Quantifying aerosol-cloud interactions using surface aerosol observations and cloud remote sensing

The first indirect effect refers to the increase in cloud albedo that occurs as a result of an increase in aerosol concentration for clouds of equal liquid water. Microphysically, the increase in albedo corresponds to enhanced cloud droplet number concentrations and smaller cloud droplet sizes. Representation of aerosol-cloud interactions (ACI) in global climate models carries one of the largest uncertainties of all climate forcing mechanisms. The first indirect effect can be difficult to quantify using observations, as it can vary substantially under different atmospheric environments and using different observational approaches. It strongly relies on simultaneous observations of cloud and aerosol microphysical properties in clouds and at the cloud base, which can be challenging to obtain without an aircraft.

In this study, we take advantage of long-term high temporal resolution data from the Atmospheric Radiation Measurement (ARM) user facility fixed sites and campaigns to evaluate various approaches to measuring ACI using surface measurements of aerosol chemical, microphysical and hygroscopic properties and remote sensing of cloud microphysical properties such as radar retrievals of effective droplet size and lidar-based retrievals of droplet number concentration. We restrict our analyses to cases where the boundary layer is neutral, no precipitation is present, and the clouds are a single layer. We quantify the sensitivity of ACI measurement with respect to variables that represent the aerosol amount (such as aerosol scattering, total aerosol number concentration and concentration of cloud condensation nuclei, CCN). In addition, we also explore how frequently the surface conditions at ARM sites and campaigns are representative of the conditions at the cloud base using a 1-D adiabatic cloud parcel model driven by aerosol microphysical and hygroscopic properties measured at the surface and radiosonde profiles of thermodynamic variables.