Abstract
AbstractEffective radiative forcing includes a contribution by rapid adjustments, that is, changes in temperature, water vapor, and clouds that modify the energy budget. Cloud adjustments in particular have been shown to depend strongly on forcing agent. We perform idealized atmospheric heating experiments to demonstrate a relationship between cloud adjustment and the vertical profile of imposed radiative heating: boundary‐layer heating causes a positive cloud adjustment (a net downward radiative anomaly), while free‐tropospheric heating yields a negative adjustment. This dependence is dominated by the shortwave effect of changes in low clouds. Much of the variation in cloud adjustment among common forcing agents such as , , solar forcing, and black carbon is explained by the “characteristic altitude” (i.e., the vertical center‐of‐mass) of their heating profiles, through its effect on tropospheric stability.
Highlights
Radiative forcing quantifies the perturbation to the Earth's energy budget associated with a particular climatic factor, such as greenhouse gases, aerosols, or solar irradiance
We find that the vertical structure of atmospheric heating explains much of the forcing agent dependence of the cloud adjustments: bottom-heavier heating causes a more positive cloud adjustment
We have demonstrated through a series of idealized experiments with vertically localized atmospheric heating that cloud-radiative adjustments are sensitive principally to the altitude of atmospheric heating caused instantaneously by the forcing agent
Summary
Radiative forcing quantifies the perturbation to the Earth's energy budget associated with a particular climatic factor, such as greenhouse gases, aerosols, or solar irradiance. “effective radiative forcing” (ERF) has become the usual metric (Myhre et al, 2014), which accounts for relatively short-timescale tropospheric adjustments in temperature, moisture, and clouds that are direct responses to the forcing, rather than being mediated by surface warming (Andrews & Forster, 2008; Gregory & Webb, 2008; Sherwood et al, 2015). Previous studies have identified a dependence of semi-direct cloud adjustments upon forcing altitude: typically positive for boundary-layer forcing, negative for free-tropospheric forcing (Amiri-Farahani et al, 2017; Ban-Weiss et al, 2012; Bellouin et al, 2020; Koch & Del Genio, 2010; Samset & Myhre, 2015; Stjern et al, 2020). This is demonstrated through comparison of idealized and common forcing experiments with a CMIP5-class climate model
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