Influence of aerosol on clouds over the Indo-Gangetic Plain, India D. M. Lal • S. D. Patil • H. N. Singh • Sachin D. Ghude • S. Tiwari • Manoj K. Srivastava Received: 13 April 2012 / Accepted: 17 April 2013 / Published online: 30 April 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Using Total Ozone Mapping Spectrometer Aerosol Index (AI) and NCEP/NCAR reanalysis clouds data for the period 1979–1992, the influence of aerosol on the clouds (low and high cloud cover) over the Indo- Gangetic Plain (IGP) in India has been brought out for the first time in the present study. AI shows increasing ten- dency over the IGP suggesting that aerosol loading over this region increased significantly during the study period. In our analysis, High Cloud Cover (HCC) shows increasing trend and Low Cloud Cover (LCC) shows decreasing trend over the IGP during the same period. During pre-monsoon season when aerosol loading is more, HCC increases in positive correlation with AI. On the other hand, LCC show decreasing trend and is anti-correlated with AI. During summer monsoon, aerosol shows increasing trend but their effect on HCC and LCC is not seen to be significant. Similarly, the role of humidity on aerosol induced changes in HCC and LCC over the IGP region was also analyzed. In the low to moderate humid areas of IGP region (western and middle IGP), increasing AI leads to increase in HCC and decrease in LCC. On the other hand, in high humid areas (eastern IGP), increase in AI does not show any significant effect on HCC, but LCC shows positive trend. Therefore, we strongly argue that increasing aerosol load- ing enhances Cloud Condensation Nuclei over the region which in turn, alters the microphysical properties of clouds by reducing the size of cloud droplets, and atmospheric humidity controls the aerosol effect on clouds. During the recent period (2005–2010), similar features have also been observed over the IGP region. Keywords Low and high cloud cover  Aerosol index  Indo-Gangetic Plain  Aerosol-cloud interaction 1 Introduction Atmospheric aerosol influences the Earth energy budget and climate forcing by absorbing and scattering radiative energy and also by altering the cloud microphysics (Kaufman and Fraser 1997). However, it is still not clear how aerosols modulate the cloud properties and affect the Earth’s climate. The catalytic effect of aerosols on cloud microphysics is quite complex. Their interaction with clouds may be influenced or obscured by regional as well as local meteorological conditions and dynamics (Ten Hoeve et al. 2011; IPCC 1995). In a recent study, Kaufman and Koren (2006) showed that an increase in the aerosol column concentration leads to increase in cloud cover and vice versa. Aerosol pollution is one of the main sources of Cloud Condensation Nuclei (CCN) (Feingold et al. 2001; Andreae et al. 2002, 2004). It is believed that variation in the con- centration of CCN can change the properties of cloud microphysics. The high concentration of different sizes of aerosols over a region, in general, is linked with climate through alternation of cloud microphysical properties by enhanced number of water cloud droplets with reduced sizes (Bre ´on et al. 2002; Feingold et al. 2003; Kaufman and Fraser 1997; Twomey 1977). Many studies, including observational and modeling on different spatio-temporal domains, investigated the relationship between aerosols D. M. Lal (&)  S. D. Patil  H. N. Singh  S. D. Ghude  S. Tiwari Indian Institute of Tropical Meteorology, Pune, India e-mail: dmlal@tropmet.res.in D. M. Lal  M. K. Srivastava Department of Geophysics, Banaras Hindu University, Varanasi, India 123 Clim Dyn (2013) 41:601–612 DOI 10.1007/s00382-013-1775-z