08/01/2012
Improved Assessment of Climate Model Clouds
Summary
Direct comparison of climate-model simulated clouds with satellite observations has been difficult because there are not direct equivalents between the model representation of clouds and what satellites are able to see. To largely solve this issue, a diagnostic tool—the Cloud Feedback Model Intercomparison Project Observation Simulator Package (COSP)—was developed by a group of scientists worldwide, including scientists at Lawrence Livermore National Laboratory (LLNL). By mimicking the satellite view of an atmospheric column with model-specified physical properties, COSP enables a meaningful comparison between modelled clouds and satellite observations overcoming the significant ambiguities in the direct comparison of model simulations with satellite retrievals. LLNL scientists, working with scientists at the National Center for Atmospheric Research (NCAR), have used COSP to assess the latest version of the NCAR/DOE Community Atmosphere Model (CAM5). Multiple independent satellite datasets and their corresponding instrument simulators in COSP were combined to systematically evaluate the model performance. Compared with the earlier atmospheric model version (CAM4), the new CAM5 model, with its more advanced physics, significantly reduces the long-standing errors in simulated clouds by increasing the total cloud fraction, decreasing optically thick clouds, and increasing mid-level clouds. The COSP diagnostics revolutionize the comparison technique, enabling consistent inter-model and observation-model comparisons. Ultimately, by better identifying model cloud biases, COSP will help to reduce uncertainty in climate predictions. This paper was included in the CCSM Earth System Model CESM1 special collection.
References
Kay, J. E., B. R. Hillman, S. A. Klein, Y. Zhang, B. Medeiros, R. Pincus, A. Gettelman, B. Eaton, J. Boyle, R. Marchand, and T. P. Ackerman. 2012. “Exposing Global Cloud Biases in the Community Atmosphere Model (CAM) Using Satellite Observations and Their Corresponding Instrument Simulators, Journal of Climate 25, 5190-5207. DOI: 10.1175/JCLI-D-11-00469.1. Community Earth System Model Special Issue.