CESM Related Projects: CPT's
Representing Key Processes
Ocean Mixing Processes, Marika Holland
Poorly understood or resolved processes in climate models lead to large uncertainty in future climate predictions. The 23 different climate models used in the 2007 Intergovernmental Panel on Climate Change (IPCC) assessment projected a wide range for the fate of arctic sea ice, from complete loss in the summer by 2020 to only slight losses by 2100. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation (Serreze et al., 2007). Thus, it is important to improve key processes related to the ice-ocean system, such as explicitly resolving leads in ice-ocean models which results in more realistic ice melt and open water formation (Holland, 2003). This, in turn, has important implications for critical climate feedbacks, such as the surface albedo feedback mechanism (e.g. Winton, 2006).
Global Ocean Models, Markus Jochum
Physical processes in the ocean span a vast range of spatial and temporal scales. The winds, tides and atmospheric buoyancy forcing of the ocean occur at lateral scales of O(100–1000km), driving basin-scale gyres, the meridional overturning circulation, and wave motions such as Kelvin, Rossby and internal waves. These processes move energy through a cascade that ultimately leads to dissipation by frictional processes, both at ocean boundaries and in the interior of the deep ocean. The cascade is modulated by the oceanic eddy field and internal waves; the later provides a clear pathway from the O(1 km) vertical scales associated with the gravest baroclinic mode, to the O(10 m) scales of finescale shear that leads to instability and turbulence. Internal wave driven mixing acts most prominently on the stratification, and as such, is the most significant diffusive process acting on the potential energy budget of the oceanic interior.
Aerosol Indirect Effects, Andrew Gettelman
The single greatest uncertainty in simulations of climate is the representation of clouds (e.g. Soden and Held 2006; Randall et al. 2007; Dufresne and Bony 2008). Among clouds, the greatest uncertainty is thought to reside in low clouds (Bony and Dufresne 2005; Webb and Co-authors 2006). Low clouds are also sensitive to increases in aerosols, but the strength of the associated radiative forcing, i.e. the aerosol indirect effects (AIEs), is poorly known (Lohmann and Feichter 2005). Such great uncertainty is associated with clouds in part because cloud processes must be almost entirely parameterized in general circulation models (GCMs). From a general perspective, the central task of parameterizing clouds is modeling subgrid variability. Subgrid variability determines mass flux and entrainment rate, and it also influences many processes of direct importance to climate: radiative fluxes, surface latent and sensible heat fluxes, and microphysical process rates.
Transition in Climate Models, Sungsu Park
In 2007 the IPCC reiterated that clouds remain the largest source of uncertainty in climate projections. Climate projections with the current generation of coupled climate models exhibit a wide range of cloud feedbacks (Soden and Held 2006). Bony and DuFresne (2005), Bony et al. (2006) and others have shown that subtropical boundary layer clouds are the biggest cause of this spread. Subtropical boundary-layer clouds also contribute to the global cloud response to anthropogenic aerosol perturbations. Uncertainties in this 'aerosol indirect effect' are the biggest issue in quantifying the relative roles of greenhouse gases vs. aerosols in producing the climate change of the last 150 years, and are a major modeling target for the next IPCC assessment. Lastly, subtropical boundary layer clouds are essential to the surface energy balance and SST distribution and are key elements in biases in seasonal/ENSO coupled model forecasts.
The Community Earth System Model (CESM) is a fully-coupled, global climate model that provides state-of-the-art computer simulations of the Earth's past, present, and future climate states.
CESM is sponsored by the National Science Foundation (NSF) and the U.S. Department of Energy (DOE). Administration of the CESM is maintained by the Climate and Global Dynamics Division (CGD) at the National Center for Atmospheric Research (NCAR).