92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 4:45 PM
Estimating the Long Term Aerosol Indirect Effect in Short Global Climate Model Integrations by Controlling for Meteorology
Room 244 (New Orleans Convention Center )
Gabriel J. kooperman, SIO/Univ. Of California, La Jolla, CA; and M. S. Pritchard, S. J. Ghan, and R. C. J. Somerville

Although greenhouse gas emissions constitute the largest component of the anthropogenic radiative forcing, aerosol particle emissions represent the largest sources of uncertainty. Unlike greenhouse gases that are well mixed and have easily measured radiative properties, aerosol particles are highly variable in space and time, have varied radiative properties, and have complex interactions with cloud microphysics. Aerosol particle concentrations are strongly impacted by natural modes of variability on many timescales that influence not only their circulation, but also cloud processes important to their production and removal. As a result, long simulations are typically required to statistically isolate the anthropogenic aerosol forcing in pairwise climate model experiments with and without pollution. In this case, the two simulations not only have different emissions, but also produce unique weather patterns and may be in different large-scale climate states. The largest source of noise in estimates of aerosol indirect effect is variability in the liquid water path (LWP), such that statistically significant differences due to aerosol perturbations can only be detected by integrating over the dominant modes that influence LWP natural variability. Here we present results from an alternate approach, which implements Newtonian relaxation ("nudging") to constrain the two simulations toward observations with identical meteorology, thus reducing differences in natural variability and dampening feedback responses, to isolate the calculation of radiative forcing. This provides a more stable global estimate of the aerosol indirect radiative forcing on shorter time scales and increases the statistical significance of the signal over large parts of the world. The approach is suggested as a strategy to bring the enhanced physics of new computationally intensive aerosol enabled climate simulation technology, for which long simulations remain prohibitive, to bear on the aerosol indirect effect problem.

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