Published by Copernicus Publications on behalf of the European Geosciences Union. 8698 J. Quaas et al.: Aerosol indirect effects -general circulation model intercomparisonbetween τ a and f cld . The short-wave total aerosol radiative forcing as simulated by the GCMs is strongly influenced by the simulated anthropogenic fraction of τ a , and parameterisation assumptions such as a lower bound on N d . Nevertheless, the strengths of the statistical relationships are good predictors for the aerosol forcings in the models. An estimate of the total short-wave aerosol forcing inferred from the combination of these predictors for the modelled forcings with the satellite-derived statistical relationships yields a global annual mean value of −1.5±0.5 Wm −2 . In an alternative approach, the radiative flux perturbation due to anthropogenic aerosols can be broken down into a component over the cloud-free portion of the globe (approximately the aerosol direct effect) and a component over the cloudy portion of the globe (approximately the aerosol indirect effect). An estimate obtained by scaling these simulated clearand cloudy-sky forcings with estimates of anthropogenic τ a and satellite-retrieved N d -τ a regression slopes, respectively, yields a global, annual-mean aerosol direct effect estimate of −0.4±0.2 Wm −2 and a cloudy-sky (aerosol indirect effect) estimate of −0.7±0.5 Wm −2 , with a total estimate of −1.2±0.4 Wm −2 .

A lack of reliable estimates of cloud condensation nuclei (CCN) aerosols over oceans has severely limited our ability to quantify their effects on cloud properties and extent of cooling by reflecting solar radiation—a key uncertainty in anthropogenic climate forcing. We introduce a methodology for ascribing cloud properties to CCN and isolating the aerosol effects from meteorological effects. Its application showed that for a given meteorology, CCN explains three-fourths of the variability in the radiative cooling effect of clouds, mainly through affecting shallow cloud cover and water path. This reveals a much greater sensitivity of cloud radiative forcing to CCN than previously reported, which means too much cooling if incorporated into present climate models. This suggests the existence of compensating aerosol warming effects yet to be discovered, possibly through deep clouds.

A prognostic equation for ice crystal number concentration together with an ice nucleation scheme are implemented in the National Center for Atmospheric Research (NCAR) Community Atmospheric Model version 3 (CAM3) with the aim of studying the indirect effect of aerosols on cold clouds. The effective radius of ice crystals, which is used in the radiation and gravitational settlement calculations, is now calculated from model-predicted mass and number of ice crystals rather than diagnosed as a function of temperature. A water vapor deposition scheme is added to replace the condensation and evaporation (C–E) in the standard CAM3 for ice clouds. The repartitioning of total water into liquid and ice in mixed-phase clouds as a function of temperature is removed, and ice supersaturation is allowed. The predicted ice water content in the modified CAM3 is in better agreement with the Aura Microwave Limb Sounder (MLS) data than that in the standard CAM3. The cirrus cloud fraction near the tropical tropopause, which is underestimated in the standard CAM3 as revealed through comparison with the Stratospheric Aerosol and Gas Experiment II (SAGE II) data, is increased by 20%–30%, and the cold temperature bias there is reduced by 1–2 K. However, an increase in the cloud fraction in polar regions makes the underestimation (by ∼20 W m−2) of downwelling shortwave radiation in the standard CAM3 even worse. A sensitivity test reducing the threshold relative humidity with respect to ice (RHi) for heterogeneous ice nucleation from 120% to 105% (representing nearly perfect ice nuclei) increases the global cloud cover by 1.4%, temperature near the tropical tropopause by 4–5 K, and water vapor in the stratosphere by 50%–80%.

[1] Aerosol indirect effects have remained the largest uncertainty in estimates of the radiative forcing of past and future climate change. Observational constraints on cloud lifetime effects are particularly challenging since it is difficult to separate aerosol effects from meteorological influences. Here we use three global climate models, including a multiscale aerosol-climate model PNNL-MMF, to show that the dependence of the probability of precipitation on aerosol loading, termed the precipitation frequency susceptibility (S pop ), is a good measure of the liquid water path response to aerosol perturbation (l), as both S pop and l strongly depend on the magnitude of autoconversion, a model representation of precipitation formation via collisions among cloud droplets. This provides a method to use satellite observations to constrain cloud lifetime effects in global climate models. S pop in marine clouds estimated from CloudSat, MODIS and AMSR-E observations is substantially lower than that from global climate models and suggests a liquid water path increase of less than 5% from doubled cloud condensation nuclei concentrations. This implies a substantially smaller impact on shortwave cloud radiative forcing over ocean due to aerosol indirect effects than simulated by current global climate models (a reduction by one-third for one of the conventional aerosolclimate models). Further work is needed to quantify the uncertainties in satellite-derived estimates of S pop and to examine S pop in high-resolution models. Citation: Wang, M., et al. (2012), Constraining cloud lifetime effects of aerosols using A-Train satellite observations, Geophys. Res. Lett., 39, L15709,