Paper in Press
GEOPHYSICAL RESEARCH LETTERS, doi:10.1029/2012GL052204
Constraining cloud lifetime effects of aerosols using A-Train satellite observations
- A new metric is shown to be a good measure of cloud lifetime effects of aerosols
- Satellite-derived Spop is significantly lower than those from global models
- Satellite observations imply a substantially smaller aerosol indirect effects
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 multi-scale aerosol-climate model PNNL-MMF, to show that the dependence of the probability of precipitation on aerosol loading, termed the precipitation frequency susceptibility (Spop), is a good measure of the liquid water path response to aerosol perturbation (λ), as both Spop and λ 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. Spop 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 (SWCF) over ocean due to aerosol indirect effects than simulated by current global climate models (a reduction by one-third for one of the conventional aerosol-climate models). Further work is needed to quantify the uncertainties in satellite-derived estimates of Spop and to examine Spop in high-resolution models.
Received 30 April 2012; accepted 11 July 2012.
Citation: (2012), Constraining cloud lifetime effects of aerosols using A-Train satellite observations, Geophys. Res. Lett., doi:10.1029/2012GL052204, in press.