Impacts of Autoconversion Scheme on Simulated Cloud Properties and Aerosol Indirect Effects Using NCAR CAM3

Catherine Chuang Lawrence Livermore National Laboratory
Jim Boyle Lawrence Livermore National Laboratory
Shaocheng Xie Lawrence Livermore National Laboratory

Category: Aerosol-Cloud-Radiation Interactions

Working Group: Cloud-Aerosol-Precipitation Interaction

Estimates of the magnitude of aerosol radiative forcing vary widely, and aerosol indirect effects remain one of the most uncertain aspects in predicting the future climate variations. Aerosol-cloud interactions begin with the direct involvement of aerosols in cloud nucleation, followed by their indirect contribution to the formation of precipitation through collision and coalescence of cloud droplets. Since the treatments of cloud microphysics in climate models are highly parameterized, it requires a thorough study to examine the range of simulated properties from different parameterizations associated with aerosol-cloud interactions. Unlike previous studies focused on climate mode simulations, our interest is in short-range model response before the development of model bias and the compensation of multiple feedback mechanisms. In this study, we modified the RK98 cloud microphysics in NCAR CAM3 to explore the sensitivity of aerosol-cloud interactions to the treatments of autoconversion over the ARM SGP site. Our sensitivity experiments indicate that the simulated LWP is very sensitive to autoconversion scheme, and a factor of two differences is noted in the mean LWP. The negative feedback of a smaller autoconversion rate on cloud fraction outweighs the influence of increased LWP on cloud forcing, resulting in a larger net TOA SW. The maximum deviation of the averaged SW cloud forcing is up to 1.92 Wm-2 with different autoconversion schemes. Calculations of sulfate indirect effects indicate that microphysical feedbacks complicate the response of climate system and then possibly yield a positive second indirect sulfate forcing that counters to the expectation of increased aerosols on net TOA SW. The calculated total sulfate indirect effects vary widely, ranging from -0.1 to -2.1 Wm-2 over SGP during the Aerosol field campaign. Since the sensitivity of aerosol indirect effects to cloud parameterizations is temporal- and spatial-dependent, integration of results over different measuring sites helps to understand the foundational physics of aerosol-cloud interactions. More aerosol field campaigns to further explore the source of uncertainties are needed.

This poster will be displayed at ASR Science Team Meeting.