Airflow and hail growth in supercell storms and some implications for hail suppression

Abstract Multiple radar and aircraft observations of a damaging supercell hailstorm in northeastern Colorado are synthesized to show that the airflow and hail growth conformed in many respects to earlier models derived by the lead author. Some features that before had to be inferred indirectly are now substantiated and elaborated upon by direct observation. As a result of our increased confidence in certain aspects of the model we have been able to draw implications regarding the feasibility of suppressing hail by different means. The observations indicate that an important stage in the growth of large hail is the entry of hailstone embryos into the edge of an intense updraught. These embryos are grown near the storm's right flank and get carried around the forward flank under the influence of a strong environmental airstream which divides at the stagnation point and flows around on either side of the main updraught. The main updraught itself is characterized by a weak‐echo vault and the embryos grow into large hail as they follow a simple up‐and‐down trajectory over it. The vault takes on added significance in that it is a symptom of inefficiency in the conversion of cloud water to precipitation. Paradoxically it is this very inefficiency that encourages the growth of large hail by minimizing the effects of competition for the available supercooled droplets. Those recirculating embryos that find their way to the edge of the vault via the embryo curtain are seen to compete unfairly by being the first to encounter the undepleted cloud water in the vault. It is difficult to suppress their growth artificially because seeding in the main updraught at economical rates produces particles that rise above the hail growth region before they can have any significant influence on the available water. The existence of a large vault implies that the flow field is so restricting the number of embryos re‐entering the main updraught that they may be sweeping up only a small proportion of the overall cloud water. Thus there is a danger that, if a mode of seeding were used which caused additional embryos to re‐enter the updraught along with the re‐entering natural embryos, this would lead to the production of additional large hailstones. Hence, at least for some storms, doubt is cast on the credibility of the competing embryo hypothesis for hail suppression, according to which the generation of additional embryos by seeding is supposed to promote competition for a restricted amount of cloud water to the extent that none can grow large.