New Primary Ice-Nucleation Parameterizations in an Explicit Cloud Model

Two new primary ice-nucleation parameterizations are examined in the Regional Atmospheric Modeling System (RAMS) cloud model via sensitivity tests on a wintertime precipitation event in the Sierra Nevada region. A model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers. The data indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C. Predicted ice concentrations from these measurements exceed values predicted by the widely used temperatures dependent Fletcher approximation by as much as one order of magnitude at temperatures warmer than −20°C. A contact-freezing nucleation model is also formulated based on laboratory data gathered by various authors using techniques that isolated this nucleation mode. Predicted contact nuclei concentrations based on the newer measurements are as much as three orders of magnitude less than values estimated by Young's model, which has been widely used for predicted schemes. Simulations of the orographic precipitation event over the Sierra Nevada indicate that the pristine ice fields are very sensitive to the changes in the ice-nucleation formulation, with the pristine ice field resulting from the new formulation comparing much better to the observed magnitudes and structure from the case study. Deposition-condensation-freezing nucleation dominates contact-freezing nucleation in the new scheme, except in the downward branch of the mountain wave, where contact freezing dominates in the evaporating cloud. Secondary ice production is more dominant at warm temperatures in the new scheme, producing more pristine ice crystals over the barrier. The old contact-freezing nucleation scheme overpredicts pristine ice-crystal concentrations, which depletes cloud water available for secondary ice production. The effect of the new parameterizations on the precipitating hydrometeors is substantial with nearly a 10% increase in precipitation across the domain. Graupel precipitation increased dramatically due to more cloud water available with the new scheme.