A novel model hierarchy isolates the limited effect of supercooled liquid cloud optics on infrared radiation

Clouds exert strong influences on surface energy budgets and climate projections. Yet, cloud physics are complex and often incompletely represented in models. For example, supercooled liquid cloud optics parameterizations are rarely incorporated into the radiative transfer models used for climate projections. Prior work has shown that incorporating these optics in longwave radiation calculations increases Arctic downwelling longwave fluxes by as much as 1.7 W m−2. Here we examine whether implementing supercooled liquid water optics in climate models for longwave radiation impacts global radiative fluxes and climate. We use a novel methodology that uses a hierarchy of dynamical constraints on the sequence of atmospheric states. In the model experiments with stronger dynamical constraints, we find that the supercooled liquid water optics increase Arctic downwelling longwave by 2.17–3.24 W m−2. In contrast, these optics increased Arctic downwelling longwave radiation by 0.36–0.68 W m−2 with dynamically unconstrained model experiments. While the optics impact was greater within the dynamically constrained models than in dynamically unconstrained models, the dynamically constrained models are also more idealized than the unconstrained models. In summary, we found a signal from supercooled liquid water optics; the influence of these optics for longwave radiation is small compared to the modeled longwave radiation variability. More broadly, this work demonstrates a novel framework for assessing the climate importance of a physics change.