Hail has been extensively observed and studied over the past few decades in numerous field projects. These projects have given great insight into the composition, mechanisms of formation, regions within a storm favorable for hail growth and storm characteristics associated with favorable hail production. In particular, the composition of hailstones following from hailstone embryo types have been studied and related to the local thermodynamic characteristics. Specifically, Knight (1981) found that cloud base temperature was associated with the most common type of hail embryo collected with warmer (colder) cloud base temperatures giving a greater fraction of frozen drops (graupel). The above finding subsequently leads to the conclusion that geographical location of storms influence hailstone embryo composition due to the significant spatial variance in climatological cloud base heights.

Our study proposes to use a next-generation bulk microphysics package within a 3-D numerical cloud model to study the sensitivity of hailstone embryo type to sounding properties (with either warm or cold cloud base temperatures). This will be done for two reasons. First, we wish to test the new microphysics model to evaluate whether is has the necessary physics (robustness) to reproduce the signal observed in nature (by Knight 1981). Next is to explain why that signal exists. Our physical hypothesis for this signal is that warmer cloud bases have a deeper region of the cloud where raindrops might be produced via warm-rain collision-coalescence growth and shedding from pre-existing wetted hail. Such storms should have a greater fraction of hail embryos from frozen drops compared to storms with colder (even subfreezing) cloud bases where the concentration of frozen raindrops (potential hail embryos) should be less.

Straka and Gilmore (2005) have developed a 24-parametric-variable, dual-moment microphysical scheme that predicts mixing ratio, total number concentration, and hydrometeor density for the hail, frozen drops, and graupel (in addition to other species). Liquid storage is additionally predicted for relevant ice species. This microphysics model will allow for evaluation of the above hypotheses as well as investigate growth trajectories that may be related to the respective fractions of hailstone embryos. The use of a double-moment scheme eliminates the guesswork associated with previous schemes where intercept and density were chosen a priori.

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