Quantifying and Decomposing the Shortwave Indirect Effect into the Twomey and Cloud Lifetime Components

Marine stratocumulus (mSc) play an important role in Earth’s radiative budget cooling the atmosphere and surface by reflecting shortwave radiation.  Increases in biomass burning and other anthropogenic activity near large mSc decks could alter Earth’s radiative balance through aerosol-cloud-radiative interactions.  Unfortunately, modeling and observations have led to little understanding of the expected impacts of aerosols on cloud’s radiative balance, also known as the indirect effect, which could have large implications for the climate sensitivity.  Four years of collocated NASA A-Train satellite retrievals are used to assess the response of mSc in the Southeast Pacific and South Atlantic to changes in aerosol concentration.  Scenes are separated into local meteorological regimes according to their local stability and the free tropospheric humidity derived from reanalyses, two factors that have been shown to influence the cloud fraction and albedo.  Shortwave indirect effects in each regime were then further decomposed into contributions from the albedo effect, whereby clouds become brighter with increased aerosol loading, and the cloud lifetime effect, whereby clouds remain in the atmosphere longer due to suppressed drizzle formation.  Results suggest the magnitude of the indirect effect, and dominance of the Twomey or cloud lifetime effect, varies by meteorological regime.