Aerosol Indirect Effects Versus Aerosol-cloud Interactions: a Top-down Approach for Reconciling Estimates (Invited Presentation)

The aerosol-cloud system cannot be understood in the context of forcers alone, but rather must be considered as a whole and continuously adjusting system. Conventional practice in quantifying aerosol effects on clouds and radiation from observations entails creating metrics that isolate the response of cloud microphysical and macrophysical properties to changes in aerosol, the forcer of interest. These metrics must then be related to changes in cloud radiative properties to estimate a cloud radiative forcing. This is a bottom-up approach that involves a long chain of processes, and that carries several different types of error at each step through to a radiative forcing estimate. If we begin with a measurement of the cloud radiative forcing itself, the system is constrained at the highest level and forcings can be quantified within the whole and adjusted system. We present a top-down method for relating changes in observed cloud radiative forcing and other cloud radiative properties to changes in aerosol to better constrain aerosol indirect effects. We show how these metrics can differ substantially from a bottom-up approach and why the top-down approach is a better constraint for climate models.