Assessing the Variability of Cloud Droplet Sensitivity to Aerosols in Height and Time

Aerosols, acting as sites for droplet formation, shape cloud radiative properties by modulating the number and size of cloud droplets. Within global climate models (GCMs), this effect has proven difficult to assess accurately due to the complexity of the process and number of variables involved. Cloud droplet activation parameterizations provide a link that has often been probed by manually perturbing aerosol concentrations within GCMs. However, the robustness and speed of this approach hinder its application to questions that might drive future development efforts; such investigations include whether the height of modeled aerosol activation shifts the outcome or the extent to which annual average sensitivities reflect the behavior of the system.

Here, a novel approach provides a means of answering these timely questions. The adjoint of the cloud droplet activation parameterization of Kumar et al. (2009) efficiently determines the sensitivity of the cloud droplet number concentration to aerosol population characteristics and meteorological parameters. Its on-line integration with the GEOS-Chem global chemical transport model allows application with 2 degrees x 2.5 degrees horizontal resolution, providing sensitivities at each model time step. The sensitivities are assessed at a range of vertical levels. Additionally, the distribution of sensitivities over time is assessed to determine the reasonableness of annually averaging cloud droplet sensitivity to aerosol concentrations given the nonlinearity of radiative effects.