Atmospheric aerosols have been found to affect the Earth’s climate in many characteristic ways. They can affect the energy balance of Earth’s atmosphere system by producing a direct or indirect change in the weather and climate system. The direct interaction of aerosols involves both scattering and absorption of radiation, and the relative importance of these processes depends on their chemical composition, refractive index, and size distribution. The indirect effect of aerosols on climate occurs when cloud optical properties are modified. Thus, the concentration, size and composition of aerosols which can act as cloud condensation nuclei determine the cloud properties, evolution and development of precipitation. Aerosols modify cloud properties and precipitation via a variety of mechanisms with varying and contradicting consequences.
Aerosols’ cloud interaction is speculated to be important to perceptive of the global climate change because clouds take part in a crucial task of controlling incoming radiations as well as outgoing radiation. A large number of studies showed that the anthropogenic aerosols change clouds and their optical properties. 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. 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. Anthropogenic 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 mechanisms. Burning of agriculture waste and deforestation are the major sources of origin of large particles in the Southern Hemisphere commonly called as ‘biomass burning’. Those aerosols are hygroscopic and can add to the CCN (cloud condensation nuclei). Recent research studies such as satellite analyses have reported persistent correlation between cloud fraction and aerosol optical depth in regions influenced by several aerosols present.
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 cloud water path (CWP). 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. 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. 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.
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 the clouds.
Sandeep Varpe
Department of Engineering Sciences
International Institute of Information Technology (I²IT), Pune
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