Study of Aerosol Indirect Effect on Shallow Warm Clouds: Results from SEAC4RS and ORACLES Field Campaigns

The impact of atmospheric aerosol on the cloud is termed the aerosol indirect effect (AIE). The Intergovernmental Panel on Climate Change (IPCC) reported the AIE to be the single largest uncertainty in global radiative forcing. The shallow warm clouds are particularly susceptible to aerosol by changes in atmospheric stability, free-tropospheric moisture, and microphysical interactions. The cloud responses to aerosol are diverse and complicated, depending on the aerosol/cloud types, meteorological conditions, and the relative locations of the aerosol/cloud layers. This study systematically examines the collocated aerosol and cloud properties sampled during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS, 2013) campaign and the ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES, 2016) field campaign. Data used in this study include the airborne high-resolution remote sensing retrieved aerosol (aerosol optical depth) and cloud (cloud top temperature, cloud top effective radius, cloud optical thickness, cloud liquid water path) properties from enhanced MODIS Airborne Simulator (eMAS), and aircraft in-situ measured aerosol (Ultra-High Sensitivity Aerosol Spectrometer and Passive Cavity Aerosol Spectrometer Probe) and cloud (Cloud Droplet Probe and 2D-Stereo probe) microphysics (number concentration and size distribution). With the availability of these data, the cloud responses to aerosols under the different atmospheric stability and free-tropospheric moisture conditions are analyzed. The aerosol-cloud interaction index (ACI) and the top of atmosphere (TOA) cloud radiative forcing (CRF) are calculated to estimate the regional aerosol indirect forcing.