Numerical Simulation of Hail Formation and Growth in a Storm with Low Supercooled Rain Water Content and the Effect of AgI Seeding on Hail Suppression

The 1 August 1981 CCOPE (Cooperative Convective Precipitation Experiment) storm is simulated using a three-dimensional compressible nonhydrostatic cloud model with two-moment bulk microphysical parameterization to investigate the mechanism of hail formation and growth and the effect of AgI seeding on hail suppression under low supercooled liquid water content conditions. The model microphysics has 2 liquid hydrometer categories (cloud droplets and rain) and 5 ice categories that are cloud ice, snow, graupel, frozen drops and hail. Frozen drops are formed by the probabilistic freezing of rain particles, and/or contact freezing resulting from collisions of rain with cloud ice crystals or active AgI particles only when the raindrop diameter is greater than 1 mm. If the raindrop diameter is smaller than 1 mm, frozen raindrops are converted to graupel. Results show that the observed storm features, such as the maximum updraft, cloud top height, radar echoes and flow structure, and liquid water content can be better simulated. Modeling results show that the maximum supercooled cloud water content and rain water content are both about 4.4 g/m3, and 29% of the hailstone embryos are frozen drops, 71% graupel. The dominant formation mechanism for graupel is contact freezing resulting from collisions of rain with cloud ice crystals or with snow flakes and autoconversion (aggregation) of snow. Once formed, accretion of cloud water becomes the dominant growth mechanism for frozen drops, graupel and hailstone. Silver iodide is inserted into cloud at 10 min, 13 min and 16 min, respectively, to compare the effects of seeding time. Seeding location is in the core of the maximum updraft, supercooled liquid water and hail water, respectively. Seeding results in substantial decreases in hail fallout, hail impact energy and rain fallout for all seeded runs. The earlier the seeding time, the less the hail. Seeding produces significant effects, resulting in more cloud ice, more graupel but fewer cloud water. These numerous smaller hailstone embryos compete beneficially for the less available supercooled cloud water resulting in fewer hailstones than the unseeded run. Accretion of cloud water by hailstones is decreased after seeding, resulting in hail amount decrease. The rain from melting of hailstones is decreased. Although more embryos are increased by seeding, the average diameter become smaller, the rain from melting of embryos is decreased.