期刊名称：Journal of Advances in Modeling Earth Systems
电子版ISSN：1942-2466
出版年度：2017
卷号：9
期号：1
页码：514-535
DOI：10.1002/2016MS000841
出版社：John Wiley & Sons, Ltd.
摘要：In model studies of aerosol‐dependent immersion freezing in clouds, a common assumption is that each ice nucleating aerosol particle corresponds to exactly one cloud droplet. In contrast, the immersion freezing of larger drops—“rain”—is usually represented by a liquid volume‐dependent approach, making the parameterizations of rain freezing independent of specific aerosol types and concentrations. This may lead to inconsistencies when aerosol effects on clouds and precipitation shall be investigated, since raindrops consist of the cloud droplets—and corresponding aerosol particles—that have been involved in drop‐drop‐collisions. Here we introduce an extension to a two‐moment microphysical scheme in order to account explicitly for particle accumulation in raindrops by tracking the rates of selfcollection, autoconversion, and accretion. This provides a direct link between ice nuclei and the primary formation of large precipitating ice particles. A new parameterization scheme of drop freezing is presented to consider multiple ice nuclei within one drop and effective drop cooling rates. In our test cases of deep convective clouds, we find that at altitudes which are most relevant for immersion freezing, the majority of potential ice nuclei have been converted from cloud droplets into raindrops. Compared to the standard treatment of freezing in our model, the less efficient mineral dust‐based freezing results in higher rainwater contents in the convective core, affecting both rain and hail precipitation. The aerosol‐dependent treatment of rain freezing can reverse the signs of simulated precipitation sensitivities to ice nuclei perturbations.
