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Study of Aerosol-Cloud Interactions for Shallow Warm Clouds
AdvisorWilcox, Eric M.
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Cumulus clouds are generally optically thick and shallow, exerting a net cooling impact on the climate system. Changes in atmospheric aerosol conditions, especially aerosol concentration increased by anthropogenic activity, can alter cloud microphysics (e.g., droplet concentration, size distribution) and cloud macrophysics (e.g., liquid water path, cloud morphology), thereby affecting cloud albedo and the Earth’s radiation budget. To deepen our understanding of aerosol-cloud-radiation interactions and to investigate the errors (e.g., due to sampling scale, remote sensing artifactual retrieval) in assessing the aerosol effects on shallow cloud and associated radiative forcing, a comprehensive study was performed by utilizing the surface station, in situ aircraft, satellite remote sensing measurements and three-dimensional radiative transfer simulations.The study over the Northern Indian Ocean revealed that more polluted clouds were substantially deeper and narrower with greater cloud liquid water path than less polluted clouds. The observed deeper clouds, mainly due to the warmer, more humid and shallower boundary layer. The narrower clouds formed in high polluted condition were caused by the intensified cloud edge evaporation effect, as a result of more and smaller cloud droplets induced by increasing aerosol concentration. The deeper and narrower clouds embedded in a high concentration of absorbing aerosols over this region contribute to a brighter atmosphere as viewed from space compared to cleaner conditions. As a consequence, the regional negative solar shortwave forcing at the top of the atmosphere due to aerosols increases in magnitude (i.e., greater cooling of regional climate) with increasing aerosol optical depth more than is contributed by just the direct effect of aerosols alone.Aerosol effects on continental shallow warm cloud were investigated by using multiple airborne and spaceborne remote sensing observations. Aerosol-cloud relationships were investigated under different meteorological conditions. Results showed that Cloud responses to aerosols were highly affected by lower tropospheric stability and free troposphere relative humidity. The Aerosol-cloud interaction index (ACI) was generally higher in an unstable and humid environment and lower under unstable and dry conditions. The total top of atmosphere cloud radiative forcing was calculated to be ~ -80 W m-2 when the ACI reached 0.3. Aerosol indirect forcing of the estimated anthropogenic portion of the aerosol due to the intrinsic aerosol effect, i.e., on cloud albedo, was estimated to be -0.63 W m-2 and the forcing due to the extrinsic aerosol effect, i.e., on cloud extent, was estimated to be -1.77 W m-2. The errors due to sampling scale and remote sensing retrieval artifacts in quantifying aerosol indirect effects on shallow warm cloud were investigated by utilizing remote sensing, in situ data and a Monte Carlo Radiative Transfer model (MCRT) designed by the author. The ACI showed a strong scale-dependent behavior, which decreases as data resolution decreases. Smoothing of aerosol and cloud fields from 1 to 10 km produced a decrease of ~ 60% in estimated ACI. The ACI estimated from 1-km resolution remote sensing data was similar to that derived from in-situ aircraft measurements. Analysis of aircraft data revealed that the aerosol humidification effect accounts for a ~ 18.7-21.8% decrease in estimate ACI. The MCRT simulation indicated that the three-dimensional radiative transfer effect from cloud side multiple scattering reduced estimated ACI by ~ 10% for a broken cloud scene.As the coupling among aerosol, cloud, radiation, and the meteorological condition is very complex, the integration of in-situ aircraft measurement, large-scale satellite observation, meteorological reanalysis data, and atmospheric modeling improves our understanding of this complex system.