Influences of winter haze on fog/low cloud over the Indo-Gangetic plains

The aerosol loading in south Asia has increased considerably because of the growing population, urbanization, and industrialization in recent years. To understand the effects of aerosols on cloud properties in this region, we have analyzed an extensive collection of aerosol and cloud properties, obtained from the Moderate resolution Imaging Spectroradiometer (MODIS) data, over the Indian subcontinent during winter 2000–2006. During these winter months, the Indo-Gangetic (IG) plains, in the northern part of India, are subjected to dense haze and fog on the basis of MODIS climatology of aerosol optical depth and cloud properties (cloud effective radius, cloud top pressure, and cloud fraction), respectively. We derive a fog/low-cloud detection scheme from MODIS level 2 data to generate distribution of fog/low cloud on a daily basis for December–January 2000–2006. Interactions between winter haze and fog/low cloud over the IG plains were analyzed by minimizing the effect of dynamical processes associated with fog formation. On the basis of the interannual variability of meteorological parameters, winter 2004–2005 was associated with favorable conditions for fog formation in terms of comparable values of relative humidity with respect to previous years. However, significantly lower fog occurrences were found in winter 2004–2005 over the IG plains from MODIS and ground observations, while higher aerosol loading was observed in 2004–2005 compared to previous years. Thus the role of higher aerosol loading in 2004–2005 was investigated, combined with the high concentration of black carbon (BC) aerosols over the IG plains, indicated by aerosol transport model, suggesting that the BC aerosols may have significant impact on the fog/low-cloud formation over the IG plains and hence less fog in winter 2004–2005.