Effect of aerosols on the properties of low-level liquid clouds over the Southern Great Plains, USA

One of the largest uncertainties in estimating the anthropogenic radiative forcing is related to the impact of atmospheric aerosols on cloud properties. This uncertainty originates mainly from the complicated nature of aerosol-cloud interaction as it is much stronger and more difficult to observe than the aerosol-radiation interaction. The estimates of radiative forcing due to changes in cloud properties vary significantly between different global climate models, highlighting the need for constraining this forcing by using observations. Moreover, it is challenging to determine the impact of aerosols on clouds from satellite observations only. In this study, we aim to improve that by combining the in-situ observations with satellite retrievals, in order to reduce uncertainties in the anthropogenic impact on clouds and the climate. The study is performed for the low-level liquid clouds over the Southern Great Plains (SGP), Oklahoma. The in-situ data on particle number concentration, large enough to act as cloud condensation nuclei (CCN), with diameter larger than 100 nm (N100) were collected for a 5-year period from the Atmospheric Radiation Measurement (ARM) observatory at SGP site. In this analysis, the level-2 collection-6 MODIS cloud property dataset (MYD06_L2) with a 1x1 km resolution was used, where only liquid, single-layer clouds with a cloud top warmer than 268 K were included. It was observed that both CER and COT increased with increasing CWP. However, at a same level of CWP, the CER was smaller, and COT was larger at high N100 concentrations, as compared to the lower N100 concentrations. This result is consistent with the hypothesis of enhanced aerosol load increasing the number concentration of CCN, which in turn leads to smaller cloud droplets.