Interpreting the Dependence of Cloud‐Radiative Adjustment on Forcing Agent

Effective 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 CO2, CH4, 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. Plain Language Summary Changes in factors such as greenhouse gas concentrations or solar irradiance affect the balance of energy coming into versus leaving the earth's atmosphere, a phenomenon known as radiative forcing. This forcing can be modified by rapid atmospheric “adjustments” that occur in temperature, humidity, and cloud cover. The cloud component in particular of these rapid adjustments strongly depends on the forcing agent, for reasons that have been unclear. 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. By understanding what happens when only a small portion of the atmosphere is heated, we show that it is possible to explain cloud adjustments to more complex forcings. We anticipate that our results will provide a physical basis to understand the causes of model‐to‐model differences in cloud adjustments. Key Points Cloud adjustment depends on the “characteristic altitude” of the atmospheric heating profile Boundary‐layer (free‐tropospheric) heating causes positive (negative) cloud adjustment Low clouds dominate the cloud adjustment dependence on forcing altitude