Numerical simulation of hail formation in the 28 June 1989 Bismarck thunderstorm: Part I. Studies related to hail production

Abstract

This study assesses the relative influence of an active warm rain process on precipitation development and hail formation in a series of cells in numerical simulations of the severe hailstorm case of 28 June 1989 from the North Dakota Thunderstorm Project. Two model runs are reported, Case I with rain autoconversion on and Case II with rain autoconversion off. Various aspects of hail production are examined within the context of these simulations. The results illustrate the importance of the microphysical path to precipitation on subsequent hail formation, especially for isolated cells. Precipitation is initiated earlier in Case I than in Case II. Frozen drops initiate the precipitating ice in Case I; for Case II, ice processes initiate the precipitating ice field and precipitation development is slower than in Case I. Pronounced differences in precipitation development, especially hail growth and production, are indicated for a relatively isolated cell undergoing rapid evolution. Much more hail mass and larger hail are produced in this cell in Case I than in Case II, with the major embryo source for hail being frozen drops in Case I and ice embryos in Case II. Differences between Case I and Case II are much less pronounced for the other cells in the simulations which experience multiple growth surges. For a strong cell embedded in the storm system, the effect of the autoconversion process is of minor significance because the surrounding cloud mass provides an ample supply of embryos in both cases. The net effect is Case II produces slightly more hail for this embedded cell. Hail growth analyses indicate that the major growth of the ice particles occurs in high liquid water regions between −5 and −35 °C, usually between −10 and −25 °C. Trajectory analyses indicate that particles which grow to relatively large size begin their major growth cycle in a very narrow ribbon-like region in an area of weak updraft near the updraft/downdraft interface on the forward flank of the storm cells, in agreement with recent observations. Model results compared favorably with recent aircraft observations of cloud properties in the neighborhood of −40 °C within fresh, vigorous deep convection. Substantial amounts of supercooled liquid water at temperatures as low as −38 °C and an abrupt transition from water to ice at slightly colder temperatures were common features in the observations and model results.