Can Ice Nucleating Particles Affect Thunderstorm Precipitation?
Many studies have suggested that surface rainfall and hailfall are affected by the number and type of cloud condensation nuclei (CCN) that the parent thunderstorms ingest from their environments, but the effects of ice nucleating particles (INPs) have been studied much less. The neglect of INP accumulation in raindrops via drop collisions, implicit to freezing relationships in most microphysics schemes used in numerical models, has likely produced unrealistic storm responses to INPs in past studies. To address this deficiency, we developed a new immersion freezing algorithm, for use in a common bulk microphysics scheme, that freezes both cloud drops and raindrops using the same observation-based immersion freezing INP (IF-INP) activity spectrum. We present idealized simulations of deep convective clouds with microphysical differences produced by independently altering IF-INP temperature dependencies and CCN number concentrations. We find little sensitivity of surface rainfall to the magnitude and slope of the IF-INP spectra compared to CCN, due to compensating processes that deplete supercooled liquid water needed for riming growth. However, in storms with warmer bases, increased IF-INP concentrations can result in more and larger hailstones at the surface, but only at low CCN concentrations. The microphysical tendencies result from the accumulation of IF-INPs in raindrops, encouraging caution in exploring aerosol-precipitation interactions in deep convection using traditional volume-dependent drop freezing relationships.