| First proposed by Twomey, the aerosol first indirect effect hypothesizes that increased aerosol concentration leads to a larger number of cloud condensation nuclei, and therefore smaller but more numerous cloud droplets, which results in greater reflection of incoming solar radiation. It is known that this phenomenon has a net effect to cool the Earth radiatively and offset a substantial amount of the warming caused by the increasing of greenhouse gases. However, the magnitude of this effect has been very uncertain. For example, discrepancies of more than a factor of 2 have been reported among various observational results. This uncertainty is a major hurdle in advancing our understanding of how humans have altered, and may in the future alter the Earth's climate. One of the difficulties in deriving the magnitude of this effect from observational data arises from the fact that the aerosol abundance often varies coherently with meteorological conditions, which makes it extremely hard to distinguish between the changes in cloud microphysical parameters caused by varying aerosol concentration and by varying meteorological conditions. Therefore, the goal of this study is to find a reliable method to extract the real strength and to narrow the uncertainty in the estimates of the indirect radiative effect of aerosols. To achieve this goal, first, a satellite visible/near-infrared algorithm is developed to retrieve cloud optical depth and effective radius simultaneously at solar wavelengths (0.63 and 1.61 mum), and a satellite microwave algorithm is developed to retrieve liquid water path in the microwave range (19 and 37 GHz). Using these algorithm we derive cloud microphysical variables in relation to the aerosol first indirect effect. Second, a drizzle index is introduced to discriminate the drizzle clouds from non-drizzle clouds from satellite, which ensures our estimation of the first indirect effect not being contaminated by precipitation related processes. Third, using an analytical model, we have explained how the coherent nature between cloud depth and aerosol concentration as observed in the northeastern Pacific causes misidentification of the aerosol first indirect effect. Finally, we have further explained that the coherent variation between aerosol abundance and meteorological conditions is the major cause responsible for the large discrepancies among various observed values of the aerosol first indirect effect published in literature. We found that clouds in clean areas tend to deviate more from adiabatic process than clouds in polluted area near the coast, which causes an artifact term in commonly-used methods for deriving the aerosol first indirect effect. By introducing a new method capable of removing this artifact, the real strength of the aerosol first indirect effect is assessed over the region of Northeast Pacific. It shows that the magnitude of the aerosol first indirect effect measured by the new parameter is about half of that originally estimated by Twomey. |
