ALBERT

Description: The Community Atmosphere Model version 6 (CAM6) released in 2018, as part of the Community Earth System Model version 2 (CESM2) modeling framework, is a major upgrade over the previous CAM5 that has been used in numerous global and regional climate studies in the past six years. Since CESM2/CAM6 will participate in the upcoming Coupled Model Intercomparison Project phase 6 (CMIP6) and is likely to be adopted in many future studies, its simulation fidelity needs to be thoroughly examined. Here we evaluate the performance of a developmental version of the Community Atmosphere Model with parameterizations that will be used in CMIP6 (CAM6α) with the default 1º horizontal resolution (0.9º × 1.25º, CAM6α-1º) and a higher resolution simulation (approximately 0.25º, CAM6α-0.25º), against various precipitation observational datasets over Asia. The CAM6α performance is also compared with CAM5 with the default 1º horizontal resolution (CAM5-1º). With the prognostic treatment of precipitation processes (which is missing in CAM5) and the new microphysics module, CAM6 is able to better simulate climatological mean and extreme precipitation over Asia, to better capture the heaviest precipitation events, to reproduce the diurnal cycle of precipitation rates over most of Asia, and to better simulate the probability density distributions of daily precipitation over Tibet, Korea, Japan and Northern China. Higher horizontal resolution in CAM6α improves simulations of mean and extreme precipitation over mountainous Sichuan and Northern China, but the performance degrades over the Maritime continent. Further diagnosis on moisture budget suggests that the physical processes leading to model improvement are different over different regions. Both upgraded physical parameterizations and higher horizontal resolution affect the precipitation response to internal variability of ocean and atmosphere (e.g. Asian monsoon index, ENSO, PDO), but the effects vary across different regions. Higher horizontal resolution degrades the model performance in simulating precipitation variability associated with the East Asian summer monsoon in the middle and lower reaches of the Yangtze River in China. The precipitation variability associated with ENSO gets better with upgraded physical parameterizations and higher horizontal resolution. Higher horizontal resolution, however, induces an opposite response to PDO in CAM6 over Southern China.

Description: This paper documents coupled simulations of two developmental versions of the Community Atmosphere Model (CAM) towards CAM6. The configuration called CAM5.4 introduces new microphysics, aerosol, and ice nucleation changes, among others to CAM. The CAM5.5 configuration represents a more radical departure, as it uses an assumed probability density function (PDF)-based unified cloud parameterization to replace the turbulence, shallow convection, and warm cloud macrophysics in CAM. This assumed PDF method has been widely used in the last decade in atmosphere-only climate simulations but has never been documented in coupled mode. Here, we compare the simulated coupled climates of CAM5.4 and CAM5.5 and compare them to the control coupled simulation produced by CAM5.3. We find that CAM5.5 has lower cloud forcing biases when compared to the control simulations. Improvements are also seen in the simulated amplitude of the Niño-3.4 index, an improved representation of the diurnal cycle of precipitation, subtropical surface wind stresses, and double Intertropical Convergence Zone biases. Degradations are seen in Amazon precipitation as well as slightly colder sea surface temperatures and thinner Arctic sea ice. Simulation of the 20th century results in a credible simulation that ends slightly colder than the control coupled simulation. The authors find this is due to aerosol indirect effects that are slightly stronger in the new version of the model and propose a solution to ameliorate this. Overall, in these early coupled simulations, CAM5.5 produces a credible climate that is appropriate for science applications and is ready for integration into the National Center for Atmospheric Research's (NCAR's) next-generation climate model.

Description: The radiative forcing from aviation-induced cloudiness is investigated by using the Community Atmosphere Model Version 5 (CAM5) in the present (2006) and the future (through 2050). Global flight distance is projected to increase by a factor of 4 between 2006 and 2050. However, simulated contrail cirrus radiative forcing in 2050 can reach 87 mW m−2, an increase by a factor of 7 from 2006, and thus does not scale linearly with fuel emission mass. This is due to non-uniform regional increase in air traffic and different sensitivities for contrail radiative forcing in different regions. CAM5 simulations indicate that negative radiative forcing induced by the indirect effect of aviation sulfate aerosols on liquid clouds in 2050 can be as large as −160 mW m−2, an increase by a factor of 4 from 2006. As a result, the net 2050 radiative forcing of contrail cirrus and aviation aerosols may have a cooling effect on the planet. Aviation sulfate aerosols emitted at cruise altitude can be transported down to the lower troposphere, increasing the aerosol concentration, thus increasing the cloud drop number concentration and persistence of low-level clouds. Aviation black carbon aerosols produce a negligible net forcing globally in 2006 and 2050 in this model study. Uncertainties in the methodology and the modeling are significant and discussed in detail. Nevertheless, the projected percentage increase in contrail radiative forcing is important for future aviation impacts. In addition, the role of aviation aerosols in the cloud nucleation processes can greatly influence on the simulated radiative forcing from aircraft-induced cloudiness and even change its sign. Future research to confirm these results is necessary.