AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
Investigating Sensitivities of Ice Crystal Concentration: The Evaluation of the Adjoint of a Physically-Based Cirrus Activation Parameterization
BENJAMIN SHEYKO, Shannon Capps, Donifan Barahona, Athanasios Nenes, Georgia Institute of Technology
Abstract Number: 437 Working Group: Aerosols, Clouds, and Climate
Abstract Ice clouds have the potential to significantly influence climate through their impact on Earth's radiative balance, stratospheric circulation, and liquid and ice water paths. Even with their climatic importance, the aerosol-cloud-climate connection is still poorly understood due to many complexities including cirrus activation. Historically efforts to understand this connection have been made through comprehensive sensitivity studies which involve making small perturbations in global climate model (GCM) inputs and observing the results of the perturbation. While being conceptually straightforward, this finite difference approximation of sensitivities is time consuming and forces the state of the system to be altered when the evaluations are made as well as lacks the ability to simultaneously evaluate output sensitivity to multiple inputs parameters. Investigating sensitivities using an adjoint has the potential to alleviate these problems.
Here we develop and evaluate the adjoint of the Barahona and Nenes (2009) cirrus activation parametrization. This physically-based activation scheme considers competition between homogeneous and heterogeneous freezing and is unique in the sense that it is able to use any ice nuclei (IN) spectrum (provides the number of crystals that freeze heterogeneously). The adjoint was validated against finite difference sensitivities and then applied globally to study the relative importance of input parameters to cirrus ice crystal number concentration for several IN spectra. Lastly sensitivities were used to determine to what extent input parameters contribute to crystal number variability, elucidating their importance spatially. The adjoint model was driven using daily averaged fields including: aerosol size distribution, ice nuclei characteristics, and meteorology from version 5.0 of the Community Atmosphere Model (CAM5).