Seasonal Dependent Impact of Ice Cloud Longwave Scattering on the Polar Climate

Abstract

Most climate models neglect cloud longwave (LW) scattering because scattering is considered negligible compared to strong LW absorption by clouds and greenhouse gases. While this rationale is valid for simulating extrapolar regions, it is questionable for the polar regions, where the atmosphere is dry and hence has weak absorption, and ice clouds that have strong scattering capability frequently occur. Using the slab-ocean Community Earth System Model, we show that ice cloud LW scattering can warm winter surface air temperature by 0.8-1.8 K in the Arctic and 1.3-1.9 K in the Antarctic, while this warming becomes much weaker in polar summer. Such scattering effect cannot be correctly assessed when sea surface temperature and sea ice are prescribed as this effect is manifested through a surface-atmosphere coupling. Cloud LW scattering is a necessity for the correct simulation of polar climate and surface radiation budget, especially in the winter. Plain Language Summary Cloud longwave scattering has never been deemed as a necessity in climate models. Out of all climate models in the IPCC fifth and sixth assessments, only three modeling centers have longwave scattering included in their models. Our study explained why the traditional wisdom of neglecting longwave scattering breaks down for the simulation of high-latitude climate in the fully coupled models. We showed the critical importance of atmosphere-surface radiative coupling for correctly assessing the role of cloud longwave scattering in the model simulation of climate mean state as well as climate changes, an issue overlooked by all previous studies. We argued that the cloud longwave scattering is a necessity in climate models, not an option.

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

“…Similar latitudinal patterns and seasonal dependence also appear in the zonalmean surface skin temperature (Figure 3a; also Figure S2 for spatial distribution) and surface downward LW flux (Figure 3b). Like what has been shown in Chen et al (2020), downward LW flux at the surface is positively correlated with surface skin temperature (𝑟 = 0.95 for DJF and 𝑟 = 0.69 for JJA). Such a seasonally dependent Arctic warming pattern is also similar to the pattern caused by the increase of CO2 (Figure S3).…”

“…Their simulations showed a 1.0~2.0 K increase in the seasonal-mean surface air temperature over the polar region, which is at least twice as fast as that in the tropics. As a cursory study, Chen et al (2020) also identified the strong correlations between the polar surface air temperature change and the change of polar surface downward LW flux. However, the analysis by Chen et al (2020) is largely confined to the polar region without examining possible connections between extra-polar and polar regions (Holland & Bitz, 2003;Stuecker et al, 2018).…”

“…This study employed the same ice-cloud optics and longwave radiation schemes as described in Chen et al (2020). In brief, a hybrid two-stream and four-stream (2S/4S) radiative transfer solver is used for longwave radiative transfer so the scattering can be included.…”

“…In this study, cloud optical properties, including cloud extinction coefficients, single-scattering albedo, and asymmetry factors, are based on a cloud particle shape model (Yang et al, 2018) consistent with that used for the Moderate Resolution Imaging Spectroradiometer (MODIS) Collection 6 operational cloud products (Platnick et al, 2015;. Details about the LW radiative transfer solver and ice-cloud topics can be found in Kuo et al (2020) and Chen et al (2020).…”

“…In this study, we incorporate the same ice-cloud LW scattering treatment in Chen et al (2020) into the Exascale Energy Earth System Model (E3SM) version 2, a flagship climate model developed by the Department of Energy (Golaz et al, 2022). LW scattering by liquid water clouds is not included here because it is estimated to be much weaker than that by ice clouds (Kuo et al, 2017).…”