Indirect impact of atmospheric aerosols in idealized simulations of convective-radiative quasi-equilibrium: double-moment microphysics

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Description

This paper will discuss an extension of the previous cloud-resolving modeling study (Grabowski 2006) 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. Current study applies sophisticated double-moment warm-rain and ice microphysics schemes (Morrison and Grabowski 2007, 2008a, 2008b) that 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, consistent with the lower water vapor content in the troposphere and 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 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 between the single-moment parameterization used by Grabowski (2006) and the double-moment microphysics parameterization. Relatively small differences between results for pristine and polluted environments highlight the differences between the process-level approach to the impact of cloud microphysics on the quasi-equilibrium state with a more appropriate system-dynamics approach that includes interactions among all processes in the modeled system.