Indirect impact of atmospheric aerosols in idealized simulations of convective-radiative quasi-equilibrium. Double-moment microphysics

The indirect impact of atmospheric aerosols (i.e., the impact through cloud processes) is one of the most uncertain aspects of the clouds-in-climate problem. This paper extends the previous cloud-resolving modeling study (reported at the 2006 AMS Radiation Conference and published in J. Climate in the same year) concerning the impact of cloud microphysics on convective-radiative quasi-equilibrium over a surface with fixed characteristics and prescribed solar input, both mimicking the mean conditions on Earth. The current study applies a sophisticated double-moment warm-rain and ice microphysics schemes which allow for a significantly more realistic representation of the impact of aerosols on precipitation processes and on the coupling between clouds and radiative transfer. Two contrasting CCN characteristics are assumed, representing pristine and polluted conditions, as well as contrasting representations of the effects of entrainment and mixing on the mean cloud droplet size.

As in the previous study, the convective-radiative quasi-equilibrium mimics the estimates of globally- and annually-averaged water and energy fluxes across the Earth's atmosphere. There are some differences from the previous study, however, consistent with the slightly lower water vapor content in the troposphere and significantly reduced lower-tropospheric cloud fraction in current simulations. There is also a significant reduction of the difference between pristine and polluted cases, from about 20 W/m**2 to about 4 W/m**2, with the difference between homogeneous and extremely inhomogeneous mixing reduced to about 2~W/m**2. An unexpected difference between previous and current simulations is the lower Bowen ratio of the surface heat flux, the partitioning of the total flux into sensible and latent components. It is shown that the change comes from the difference in the representation of rain evaporation in the sub-cloud layer in the single- and double-moment microphysics schemes. The difference affects the mean air temperature and humidity near the surface, and thus the Bowen ratio. The differences between various simulations are discussed contrasting the process-level approach to the impact of cloud microphysics on the quasi-equilibrium state with a more appropriate system-dynamics approach. The key distinction is that the latter includes interactions among all processes in the modeled system.