The sensitivity of simulated convective storms to variations in prescribed microphysics parameters

Sensitivity studies conducted within the context of an eight-dimensional parameter space numerical simulation study reveal the susceptibility of storm morphology and intensity to changes in the amplitudes of some important prescribed microphysical parameters. Among parameters tested are graupel and hail mean-mass diameters, particle number concentrations, and the shape parameters of the assumed Gamma-function particle distributions.

Findings are that the general statistics of updraft speeds and other dynamic parameters of the storms are relatively insensitive to the microphysics changes tested. However, the form and arrangement of simulated storm cells does show significant sensitivity to the microphysics parameter changes. The lack of sensitivity is partly due to counteracting effects. For example, increasing the shape parameter in the gamma distribution increases the rate of collection of cloud water, but also increases the rate of evaporation of rain, leading to only a small change in precipitation rate. Larger hailstones have lower collection rates per unit mass, but this allows more cloud water to be available for collection by graupel, whose peak concentration is at a slightly higher altitude. Although the larger hail and graupel together collect less cloud water, the larger hail melts more slowly, resulting in less evaporation of rain, and only a small change in precipitation rate.