How does Tropospheric Chemistry Affect the Cloud Radiative Effect?

One of the greatest uncertainties in current climate models is the cloud-climate radiative feedback. The cloud-climate radiative feedback is the how the radiative flux out of the top of the atmosphere changes due to the response of climate warming on clouds. The cloud-climate radiative feedback due to anthropogenic warming is estimated to be -0.55 Wm-2 according to the AR5 IPCC report, but with uncertainty included this feedback ranges from -1.33 Wm-2 to as little as -0.06 Wm-2. This feedback is intimately connected to the cloud radiative effect (CRE). The net cloud radiative effect (CRE) is the combined effect of low and high clouds on the energy budget of the Earth system. Low clouds tend to reflect solar radiation, while high clouds tend to trap the outgoing infrared radiation absorbed and re-emitted by the Earths surface. In this study we compare a Community Earth System Model (CESM2) case with fully coupled full atmospheric chemistry to a simple version of the same case that only has stratospheric and mesospheric coupled chemistry enabled. In the fully coupled chemistry case the treatment of secondary organic aerosols has been improved. The mean differences were analyzed and tested for statistical significance. Between the two cases, there is a difference in the net CRE ranging between -10 Wm-2 to 15 Wm-2 with significant regional differences across the globe. There are regional differences in the surface temperature ranging between -5 K to 5 K. There are significant differences in other global climate parameters like precipitation and sea level pressure. The improvements to aerosol treatment in the model do not influence the direct aerosol effect as much as it influences the indirect aerosol effect. The coupled chemistrys influence on the indirect aerosol effect also seems to influence the global albedo and dynamics. One area of further investigation will be based around the influence of the fully coupled chemistry and aerosol treatment on the deposition of aerosols onto sea-ice surfaces.