A Refined Understanding of the Cloud Longwave Scattering Effects in Climate Model

Abstract

Because strong absorption of infrared radiation by greenhouse gases is more significant than the cloud longwave (LW) scattering effect, most climate models neglect cloud LW scattering to save computational costs. However, ignoring cloud LW scattering directly overestimates the outgoing longwave radiation (OLR). A recent study performed slab-ocean model simulations in the Community Earth System Model and showed that such radiative flux changes due to ice cloud LW scattering can affect the polar surface climate more than other climate zones. In this study, we included the same ice cloud LW scattering treatment in the Exascale Energy Earth System Model (E3SM) version 2 and ran fully-coupled simulations to assess the impact of ice cloud LW scattering on global climate simulation. Including ice cloud LW scattering leads to ˜2 W mˆ-2 instantaneous OLR reduction in the tropics, more than the OLR reduction in other climate zones. Strong surface warming occurs in the Arctic, which is dominantly caused by the polar amplification resulting from the radiative forcing caused by ice cloud LW scattering. In the tropics, when the ice cloud LW scattering effect is included, more liquid clouds form in the middle troposphere, high clouds in the convection zone are lifted, anvil clouds retreat, and stratiform low cloud fraction increases. Most of these effects are similar to the cloud response to the increase of well-mixed greenhouse gases. The present study suggests that the ice cloud LW scattering effect must be incorporated into climate simulations.

Uncertainties in cloud-radiative heating within an idealized extratropical cyclone

“…Although advances in radiation solvers can account for both 3D radiative effects and cloud subgrid variability, the lack of knowledge about the shape and surface roughness of ice particles continues to introduce large variability in the CRH and remains a critical challenge to better represent ice-optical properties in models. Future work in the direction of ice-optical properties should also address the problem that in many current models including ICON, the ice crystal effective radii are not treated consistently between the microphysics and radiation schemes (Sullivan and Voigt, 2021), and longwave cloud scattering is neglected (Fan et al, 2022). In view of our results, we believe that future studies should focus on the impact of uncertainties in the ice-optical properties on the dynamics and predictability of extratropical cyclones.…”

Improved Temperature‐Dependent Ice Refractive Index Compilation in the Far‐Infrared Spectrum

“…Nonetheless, work exploiting airborne observations of far‐IR radiances in the presence of cirrus clouds demonstrate an inability to simulate measurements consistently across the far‐IR and Mid‐IR spectrum using available ice‐cloud bulk optical property data sets (Bantges et al., 2020; Cox et al., 2010). This is a serious issue, especially given recent modeling work highlighting the marked impact of longwave scattering in the presence of ice cloud on atmospheric heating rates (e.g., Fan et al., 2023; Kuo et al., 2017; Ren et al., 2020), motivating the development of an improved database of ice cloud optical properties.…”