Seasonal Remote-Sensing Observations of Aerosol-Cloud Relationships under Varied Water Vapor Conditions

Understanding the aerosol-cloud relationship is critical for reducing uncertainties in climate projections, especially over regions that experience complex aerosol dynamics from both natural and anthropogenic sources. This study aims to investigate how aerosols influence cloud properties under varying water vapor conditions over nine urban and rural subregions of the Eastern Mediterranean, disentangling the seasonal and annual relationships between aerosols and cloud parameters while distinguishing the effects of different aerosol types. For this purpose, Aerosol Optical Depth (AOD) at 550 nm, Water Vapor (WV) under clear-sky conditions, Cloud Cover (CC), Cloud Optical Depth (COD), and Cloud Top Pressure (CTP) from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard the Aqua satellite, for the time period July 2002-December 2012. Additionally, anthropogenic (AODanthr) and dust AOD (AODdust) datasets, constructed through a synergy of satellite observations, chemical transport modeling, and reanalysis products were utilised. Our results reveal consistent increases in CC with increasing total, anthropogenic, and dust aerosol loading across all regions, seasons, and water vapor levels, supporting the hypothesis of aerosol-induced cloud invigoration. COD was found to increase with AOD when AOD < 0.5 but remain steady or decline for higher AOD levels, independent of water vapor concentration. Furthermore, anthropogenic aerosols tend to enhance COD more strongly than dust aerosols. Seasonal differences in cloud height were also observed: in spring and summer, CTP decreases (indicating higher cloud tops) with increasing AOD and CC, while in autumn and winter, under high water vapor conditions, aerosol loading leads to higher clouds.