The dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component of the GFDL global coupled model CM3

Abstract

The Geophysical Fluid Dynamics Laboratory (GFDL) has developed a coupled general circulation model (CM3) for atmosphere, oceans, land, and sea ice.The goal of CM3 is to address emerging issues in climate change, including aerosol-cloud interactions, chemistry-climate interactions, and coupling between the troposphere and stratosphere.The model is also designed to serve as the physical-system component of earth-system models and models for decadal prediction in the near-term future, for example, through improved simulations in tropical land precipitation relative to earlier-generation GFDL models.This paper describes the dynamical core, physical parameterizations, and basic simulation characteristics of the atmospheric component (AM3) of this model.Relative to GFDL AM2, AM3 includes new treatments of deep and shallow cumulus convection, cloud-droplet activation by aerosols, sub-grid variability of stratiform vertical velocities for droplet activation, and atmospheric chemistry driven by emissions with advective, convective, and turbulent transport.AM3 employs a cubed-sphere implementation of a finite-volume dynamical core and is coupled to LM3, a new land model with eco-system dynamics and hydrology.Most basic circulation features in AM3 are simulated as realistically, or more so, than in AM2.In particular, dry biases have been reduced over South America.In coupled mode, the simulation of Arctic sea ice concentration has improved.AM3 aerosol optical depths, scattering properties, and surface 1 clear-sky downward shortwave radiation are more realistic than in AM2.The simulation of marine stratocumulus decks and the intensity distributions of precipitation remain problematic, as in AM2.The last two decades of the 20th century warm in CM3 by .40o C relative to 1881-1920.The Climate Research Unit (CRU) analysis of observations show warming of .56o C over this period.Although CM3 includes anthropogenic cooling by aerosol-cloud interactions, its warming by late 20th century is only slightly less realistic than in CM2.1, which warmed .66o C and did not include aerosol-cloud interactions.The improved simulation of the direct aerosol effect (apparent in surface clear-sky downward radiation) in CM3evidently acts in concert with its simulation of cloud-aerosol interactions to limit greenhouse gas warming in a way that is consistent with observed global temperature changes.