Colloquium: October 5, 2007, 3:00 pm, EN6085A

Aerosols can have a noticeable impact on cloud radiative properties and on precipitation.  Aerosols are likely to also modulate the lifetime of boundary layer clouds.   All three interactions impact the earth radiation budget. It is however difficult to document the aerosol/lifetime impact from observations. Indeed, pristine and polluted aerosol states correspond to different air masses and hence to different vertical profiles of moisture and stability. The accuracy and resolution of meteorological measurements is however insufficient for distinguishing cloud variations caused by aerosols from those caused by meteorology. 

Several investigators have explored the interplay between aerosol and meteorology within boundary layer clouds; most commonly with large eddy simulation models.   The model can include parameterizations of turbulence, radiative transfer, surface fluxes, droplet activation, condensational growth, collection and sedimentation of precipitation. The results of such studies are contradictory.  Depending on the meteorological forcing an aerosol concentration increase can lead to either an increase or a decrease of vertically-integrated cloud properties. 

These previous studies were mainly focused on short time intervals, corresponding to either nocturnal or diurnal situations. In this study, the aerosol impacts are examined in the context of the boundary layer diurnal cycle, using 36 hours large eddy simulations of boundary layers initialized with either pristine or polluted aerosol conditions. These simulations corroborate the previous findings that aerosol-induced liquid water path changes are sensitive to the meteorological forcing.  Our work shows that during the day, enhanced entrainment, inhibition of drizzle evaporation below cloud base and reduced sensible heat flux from the surface lead to a more pronounced decoupling of the boundary layer.   This is most pronounced for the polluted cloud such that their liquid water path, during the daytime, is always smaller than that of the pristine cloud.

These findings are used to propose how observational studies can be designed for validating models of aerosol impacts on the properties (radiation, precipitation and lifetime) of boundary layer clouds. It is also shown how these simulations can be used as a benchmark to test the ability of simpler models (0D or 1D) to reproduce the key features of aerosol/cloud interactions.