The Impact of Changing the Mean Hail Diameter on Simulated Supercell Storms

One of the goals when seeding clouds is to reduce the hail size, the end result of which is considered beneficial. However, there may be secondary effects that occur as a result of changing the hail size, the results of which may not be beneficial. Numerical simulations have been performed in order to determine the possible effects that changing the hail size may have on supercell storm dynamics.The simulations have been conducted using the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University. A single grid with grid spacing of 1km in the horizontal and variable spacing in the vertical was utilized. Convection was initiated using a warm (3k perturbation), moist (20% perturbation) bubble. The model was initialized horizontally homogeneously using a sounding typical of convective days over Oklahoma. Both single-moment and two-moment bulk microphysics are used. The microphysical species represented include vapor, cloud droplets, rain, pristine ice, snow, aggregates, graupel and hail. Model runs using a mean hail diameter of 3mm, 5mm, 1cm and 2cm were performed. Apart from a change in the mean hail diameter, the model runs were otherwise identical.

The simulations reveal that changing the mean hail diameter has several effects on supercell storm dynamics. Increasing the mean hail diameter produced increasingly warmer cold pools. Associated with this is a change in the longevity of the left-mover. As the mean hail diameter was increased from 3mm to 2cm, so the lifetime of the left-mover increased from about 20 minutes to over two hours. Increasing the mean hail diameter also influenced the distribution of hail with respect to the updraft. With the larger mean hail diameters, the fall velocities are greater, and the hail is situated closer to the updraft, producing a storm that has many of the characteristics of a high-precipitation supercell. The hail is situated further from the updraft in the smaller mean diameter cases, and the storm resembles a ow-precipitation supercell type. Another storm aspect that is influenced by the hail size is the low-level vertical vorticity. The simulations reveal that decreasing the mean hail diameter results in increasing the low-level vertical vorticity. Such a response could have important implications for cloud seeding and severe weather. Reasons for the effects discussed above will be presented as well as the results from the two-moment simulations.