Abstract
Abstract. Secondary ice production via processes like rime splintering, frozen droplet shattering, and breakup upon ice hydrometeor collision have been proposed to explain discrepancies between in-cloud ice crystal and ice-nucleating particle numbers. To understand the impact of this additional ice crystal generation on surface precipitation, we present one of the first studies to implement frozen droplet shattering and ice–ice collisional breakup parameterizations in a mesoscale model. We simulate a cold frontal rainband from the Aerosol Properties, PRocesses, And InfluenceS on the Earth's Climate campaign and investigate the impact of the new parameterizations on the simulated ice crystal number concentrations (ICNC) and precipitation. Near the convective regions of the rainband, contributions to ICNC can be as large from secondary production as from primary nucleation, but ICNCs greater than 50 L−1 remain underestimated by the model. The addition of the secondary production parameterizations also clearly intensifies the differences in both accumulated precipitation and precipitation rate between the convective towers and non-convective gap regions. We suggest, then, that secondary ice production parameterizations be included in large-scale models on the basis of large hydrometeor concentration and convective activity criteria.
Highlights
Cloud microphysics mediate precipitation formation, either from the in-cloud liquid or ice phase
In clouds with high cloud condensation nuclei (CCN) concentrations, precipitation is more likely to initiate in the ice phase because cloud droplets will be smaller and less likely to grow to sedimentable size
We investigate the impact of these parameterizations on the simulated ice crystal number concentrations (ICNC) and surface precipitation in a case study
Summary
Cloud microphysics mediate precipitation formation, either from the in-cloud liquid or ice phase. Accretional growth is required, be it collision-coalescence of liquid droplets, droplet riming on ice hydrometeors, or ice crystal aggregation. In clouds with high cloud condensation nuclei (CCN) concentrations, precipitation is more likely to initiate in the ice phase because cloud droplets will be smaller and less likely to grow to sedimentable size. This ice-initiated precipitation occurs often over the continents, where aerosol loadings are higher (e.g., Mülmenstädt et al, 2015; Lohmann, 2017), and in convective clouds for which the vertical motions are strong enough to carry droplets above the freezing level. Cold phase initiation has been associated with the top 10 % of heavier rains according to data from the Tropical Rainfall Measuring Mission (Lau and Wu, 2011), and precipitation indices have
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