The Mean Climate of the Community Atmosphere Model (CAM4) in Forced SST and Fully Coupled Experiments

Abstract

The Community Atmosphere Model, version 4 (CAM4), was released as part of the Community Climate System Model, version 4 (CCSM4). The finite volume (FV) dynamical core is now the default because of its superior transport and conservation properties. Deep convection parameterization changes include a dilute plume calculation of convective available potential energy (CAPE) and the introduction of convective momentum transport (CMT). An additional cloud fraction calculation is now performed following macrophysical state updates to provide improved thermodynamic consistency. A freeze-drying modification is further made to the cloud fraction calculation in very dry environments (e.g., the Arctic), where cloud fraction and cloud water values were often inconsistent in CAM3. In CAM4 the FV dynamical core further degrades the excessive trade-wind simulation, but reduces zonal stress errors at higher latitudes. Plume dilution alleviates much of the midtropospheric tropical dry biases and reduces the persistent monsoon precipitation biases over the Arabian Peninsula and the southern Indian Ocean. CMT reduces much of the excessive trade-wind biases in eastern ocean basins. CAM4 shows a global reduction in cloud fraction compared to CAM3, primarily as a result of the freeze-drying and improved cloud fraction equilibrium modifications. Regional climate feature improvements include the propagation of stationary waves from the Pacific into midlatitudes and the seasonal frequency of Northern Hemisphere blocking events. A 1° versus 2° horizontal resolution of the FV dynamical core exhibits superior improvements in regional climate features of precipitation and surface stress. Improvements in the fully coupled mean climate between CAM3 and CAM4 are also more substantial than in forced sea surface temperature (SST) simulations.

Impact of Surface Forcing on Southern Hemisphere Atmospheric Blocking in the Australia–New Zealand Sector

“…The North Atlantic blocking frequency after this bias correction of the N96 simulation was comparable to a high-resolution HadGEM3 run at N216 resolution that did not have the erroneous North Atlantic SST features (Scaife et al 2011). Similarly, Neale et al (2013) attribute improvements in the representation of North Atlantic blocking in the latest version of the Community Climate System Model, version 4 (CCSM4), to a better representation of North Atlantic SST owing to a more realistic separation of the Gulf Stream.…”

Seasonal differences in the response of Arctic cyclones to climate change in CESM1

“…It consists of coupled atmosphere, ocean, land, and sea ice components all at approximately 1° horizontal resolution. Treatment of the atmospheric dynamics in this model is similar to the previous version of the model, CAM4, described in Neale et al (2013).…”

The impact of changes in parameterizations of surface drag and vertical diffusion on the large-scale circulation in the Community Atmosphere Model (CAM5)

“…It should be pointed out that the fact that we use climatological sea surface temperatures might affect the variability that could influence the simulated blocking frequency. However, the blocking pattern in NoTMS is similar to that in the previous version of the Community Atmosphere Model, CAM4 (Neale et al 2013). It is therefore likely that the improvement in blocking frequency between CAM4 and CAM5 is largely due to the TMS parameterization.…”