Cloud tunings in a coupled climate model and their impact on 20th century warming

Cloud tunings in a coupled climate model and their impact on 20^th century warming

Jean-Christophe Golaz, Larry W. Horowitz, Hiram Levy II

NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ

General circulation models (GCM) are well known for incorporating a number of adjustable parameters in their cloud formulations. These parameters arise from uncertainties in cloud processes and are tuned to achieve a desired radiation balance. They control processes such as cloud formation and erosion, precipitation efficiency, and cloud droplet nucleation. A given radiation balance can be achieved by multiple combinations of these parameters.

We investigate the impact of cloud tunings in the GFDL CM3 coupled climate model by constructing two alternate model configurations (CM3c, CM3w) that achieve the desired radiation balance using different combinations of cloud parameters. All cloud parameters are within the range of other GFDL models. Although the three CM3 model configurations exhibit similar present-day climate, the magnitude of their aerosol indirect effects differs by +/- 0.6 Wm^-2. This spread in the indirect effect translates into significantly different temperature evolution over the course of the 20^th century (Figure 1). This leads us to conclude that the GCM predicted 20^th century warming is, to a large degree, tunable.

Figure 1: time evolution surface air temperature anomaly relative to 1881-1920. Observations are in black (GISS) and gray (HadCRUT3). Model results are ensemble mean (5 members each) for the reference model in green (CM3) and configurations with alternate cloud tunings in red (CM3w) and blue (CM3c).