Moderate resolution imaging spectroradiometer (MODIS) data have been analyzed over four different regions (Yellow sea, Korean inland, East Sea, and South Sea) in Republic of Korea to investigate the seasonal variability of aerosol-cloud properties and aerosol indirect effect during the past decade (2000–2009). Aerosol optical depth (AOD) was found to be consistently high during spring. Cloud ice radius (CIR) also showed higher values during spring, while an enhancement in cloud water radius (CWR) and fine mode fraction (FMF) was observed during summer. AOD and aerosol index (AI) were found to be higher during January to June. However, FMF and CWR showed enhancement during July to December. Aerosol indirect effect (AIE) in each year has been estimated and found to be showing positive and negative indirect effects. The AIE for fixed cloud ice path (CIP) showed positive indirect effect (Twomey effect) over Yellow sea, while the AIE for fixed cloud water path (CWP) showed a major negative indirect effect (anti-Twomey effect) over all regions. During Changma (summer monsoon) period, the AIE for both CIP and CWP showed dominant anti-Twomey effect in middle and low level clouds, indicating the growth of cloud droplet radius with changes in aerosols, enhancing the precipitation. 1. Introduction Aerosols are known to impact the formation and the life cycle of clouds by acting as cloud condensation nuclei (CCN) or ice nuclei (IN). A wide range of measurements shows that anthropogenic aerosols induce changes in clouds and their optical properties. These changes are popularly known as aerosol indirect effect (AIE) [1–3]. It is important to understand and quantify the microphysical impact of both natural and anthropogenic aerosols on clouds, in order to understand and predict climate change. The main identified AIEs include the cloud albedo effect [4] and cloud life time effect [5]. The “Twomey effect” (positive indirect effect) refers to a decrease in a cloud effective radius with increasing aerosol content for a fixed liquid water path. Contrary to this effect, an increase in the cloud droplet size with the aerosol load, or an “anti-Twomey effect” (negative indirect effect), was also reported in some parts of the world for certain environmental conditions [6]. It has also been reported that the AIE could be an important factor in modulating the response of large scale systems such as monsoons [7–9]. However, studies on AIE and its influence on precipitation are sparse across the world [10–12]. Aerosols in Asian regions are known to be an important factor in
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