Clouds have a significant but complicated effect on Earth’s climate. They shield the planet from incoming solar radiation, reflecting sunlight back into space. Clouds also insulate Earth like a blanket, emitting infrared radiation down to the surface while blocking radiation emitted by the surface from escaping to space.
Despite their obvious importance for weather and climate, the complexity of cloud shapes, their ephemeral nature, and the extreme variability in how they form and how long they last make modeling clouds and their interaction with radiation very difficult. Historically, radiation schemes in weather and climate models have ignored the flow of radiation through cloud sides because the only schemes that could accurately capture these three-dimensional (3-D) effects were much too computationally costly for use in global models.
In a step toward less computationally costly methods, Schäfer et al. present a radiation scheme called the Speedy Algorithm for Radiative Transfer Through Cloud Sides (SPARTACUS). The researchers used an isolated, homogeneous, isothermal, cubic cloud as a benchmark to develop the scheme’s ability to capture infrared radiation emitted from clouds.
Theoretically, 3-D effects within this reference cloud are expected to increase the cloud radiative effect by a factor of 3. SPARTACUS was able to capture this effect, as long as the movement of energy throughout the cloud was accounted for. The authors parameterized the energy fluxes at cloud sides and presented an empirical adjustment to roughly account for the fact that in cloud clusters, neighboring clouds can absorb emitted radiation from each other, influencing the overall cloud radiative effect.
In a companion paper, Hogan et al. further explore the modifications necessary to incorporate SPARTACUS into weather and climate models. To ensure that the SPARTACUS scheme incorporated 3-D effects, the authors modified two-stream equations, which usually assume radiation to flow just up and down, to include horizontal radiative movement as well.
The study revealed that the 3-D radiative effects significantly influence how cumulus clouds influence Earth’s energy budget, but there is still room for improvement in the treatment of cloud clustering. According to the authors, more observations of cloud geometry and clustering in the real world will help researchers determine the effect of 3-D radiative transfer on the solar and infrared radiation on a global scale. (Journal of Geophysical Research, doi:, doi:10.1002/2016JD024876, 2016)
—Kate Wheeling, Freelance Writer