reduce the likelihood of rainfall onset, a major airborne experiment for weather modification took place by seeding hygroscopic agents to warm clouds to reduce cloud droplet size. The effectiveness of seeding is examined, mainly for stratiform clouds with patchy small convective cells. A radar-domain-index algorithm (RD1) was proposed to analyze the seeding effect. The threshold strategy and the tracking radar echo by correlation (TREC) technique was applied in the domain selection. Factors analyzed include echo reflectivity parameters such as the mean and maximum echo intensity, the anomaly percentage of the grid number of effective echoes, the fractional contribution to the total reflectivities, and the vertically integrated liquid water content (VIL) during and after the seeding process. About 12 minutes after seeding ended, the composite reflectivity of seeded clouds decreased to a minimum (< 10 dBz} and the VIL of seeded clouds was -0.2 kg m*3. The echo top height dropped to -3.5 km, and the surface echoes were also weakened. By contrast, there was no significant variation in these echo parameters for the surrounding non-seeded clouds. The seeded cell appeared to have the shortest life cycle, as revealed by applying the cloud-cluster tracking method. The airborne CDP measured cloud number concentration, effective diameter and liquid water content gradually increased since the cloud seeding start. This probably because the hygroscopic growth by agent particles and collision-coalescence by small cloud droplets. However, these parameters sampled at -40 min after seeding decreased significantly, which probably due to the excessive seeding agents generated a competition for cloud water and thus suppressing cloud development and precipitation. Overall, the physical phenomenon was captured in this study, but a more quantitative in-depth analysis of the underlying principle is needed.

Introduction

Weather modification, mainly by cloud seeding, is a common technique of changing the amount or intensity of precipitation. Cloud seeding activities include dispersing agents to a cloud by ground-based (Dessens, 1998), rockets (Warburtonetal., 1982; Radinovic and Curie, 2007), and aircraft (Jung et al., 2015; French el al., 2018). The seeding agents can serve as cloud condensation nuclei (CCN) to advance the collision-coalescence process in warm clouds (Jensen and Lee, 2008; Jung et al., 2015), or serve as ice nuclei (IN) to convert liquid water into ice crystals and strengthen vapor deposition, riming, and aggregation processes in super-cooled clouds. The theories behind hygroscopic and glaciogenic cloud seeding have been well documented (Schaefer, 1946; Vonnegut, 1947; Bowen, 1952), but the actual effect in practice remains highly uncertain and even controversial (Council, 2003).

Many laboratory, modeling, and field experimental studies on cloud seeding have been conducted for more than a half century, assessing the effectiveness of cloud seeding is very challenging due to notorious difficulties in gaining convincing scientific evidences. The randomized evaluation of cloud seeding based on multiple samples has been performed with relatively high support and confidence (Gagin and Neumann, 1981; Silverman, 2001). However, conducting a long-term, well-designed, and randomized cloud seeding experiment is fraught with difficulties and uncertainties (Guo et a!., 2015). Relative to modeling and statistical evaluations, much fewer have been done to acquire direct observational evidences in field experiment of the effectiveness of cloud seeding (Kerr, 1982; Mather et al., 1997; Silverman, 2003). Encouraged by some recent successes (Tessendorf et a!., 2012, 2018), we have attempted to investigate the effectiveness of cloud seeding by exploring different evaluation methods.

Presented here is a study of assessing the cloud seeding effect by injecting hygroscopic particles into a convective cell in a warm stratocumulus cloud for the prevention of rainfall. Hygroscopic seeding to promote the drop collision-coalescence process in liquid-water clouds has been investigated for some time (Bowen, 1952). Rosenfeld et al. (2010) concluded that hygroscopic seeding was generally guided by three conceptual models: seeding with large CCN that serve as embryos for raindrops, acceleration of the coalescence process via the competition effect, and widening of cloud drop size distribution though the tail effect. However, hygroscopic materials of different properties, different concentration and size distribution may have positive or negative responses to cloud seeding. Previous studies (Bruintjes. 2003: Belyaeva et al., 2013) have shown that introducing a certain amount of CCN into clouds could broaden the cloud droplet spectrum at the initial stage of condensation, intensify coagulation during the formation of precipitation, and enhance the lifetime of convective clouds by changing their vertical structure. For example, flares generate giant hygroscopic particles which could shift the cloud drop size distribution toward large sizes, thereby promoting the coalescence process and enhancing precipitation (Tzivion et al., 1994; Cooper et al., 1997). A modeling study (Segal et al., 2004) showed that hygroscopic particles with diameters (/)) of 3-6 urn are optimal for enhancing precipitation in liquid-water clouds. Conversely, high concentrations of small hygroscopic particles may suppress precipitation (Rosenfeld et al., 2008). The increasing CCN from anthropogenic pollution causes higher cloud drop concentration and narrower droplet spectrum, leading to suppressed drizzle formation and prolonged stratiform clouds (Bruintjes, 2003). They also produce brighter clouds that are less efficient in precipitation (Albrecht, 1989). Some modeling studies on hygroscopic seeding have suggested similar effects, such as Yin et al. (2000) who reported that particles with D less than 2 urn had a negative effect on rain development in convective clouds.