Using Radiative Convective Equilibrium to Explore Clouds and Climate in the Community Atmosphere Model
The atmospheric circulation of the tropics is integral to the global climate and the response to external forcing. Moist physics – i.e., clouds, convection, and rainfall – in turn, play an important role in the tropical climate. Climate models differ in their representation of these features, leading to different solutions and contributing to uncertainty in future climate projections. One useful tool for better understanding differences among models and informing model development efforts is to analyze idealized configurations and experiments. This paper explores two different numerical representations of atmospheric physics within a single climate modeling framework using an idealized configuration relevant to tropical regions: radiative-convective equilibrium (RCE). The goal is to determine the impact of the differing physics on the characteristics of clouds, rainfall, and circulations as well as the tropical atmospheric response to warming.
The two representations of atmospheric physics are provided by two generations of the Community Atmosphere Model (CAM5 and CAM6). The RCE experiments remove land and ice, homogenize solar irradiance and surface temperature, and remove the influence of planetary rotation. Three surface temperatures are used to investigate the climate sensitivity (295, 300, 305K). Overall, CAM5 simulates more extreme precipitation and larger global average precipitation. Differences in the structure of clouds, particularly the amount and vertical location of cloud liquid, exist between the models and are likely related to distinct representations of shallow convection and boundary layer processes. Both CAM5 and CAM6 simulate similar peaks in cloud fraction, relative humidity, and cloud ice, linked to the usage of the same deep convection parameterization. These anvil clouds rise and decrease in extent in response to surface warming. Extreme precipitation, aggregation of convection, and climate sensitivity increase with warming in both CAM5 and CAM6. This analysis provides a benchmark for future studies that explore clouds, convection, and climate in CAM with the RCEMIP protocols now available in the Community Earth System Model.
The two representations of atmospheric physics are provided by two generations of the Community Atmosphere Model (CAM5 and CAM6). The RCE experiments remove land and ice, homogenize solar irradiance and surface temperature, and remove the influence of planetary rotation. Three surface temperatures are used to investigate the climate sensitivity (295, 300, 305K). Overall, CAM5 simulates more extreme precipitation and larger global average precipitation. Differences in the structure of clouds, particularly the amount and vertical location of cloud liquid, exist between the models and are likely related to distinct representations of shallow convection and boundary layer processes. Both CAM5 and CAM6 simulate similar peaks in cloud fraction, relative humidity, and cloud ice, linked to the usage of the same deep convection parameterization. These anvil clouds rise and decrease in extent in response to surface warming. Extreme precipitation, aggregation of convection, and climate sensitivity increase with warming in both CAM5 and CAM6. This analysis provides a benchmark for future studies that explore clouds, convection, and climate in CAM with the RCEMIP protocols now available in the Community Earth System Model.