Zonal variations of the vertical distribution of atmospheric aerosols over the Indian region and the consequent radiative effects

Abstract

Abstract. The vertical structure of atmospheric aerosols over the Indian mainland and the surrounding oceans and its spatial distinctiveness are characterized using long-term (2007–2020) spaceborne lidar observations, satellite-retrieved aerosol optical depths and assimilated aerosol single scattering albedo. The consequence of these on the spatial distribution of aerosol-induced atmospheric heating is estimated using radiative transfer calculations. The results show strong, seasonally varying zonal gradients in the concentrations and vertical extent of aerosols over the study region. In general, while over the oceans, aerosol concentrations decrease rather monotonically with increase in altitude (from its highest value near the surface), over the mainland, the concentrations initially increase from the surface to about 1 km before decreasing towards higher altitudes, in all seasons over Central India and during summer monsoon season in northern India. This is attributed to the seasonal variations in the source strengths and the atmospheric boundary layer dynamics. Compared to the surrounding oceans, where the vertical extent of aerosols is confined within 3 km, the aerosol extinction coefficients extend to considerably higher altitudes over the mainland, reaching as high as 6 km during pre-monsoon and monsoon seasons. Longitudinally, the vertical extent is highest around 75° E and decreasing gradually on either side over the peninsular India. In the west, the concentrations and vertical extent of aerosols are highest during summer/monsoon due to the lofting and strong advection of mineral dust and sea salt aerosols. Particulate depolarization ratio profiles affirm the ubiquity of dust aerosols in western India during monsoon. Dust aerosols are distributed all the way from surface to 6 km over the north-western semi-arid regions. While the presence of low-altitude dust aerosols decreases further east, the high-altitude (above 4 km) dust layers are observed to remain aloft throughout the year with seasonal variations in its zonal distribution over north-western India. Southern peninsular India and its surrounding oceans are marked with high-altitude (around 4 km) dust aerosols during the monsoon season. Radiative transfer calculations show that these changes in vertical distribution of aerosol loading and types result in enhanced atmospheric heating at the lower altitudes during pre-monsoon, with prominent heating within 2–3 km throughout the Indian region. These results will have large implications for aerosol-radiation interactions in regional climate simulations.