DescriptionAtmospheric aerosols and clouds present several scientific, mathematical, and computational challenges in the development of large-scale earth system models. Aerosols and clouds are inherently multiscale, as they characterize the effects of submicron-scale particles on observed local, regional, and global phenomena. While the understanding of aerosols and clouds and their atmospherically relevant mixed-phase physicochemical processes has recently grown dramatically, it has become clear that related software packages must be extended or replaced. Software design for these sophisticated models has an ever greater impact on stability, extensibility to new science, and user experience. Meanwhile, exascale computing platforms offer more simulation capability but do not alleviate the curse of dimensionality. Given the high stakes of understanding our changing climate, we must rigorously answer the question of whether our models are "correct" and quantify our confidence in our answer. This symposium examines the challenges and implications inherent to two critical areas of concern in aerosol science and cloud physics: (1) the computational stability, accuracy, and representation of complex aerosol and cloud representations in weather and climate models; (2) software development packages that support robust, scalable, portable, and testable models.