**1. Introduction**

Atmospheric aerosols have been found to affect the earth's climate in many characteristic ways [1, 2]. They can affect the energy balance of the earth–atmosphere system by producing a direct or indirect change in the weather and climate system [3]. The direct interaction of aerosols

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

involves both scattering and absorption of radiation, and the relative importance of these processes depends on their chemical composition, refractive index, and size distribution [4, 5]. The indirect effect of aerosols on climate occurs by modifying the cloud optical properties [6]. Thus, the concentration, size, and composition of aerosols which can act as cloud condensation nuclei determine the cloud properties, evolution, and development of precipitation [7]. Aerosols modify cloud properties and precipitation via a variety of mechanisms with varying and contradicting consequences [8].

Cloud interactions with aerosols are hypothesized to be critical to understanding the climate change since clouds play a pivotal role in controlling incoming and outgoing radiation [9]. A large number of studies showed that the anthropogenic aerosols change clouds and their optical properties [4, 10, 11]. Atmospheric aerosols change the concentration and size of the cloud droplets which in turn lead to a change in cloud albedo, its lifetime and thereby affect the precipitation [6, 12]. Also, the reduction in cloud effective radius due to the increase in cloud droplet number concentration (CDNC) leads to the increase in cloud lifetime. The possible repercussion of this process is to decrease the rate of surface evaporation which results in stable and drier atmosphere as a result of the reduction in cloud formation [12]. Anthropo‐ genic aerosols influence mixed‐phase clouds in a number of ways and needs comprehensive study to understand the precise phenomenon. A great number of studies were conducted on the possible modification of cloud properties via the interaction with atmospheric aerosol particles, as this may lead to important changes in the Earth's climate. Biomass burning aerosols have been shown to affect clouds through both microphysical and radiative mecha‐ nisms [7, 13]. Biomass burning, from both deforestation and annual agricultural burning, is the largest anthropogenic source of such particles in the Southern Hemisphere. Biomass burning aerosols are hygroscopic and can serve as cloud condensation nuclei [14, 15]. More recently, satellite analyses have revealed a persistent correlation between cloud fraction and aerosol optical depth in regions influenced by marine aerosol, smoke, dust, and industrial air pollution [4].

The first indirect effect known as the Twomey effect produces the reduction in cloud effective radius due to the increase in aerosol loading for fixed liquid water path (LWP). Opposite of this effect (i.e., as aerosol loading increases cloud effective radius also increases) were observed over some parts of the world in certain environmental conditions [16]. The Twomey effect and Albrecht effect (i.e., lifetime effect) facilitate cooling of the atmosphere by increasing cloud optical depth (COD) and cloud fraction (CF), respectively [17]. This causes a reduction in the net solar radiation at the top of the atmosphere and hence at the surface. Several other studies have pointed out that the aerosol–cloud interactions are not determined by aerosols alone, but the regional meteorological conditions can play a significant role in this relationship [18]. Comparison of the modeled results and MODerate Resolution Imaging Spectroradiometer (MODIS) retrievals for aerosol indirect effect investigated by Myhre et al. [10] and Storelvmo et al. [18] showed a negative correlation between AOD and cloud effective radius (CER) while a positive correlation was observed between AOD and cloud optical depth (COD).

Extensive studies were conducted on various mechanisms of cloud properties through the interaction of atmospheric aerosol particles with cloud parameters which further influence the earth's climate. It was found that at low AODs, cloud optical depth (COD) increases with increasing AOD while COD decreases with increasing AOD at higher AODs. This increase was attributed to a combination of microphysical and dynamical effects, while the decrease was due to the dominance of radiative effects that thin and darken clouds [19]. The AOD and cloud fraction correlation increases for those regions which have more particulate matter due to dust, biomass, industrial, and domestic activities [20].

In the present study, we have analyzed 11 years of MODIS aerosol and cloud products at the selected stations over Western Himalayan (75°E–80°E; 29°N–33°N) and Deccan Plateau (73°E– 75°E; 16°N–19°N) regions. The selected stations over these regions comprised of (1) Dharma‐ shala (DSL), (2) Mandi (MND), (3) Shimla (SML), (4) Ludhiana (LDN), (5) Patiala (PTL), (6) Muzaffarnagar (MZR), (7) Pune (PUN), (8) Satara (STR), and (9) Kolhapur (KPR) (abbreviated hereinafter as DSL, MND, SML, LDN, PTL, MZR PUN, STR, and KPR, respectively). Western Himalaya is desert dust dominated region while Deccan Plateau is dominated by fossil fuel and biomass burning. The study deals with the spatiotemporal variations of the MODIS retrieved aerosol and cloud products at the nine selected stations over Western Himalayan and Deccan Plateau regions to bring out their salient features. The data were also employed to investigate aerosol–cloud interaction and to quantify the aerosol indirect effect (AIE) over these regions.
