Abstract
Abstract. Clouds constitute a large uncertainty in global climate modeling and climate change projections as many clouds are smaller than the size of a model grid box. Some processes, such as the rates of rain and snow formation that have a large impact on climate, cannot be observed. The uncertain parameters in the representation of these processes are therefore adjusted in order to achieve radiation balance. Here we systematically investigate the impact of key tunable parameters within the convective and stratiform cloud schemes and of the ice cloud optical properties on the present-day climate in terms of clouds, radiation and precipitation. The total anthropogenic aerosol effect between pre-industrial and present-day times amounts to −1.00 W m−2 obtained as an average over all simulations as compared to −1.02 W m−2 from those simulations where the global annual mean top-of-the atmosphere radiation balance is within ±1 W m−2. Thus tuning of the present-day climate does not seem to have an influence on the total anthropogenic aerosol effect. The parametric uncertainty regarding the above mentioned cloud parameters has an uncertainty range of 25% between the minimum and maximum value when taking all simulations into account. It is reduced to 11% when only the simulations with a balanced top-of-the atmosphere radiation are considered.
Highlights
Uncertainties in climate change projections stem from uncertainties in emission scenarios, structural uncertainties that measure the range of the mean responses in different models, internal variability and parametric uncertainties that are induced by uncertainties in the model parameters (Cox and Stephenson, 2007; Hawkins and Sutton, 2009)
In this paper we investigated the impact of key tuning parameters in the cloud scheme on the present-day climate and on the anthropogenic aerosol effect
They are, for instance, associated with shallow convection, inhomogeneity of liquid water clouds, horizontal diffusion and launching of gravity waves. These parameters were never varied in the context of tuning the cloud microphysical scheme (Lohmann et al, 2007) and were not investigated in this paper
Summary
The scenario uncertainty increases with time of climate projections into the future because the scenarios depend on the demographic evolution, socio-economic development and technological changes and renovations. In this paper we investigate the parametric uncertainty of tunable parameters related to key cloud processes and radiative properties in terms of the present-day climate and for the total anthropogenic aerosol effect between pre-industrial times and the present-day. Whereas the studies by Pan et al (1998) and Haerter et al (2009) evaluated the parametric uncertainty only for the direct aerosol effect and for a simple estimate of the cloud albedo effect in response to sulfate aerosols, here we take the fast feedbacks (Lohmann et al, 2010), such as the cloud lifetime effect, the semi-direct effect and aerosol effects on mixed-phase and ice clouds, from the three major anthropogenic aerosols (sulfate, black carbon and organic carbon) into account. The estimates of the parametric uncertainty on the total anthropogenic aerosol effect in this paper are based on advanced aerosol-cloud microphysics interactions
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
