Numerical simulation of hail formation in the 28 June 1989 Bismarck thunderstorm: Part II, cloud seeding results

Abstract

The Institute of Atmospheric Sciences' two-dimensional, time-dependent (2DTD) hail category model is used to simulate the effects of various cloud seeding scenarios on the North Dakota Thunderstorm Project (NDTP) hailstorm of 28 June 1989. Two model cases (Case I, rain autoconversion on, and Case II, rain autoconversion off) are tested. Seeding tests concentrate on an intense isolated cell undergoing rapid evolution (Cell 3). Silver iodide seeding done early in the life of Cell 3 at cloud top, near the −10 °C level, produces significant effects. The seeding makes the cloud initiate ice earlier, resulting in increased development of small- and intermediate-sized ice particles at earlier times in the seeded runs for both Cases I and II. The initial stages of embryo production to a large extent determine the character of hail development in Cell 3 due to the limited cell lifetime. Seeding later in the life of Cell 3 is ineffective in modifying hail production because the cell has already developed abundant amounts of ice. For Case I, a positive hail suppression effect is obtained. More intermediate-sized ice particles are formed in the seeded run. These numerous hailstone embryos then compete beneficially for the available supercooled water resulting in the formation of numerous small hailstones, many of which melt before reaching the ground. Such seeding results in substantial decreases in hail fallout (44%) and hail impact energy (58%) for Cell 3. The rain from the seeded cell is increased by about 3% and from the entire system by about 7%. For Case II, with the coalescence process turned off, there is more cloud water and cloud ice but fewer precipitating ice particles than in Case I. The earlier formation and increased amounts of intermediate-sized ice particles in the seeded run allow the particles to grow in the plentiful supercooled water region resulting in more hail particles. Earlier formation of precipitating ice also initiates the rain process earlier. Seeding increases both rain and hail in this situation. The results indicate a 340% increase in rain and a 280% increase in hail in the seeded run for Cell 3, although the precipitation amounts are quite small. In the seeded run, the rain from the entire storm system is increased by 16% and hail by 22%. Seeding near the cloud base of an embedded cell produces only minor differences between the seeded and unseeded run because the surrounding cloud mass provides an ample supply of embryos in both the natural and the seeded runs. A crude simulation of the effects of hygroscopic seeding indicates a slight hail suppression effect for the relatively isolated cell, but increased rain and hail from the embedded cell. These results demonstrate the important role that the microphysical path to precipitation has not only on hail formation, but also on the potential for successful hail suppression.