Investigating the sensitivity of crystal number to aerosol and meteorological fields in CAM 5.1 using the adjoint of a physically based cirrus formation parameterization

Ice clouds play a central role in the climate system affecting Earth's radiative balance, tropospheric water distribution, and stratospheric water vapor content. Despite their climatic relevance, understanding the link between aerosol/meteorological fields and cirrus formation on a global scale remains a considerable task. Attempts to understand this link have been made through sensitivity studies where the impact of perturbations to aerosol/meteorological fields and model parameters on crystal number is quantified. Although straightforward, this way of investigating crystal number's dependence on model fields/parameters is time consuming and forces the state of the system to be altered when the evaluations are made. In addition studies like these provide limited quantification of the time evolution of crystal number's sensitivity to the model state. Investigating sensitivities using the adjoint of the cirrus formation parameterization itself can alleviate these shortcomings. Here we present the development and application of the adjoint of the Barahona and Nenes (2009) cirrus formation parameterization. This scheme allows for competition between homogeneous and heterogeneous freezing and is unique in the sense that it is able to consider any ice nuclei (IN) spectrum (provides the number of crystals that freeze heterogeneously). The adjoint was then driven using daily averaged fields including: aerosol size characteristics, ice nuclei freezing characteristics, and meteorology from version 5.1 of the Community Atmosphere Model (CAM5.1) to study the importance of model parameters to cirrus ice crystal number globally. Sensitivities were used to determine the relative impact of model parameters on crystal number variability, elucidating parameter importance spatially.